Intro to Computer Science I
Chapter 4
Classes, Objects, and Methods
OOP Concepts
1
Strings (1)
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A String is a sequence of 0 or more
characters
Every string has a length associated with it
An empty string has length 0
Each character is stored internally as a 16-bit
Unicode character (16-bit integer)
Each character in a string has an index
associated with it.
2
Strings (2)
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Literal string: sequence of 0 or more
characters delimited by double quote
characters
The double quotes
Example: "Hello"
are not part of the string
Empty string: ""
Memory model:
index
begins at 0
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H
e
l
l
o
0
1
2
3
4
3
Substring
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A substring is constructed by specifying a
subsequence of characters
H
e
l
l
o
0
1
2
3
4
e
l
l
0
1
2
A substring is a
string
4
Constructing a literal string
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Strings are objects
String greeting = "Hello World";
greeting
String
Hello World
object reference
variable
object
reference
object
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5
String expressions
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Strings can be concatenated together
Numeric values can be automatically
converted to strings
The + sign is used to denote addition and
also string concatenation
RULE: In a + b if one of a or b is a string
then the other will be converted to a string if
necessary
6
String expression examples
String first = "William";
String middle = "James";
String last = "Duncan";
String fullName =
first + " " + middle + " " +
last;
fullname has the value "William James Duncan"
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7
Mixed string expressions (1)
String area = "Area: " +
Math.sqrt(2.0);
The value of area is
"Area: 1.4142135623730951"
You can try this in BeanShell and use print(area);
to see the result
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8
Mixed string expressions (2)
"The sum of " + x + " and " +
y + " is " + x + y
if x is 3 and y is 4 then the result of this expression is
"The sum of 3 and 4 is 34"
What's wrong?
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9
Length of a string
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Prototype:
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int length()
Example:
String name = "Harry";
int len = name.length();
The value of len
will be 5
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10
Number to string conversion

Examples
String s1 = "" + age;
String s2 = "" + area;
If age is 34 and area is 2.34 then
s1 and s2 will have the values
"34" and "2.34"
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11
Extracting characters
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Prototype:
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char charAt(int index)
Example
String s = "Hello";
char c = s.charAt(1);
Here the value of c is 'e'
(index begins at 0)
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12
Substring construction
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Prototypes
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String substring(int firstIndex)
String substring(int firstIndex,
int lastIndexPlusOne)
int d = 531452;
String sd = "" + d;
int len = sd.length();
sd = "$" + sd.substring(0,len-3)
+ "," + sd.substring(len-3);
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result is
"$531,452"
13
Trimming spaces
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Prototype

String trim()
String s = "
s = s.trim();
Hello
";
result is
"Hello"
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14
String case conversion
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upper/lower case conversion
Prototypes
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String toLowerCase()
String toUpperCase()
String test = "Hello";
String upper = test.toUpperCase();
String lower = test.toLowerCase();
char first = test.toLowerCase().charAt(0);
results are "HELLO", "hello" and "h"
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15
String search methods
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Searching for one string inside another
signature
Prototypes
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int
int
int
int
indexOf(char ch)
indexOf(char ch, int startIndex)
indexOf(String sub)
indexOf(String sub, int startIndex)
Method Overloading: these methods
all have the same name but they have
different signatures
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16
String search examples
String target = "This is the target string";
target.indexOf('u')
target.indexOf("the")
target.indexOf("target",13)
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value is -1
value is 8
value is -1
17
Displaying numbers, strings
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Some prototypes:
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void
void
void
void
void
void
void
print(int n)
print(double d)
print(String s)
println(int n)
println(double d)
println(String s)
println()
print but don't
move to next line
print and move
to next line
Output appears in the terminal window
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18
Examples:
double area = 3.14159;
System.out.println("Area: " + area);
double area = 3.14159;
System.out.print("Area: ");
System.out.println(area);
These produce the same results
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19
Examples:
System.out.println("Hello");
System.out.print("Hello\n");
\ is called the
escape character
These produce the same results since \n is interpreted as the newline
character
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20
Examples:
System.out.println("\"\\Hello\\\"");
Displayed result is
"\Hello\"
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21
The BlueJ terminal window
put this method in
CircleCalculator
class
public void display()
{ System.out.println("Radius = " + radius);
System.out.println("Area = " + area);
System.out.println("...");
}
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22
The toString method
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Prototype
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Purpose
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return a string representation of an object
Default representation
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public String toString()
toString is special: every class has a default
toString method with a string representation
that is not very useful so we normally provide
our own version of this method.
23
Default toString example
bsh % addClassPath("c:/book-projects/chapter3");
bsh % CircleCalculator circle =
new CircleCalculator(3.0);
bsh % String rep = "toString gives " + circle;
bsh % print(rep);
toString gives CircleCalculator@4d1d41
bsh % rep = "toString gives " + circle.toString();
bsh % print(rep);
Not very
toString gives CircleCalculator@4d1d41
meaningful
bsh % print(circle);
CircleCalculator@4d1d41
Using the object name circle in a string expression is
equvalent to using circle.toString()
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Define our own toString
public String toString()
{
return "CircleCalculator[radius=" +
radius + ", area=" + area + ", circumference="
+ "]";
}
now the displayed
result is meaningful
add this method
to CircleCalculator cass
print(circle);
CircleCalculator[radius=3.0, area=28.274333882308138,
circumference=18.84955592153876]
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25
Why toString ?
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It is useful to write a toString method for
every class. The toString representation of
an object obj can be displayed using
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System.out.println(obj);
This is useful for debugging classes (finding
logical errors) since it provides an easy way
to display the state of any object.
26
Formatting data (Java 5)
The String class contains a static method with prototype
public static String format(String f,
Object... args)
Here f is the format string and Object... args represents the
argument list of values to be formatted.
There is also a new printf method that can be used with a
format string that has the prototype
public void printf(String f, Object... args)
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Format codes (Java 5)
Format codes begin with %
Here are some useful ones (see javadoc for complete list)
%5d
%-5d
%-20s
%15.5f
%.5f
%20.8e
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format an integer right justified in field of width 5
format at integer left justified in a field of width 5
format s string left justified in a field of width 20
format a floating point number right justified in a field
of width 15 using fixed format rounded to 5 digits after
the decimal point
format a floating point nuber in a field that just fits
using fixed format rounded to 5 digits after the decimal
point
format a floating point number right justified in a field
of width 20 using exponential (scientific) format
rounded to 8 digits after the decimal point
28
Format example (1)
The statements
int i = 3;
double pi = Math.PI;
String end = "End";
String f =
String.format("answer: %5d%15.5f%10s\n",
i, pi, end);
System.out.println(f);
produce the output
answer:
3
%5d
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3.14159
%15.5f
End
%10s
29
Format example (2)
The statements
int i = 3;
double pi = Math.PI;
String end = "End";
System.out.printf("answer: %5d%15.5f%10s\n",
I, pi, end);
produce the same output using printf
answer:
3
%5d
3.14159
%15.5f
End
%10s
To reuse formats use the format method in the String class
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30
Format example(3)
In the CircleCalculator class we could include the following
display method that formats the results.
public void display()
{
System.out.printf("Radius = %.5f\n", radius);
System.out.printf("Area = %.5f\n", area);
System.out.printf("Circumference = %.5f\n",
circumference);
}
to display the values rounded to 5 digits after the decimal point
in fields that just fit.
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31
Intro to Computer Science I
Chapter 4
Example classes that use the
String class
32
Design, Implement, Test
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Begin with an English description of a class
Design the class by deciding what
constructors and methods it should have.
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This is called writing the public specification or
public interface.
Write the complete class by providing the
implementation
Test the class by itself using BlueJ
33
BankAccount class description
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A BankAccount object should represent a
bank account using an account number, an
owner name, and a current balance.
There should be a constructor for creating a
bank account given these values.
There should be methods to withdraw or
deposit a given amount and the usual "get
methods" for retrurning the account number,
owner name, and balance.
34
BankAccount class design
public class BankAccount
{
// instance data fields go here
public BankAccount(int accountNumber,
String ownerName, double initialBalance) {...}
public void deposit(double amount) {...}
public void withdraw(double amount) {...}
public int getNumber() {...}
public String getOwner() {...}
public double getBalance() {...}
public String toString() {...}
}
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How will we use the class
Construct an object
BankAccount account =
new BankAccount(123, "Fred", 125.50);
withdraw $100
account.withdraw(100);
display balance
System.out.println("Balance is " +
account.getBalance());
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36
formal and actual arguments
Actual arguments in constructor call expression
BankAccount myAccount = new BankAccount(123, "Fred", 125.50);
Formal arguments in constructor prototype
public BankAccount(int accountNumber,
String ownerName, double initialBalance)
{
...
}
Same idea applies to method call expressions and prototypes
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Implementing the class (1)
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instance data fields
private int number;
private String name;
private double balance;
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38
Implementing the class (2)
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constructor implementation
public BankAccount(int accountNumber,
String ownerName, double initialBalance)
{
number = accountNumber;
name = ownerName;
balance = initialBalance;
}
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39
Implementing the class (3)
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deposit, withdraw (mutator methods)
public void deposit(double amount)
{
balance = balance + amount;
}
public void withdraw(double amount)
{
balance = balance - amount;
}
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40
Implementing the class (4)
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get methods (enquiry methods)
public int getNumber()
{ return number;
}
public String getName()
{ return name;
}
public double getBalance()
{ return balance;
}
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41
Implementing the class (5)
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toString method
public String toString()
{ return "BankAccount["
+ "number=" + number
+ ", name=" + name
+ ", balance=" + balance + "]";
}
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42
BankAccount class (1)
public class BankAccount
{
private int number;
private String name;
private double balance;
public BankAccount(int accountNumber,
String ownerName, double initialBalance)
{
number = accountNumber;
name = ownerName;
balance = initialBalance;
}
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43
BankAccount class (2)
public void deposit(double amount)
{
balance = balance + amount;
}
public void withdraw(double amount)
{
balance = balance – amount;
}
public int getNumber()
{
return number;
}
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44
BankAccount class (3)
public String getName()
{ return name;
}
public double getBalance()
{ return balance;
}
public String toString()
{ return "BankAccount[number=" + number
+ ", name=" + name
+ ", balance=" + balance + "]";
}
}
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45
BankAccount with BeanShell
addClassPath(
"c:/book-projects/chapter4/bank-account");
BankAccount account =
new BankAccount(123, "Fred", 125.50);
account.withdraw(100);
print(account.getBalance());
25.5
account.deposit(100);
print(account.getBalance());
125.5
print(account);
BankAccount[number=123, name=Fred,
balance=125.5]
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BankAccount with BlueJ
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InitialsMaker description
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An InitialsMaker object uses the first
and last name of a person to produce the
initials in uppercase. For example, if the
name is Henry James or henry james then
the initials are HJ.
48
InitialsMaker design
public class InitialsMaker
{
// instance data fields go here
public InitialsMaker(String firstName,
String lastName) {...}
public String getInitials() {...}
public String toString() {...}
}
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49
Implementing the class (1)
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instance data field
private String initials;

Another possibility
private String firstName;
private String lastName;
private String initials;
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50
Implementing the class (2)
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Constructor
public InitialsMaker(String firstName,
String lastName)
{
initials = firstName.substring(0,1);
+ lastName.substring(0,1);
initials = initials.toUpperCase();
}
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Implementing the class (3)
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get method
public String getInitials()
{
return initials;
}
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toString method
public String toString()
{
return "InitialsMaker[initials="
+ initials + "]";
}
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InitialsMaker with BeanShell
addClassPath(
"c:/book-projects/chapter4/strings");
InitialsMaker maker =
new InitialsMaker("harry", "James");
print(maker.getInitials());
HJ
print(maker);
InitialsMaker[initials=HJ]
Test all four possibilities for the names:
"Henry James", "Henry james", "henry James'", "henry james"
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PasswordGenerator description
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A PasswordGenerator object generates
random 7 character passwords. The first four
characters should be lower case letters and
the last three characters should be digits 0
to 9.
54
Random number generation
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In the Math class there is the random()
method that returns a real number 0  r  1
In the java.util package there is a Random
class that does what we want.
generated
Constructors and methods sequence
depends on current

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time in milliseconds
public Random()
public Random(long seed)
public int nextInt()
repeatable by using
0  i  n 1
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the same
seed again
55
Random digits and characters
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nextInt(10)
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generates random integer 0,1,...,9 which
can be converted to a character
nextInt(26)

generates random integer 0,1, ...,25 which
can be used as an index into the string
index 0
"abcdefghijklmnopqrstuvwxyz"
to generate a random letter
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index 25
56
PasswordGenerator design
public class PasswordGenerator
{
// instance data fields go here
public PasswordGenerator() {...}
public PasswordGenerator(long seed) {...}
public String next() {...}
}
Each call to next produces
a random password
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Using the class
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First construct PasswordGenerator object
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Each call to next returns a random password
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PasswordGenerator gen =
new PasswordGenerator();
String p1 = gen.next();
String p2 = gen.next();
String p3 = gen.next();
58
Implementation (1)

data fields
static class data feld
(not associated with any object)
private static final String LETTERS =
"abcdefghijklmnopqrstuvwxyz";
private Random random;
Instance data feld
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Implementation (2)

Constructors
public PasswordGenerator()
{
random = new Random();
}
public PasswordGenerator(long seed)
{
random = new Random(seed);
}
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Implementation (3)

The next method
public String next()
{ int index;
String password = "";
index = random.nextInt(26);
password = password +
LETTERS.substring(index, index + 1);
// repeat these two statements three more times
index = random.nextInt(10);
password = password + index;
// repeat these two statements two more times
return password;
}
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Import statement (1)
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
The Random class is not in the standard
java.lang package that is automatically
imported into every java class
It is in java.util and needs to be
imported excplicity using

import java.util.Random;
This statement goes OUTSIDE
the class declaration at the top
of the file
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Import statement (2)
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Another variation is
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
import java.util.*;
This can import and class in the java.util
package
If the import statement is not used then it is
necesary to use the fully qualified name

java.util.Random
of the Random class instead of the short
name
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Testing the class
addClassPath("c:/book-projects/chapter4/strings");
PasswordGenerator gen = new PasswordGenerator();
print(gen.next());
avfi637
print(gen.next());
iqde665
gen = new PasswordGenerator(); // make a new one
print(gen.next());
zuwe456
gen = new PasswordGenerator(123);
print(gen.next());
eomt574
gen = new PasswordGenerator(123);
print(gen.next());
eomt574
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Test with BlueJ
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Construct an object
Use the next method repeatedly from the
object menu
Can also insert the simple method
public test()
{
System.out.println(next());
}
this.next()
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65
The "this" object
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When you write a class and call a method
from the same class what is the object on
which the method is invoked?
Method call expression with an
Compare
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

gen.next();
this.next();
next();
explict object called gen
Method call expression from
within the
PasswordGenerator class
If object name is omitted for an instance
method the compiler assumes that the
method is in the same class
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Association & Aggregation (1)
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
Programs normally consist of interacting
objects from several classes.
Types of classes
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built-in classes such as String or Random
classes obtained from someone else
classes you write yourself such as
CircleCalculator
The terms association and aggregation are
used to describe how classes can relate to
each other.
67
Association (1)
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
Some classes such as CircleCalculator
from Chapter 3 are not related with any
other classes.
Others depend on other classes
Example

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PasswordGenerator uses (depends on) the
String and Random classes
This dependence of one class on others is
called association
68
Association (2)

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
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The PasswordGenerator class is
associated with, or uses, or depends on the
String and Random classes.
The relationship is not symmetric
For example, we don't say that the String
class is associated with the
PasswordGenerator class
The String class is designed to be used by
other classes
69
Association (3)
Class A is associated with class B if A uses
B. This can occur in several ways.


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
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An object of B is used as a local variable in a
constructor or method in A
An object of B is used as a method or
constructor argument in A
An object of B is used as a return value of a
method in A
An object of B is an instance data field in A
70
Aggregation



Aggregation (sometimes called composition)
refers to the important last case that an
object of B is used as an instance data field
of A.
This is gives rise to an object hierarchy
Example

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Random class is used as instance data fields in
PasswordGenerator
71
TriangleCalculatorTester
public class TriangleCalculatorTester
{ public TriangleCalculatorTester()
Can omit constructor
{ }
public void doTest(double a, double b, double g)
{
TriangleCalculator tri =
Association
new TriangleCalculator(a,b,g);
System.out.println("Sides: " + tri.getA()
+ ", " + tri.getB() + ", " + tri.getC());
System.out.println("Angles: " + tri.getAlpha()
+ ", " + tri.getBeta() + ", "
+ tri.getGamma());
System.out.println("Angle sum is " +
tri.checkAngleSum());
}
In BlueJ a dotted arrrow indicates association
}
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Association in BlueJ
the dotted arrow
indicates
association
TriangleCalculatorTester is associated
with (uses) TriangleCalculator
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doTest
produces this
output
73
Point class design
public class Point
{
private double x, y;
public Point() {...}
public Point(double x, double y){...}
public double getX() {...}
public double getY() {...}
public String toString() {...}
}
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Point class implementation (2)
public class Point
{
private double x, y;
public Point()
{
x = y = 0.0;
}
public Point(double x, double y)
{ this.x = x;
this.y = y;
}
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Point class implementation (2)
public double getX()
{ return x;
}
public double getY()
{ return y;
}
public String toString()
{ return
"Point[" + x + ", " + y +"]";
}
} // end of class
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76
Testing the Point class
addClassPath("c:/book-projects/chapter4/geometry");
import Point; // necessary or we get java.awt.Point
Point origin = new Point(0,0);
Point p = new Point(1,2);
print(origin);
Point[0.0, 0.0]
print(p);
Point[1.0, 2.0]
print(p.getX());
1.0
print(p.getY());
2.0
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Designing a circle class


We want to develop a simple Circle class
to describe geometric circles in terms of their
center (x,y) and radius r
Method 1


Method 2 (using aggregation)

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Use three double type instance data fields for
the center coordinates and the radius
Use two instance data fields. One is a Point
object for the center and the other is the radius
78
Circle class design (method 1)
public class Circle
{
private double x, y, radius;
public Circle() {...}
public Circle(double x, double y,
double r) {...}
public double getX() {...}
public double getY() {...}
public double getRadius() {...}
public String toString() {...}
}
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Circle class design (method 2)
public class Circle
{
private Point center;
private double radius;
aggregation
public Circle() {...}
public Circle(double x, double y,
double r) {...}
public Circle(Point c, double r) {...}
public Point getCenter() {...}
public double getRadius() {...}
public String toString() {...}
}
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Implementation (1)

Constructors
public Circle(Point p, double r)
{ center = p;
radius = r;
}
public Circle(double x, double y,
double r)
{ center = new Point(x,y);
radius = r;
}
public Circle()
{ center = new Point();
radius = 1;
}
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81
Implementation (2)

Methods
public double getRadius()
{ return radius;
}
public Point getCenter()
{ return center;
}
public String toString()
{
return "Circle[" + center + ", "
+ radius + "]";
}
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Testing the class (BeanShell)
addClassPath("c:/book-projects/chapter4/geometry");
import Point;
Point center = new Point(3,4);
Circle c1 = new Circle();
Circle c2 = new Circle(center, 5);
Circle c3 = new Circle(3,4,5);
print(c1);
multiple method
Circle[Point[0.0, 0.0], 1.0]
call expressions
print(c2);
Circle[Point[3.0, 4.0], 5.0]
print(c3);
Circle[Point[3.0, 4.0], 5.0]
double x = c2.getCenter().getX();
print(x);
3.0
BGA
83
Testing the class (BlueJ)
Point object for (3,4)
BGA
use it here
as
constructor
argument
84
CircleTester (1)
public class CircleTester
{
public CircleTester()
{
}
can omit constructor
Construct 3 circle
objects
public void doTest()
{
Point center = new Point(3,4);
Circle c1 = new Circle();
Circle c2 = new Circle(center, 5);
Circle c3 = new Circle(3,4,5);
BGA
85
CircleTester (2)
System.out.println("c1 = " + c1);
System.out.println("c2 = " + c2);
System.out.println("c3 = " + c3);
double radius = c2.getRadius();
double x = c2.getCenter().getX();
double y = c2.getCenter().getY();
display
using
toString
aggregation
System.out.println("Radius = " + radius);
System.out.println("Center x = " + x);
System.out.println("Center y = " + y);
} // end of doTest method
} // end of CircleTester class
BGA
86
Other Library classes
Dates and times
Calendars
Currency formatting
numeric formatting
87
Date class


Represents dates as the number of
milliseconds since January 1, 1970,
00:00:00 GMT as a long value
Example

1055681816162 =
Sun Jun 15 08:56:57 EDT 2003
toString provides
this
BGA
88
Date class specification

Some of the constructors and methods are
public Date()
date right now
public Date(long date)
public long getTime()
public void setTime(long date)
public String toString()
Date class is in package java.util
BGA
89
Date example (BeanShell)
import java.util.Date;
We are 5 hours
Date now = new Date();
ahead of
print(now);
midnight GMT
Sun Jun 15 08:56:56 EDT 2003
long t = now.getTime();
print(t);
1055681816162
Date first = new Date(0L);
print(first);
Wed Dec 31 19:00:00 EST 1969
first.setTime(0L + 1000L * 60L * 60L * 24L);
print(first);
Thu Jan 01 19:00:00 EST 1970
milliseconds in a day
BGA
90
SimpleDateFormat class


We need to be able to display dates in
various formats.
This class is in
SimpleDateFormat can do this
package
Constructors and methods
java.text
public SimpleDateFormat()
public SimpleDateFormat(String pattern)
public String format(Date d);
Convert a Date object to the
format specified by the constructor
BGA
91
Example (BeanShell)
import java.util.Date;
import java.text.SimpleDateFormat;
Date now = new Date();
SimpleDateFormat f1 = new SimpleDateFormat();
String n1 = f1.format(now);
print(n1);
6/15/03 8:56 AM
SimpleDateFormat f2 =
new SimpleDateFormat("dd/MM/yyyy");
print(f2.format(now));
15/06/2003
SimpleDateFormat f3 =
new SimpleDateFormat("HH:mm:ss z");
print(f3.format(now));
08:56:56 EDT
BGA
92
Calendar class



BGA
A higher level class than Date that knows
about year, month, day of month, day of
year, etc.
Java can use any of the world's calendars
depending on the locale.
For us this is the Gregorian calendar
93
Constructing a Calendar
A constructor is not used to create a Calendar object.
Instead a static class methodis used. Method prototype is
public static Calendar getInstance()
Recall that a static method in a class is not associated with any objects
of the class. The Math class methods were like this.
Compare Math.sqrt with Calendar.getInstance
Calendar now = Calendar.getInstance();
For a static (class) method
the class name is used
instead of an object name
BGA
This is a static method
call expression
94
Instance versus static method

Instance method call expression
objectName.methodName(actualArgumentList)

Static method call expression
ClassName.methodName(actualArgumentList)
BGA
95
Some Calendar methods
public Date getTime()
public int get(int field)
public void set(int field, int value)
public void set(int year, int month,
int day)
Calendar fields are integer constants such as Calendar.YEAR,
Calendar.MONTH, Calendar.DAY_OF_MONTH
BGA
96
Using Calendar class (1)
import java.util.Date;
factory
import java.util.Calendar;
method
Calendar now = Calendar.getInstance();
Date time = now.getTime();
print(time);
Sun Jun 15 10:04:12 EDT 2003
print(now.get(Calendar.YEAR));
2003
print(now.get(Calendar.MONTH)); // Jan is month 0
5
print(now.get(Calendar.DAY_OF_MONTH));
15
print(now.get(Calendar.DAY_OF_WEEK)); // Sunday is 1
1
BGA
97
Using Calendar class (2)
print(now.get(Calendar.DAY_OF_YEAR));
166
Calendar christmas = Calendar.getInstance();
int year = christmas.get(Calendar.YEAR);
christmas.set(year, Calendar.DECEMBER, 25);
print(christmas.getTime());
Thu Dec 25 11:13:01 EST 2003
BGA
98
Leap years
import java.util.Calendar;
Calendar feb2003 = Calendar.getInstance();
feb2003.set(2003, Calendar.FEBRUARY, 1));
Calendar feb2004 = Calendar.getInstance();
feb2004.set(2004, Calendar.FEBRUARY, 1));
print(feb2003.getActualMaximum(
Calendar.DAY_OF_MONTH));
28
print(feb2004.getActualMaximum(
Calendar.DAY_OF_MONTH));
29
BGA
99
Person class (1)
import java.util.Calendar;
public class Person
{
private String name;
private int birthYear;
public Person(String name, int birthYear)
{ this.name = name;
this.birthYear = birthYear;
}
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100
Person class (2)
public String getName()
{ return name;
}
public int getBirthYear()
{ return birthYear;
}
Using a Calendar
object to determine
the person's age
public int age()
{
Calendar now = Calendar.getInstance();
return now.get(Calendar.YEAR) - birthYear;
}
} // end of class
BGA
101
CalendarMonth class (1)




BGA
Suppose we want to display a calendar for a
given year and month.
We don't need the full complexity of the
Calendar class
We can write a simpler version called
CalendarMonth called an adapter class
An adpater class is a class which makes
another class easier to use in specific
problems
102
CalendarMonth class (2)

We need the following functionality





BGA
only the year and month parts of a Date are required
We need the day of the week for the first day of the
month
We need to know the number of days in the month
properly accounting for leap years in February
We need the names of the months so that we can print
headings.
This class is called an adapter class since it
provides a simpler more specialized version of a
more complicated class.
103
Class design (specification)
public class CalendarMonth
calendar
for this month
{
public CalendarMonth() {...}
public CalendarMonth(int year,
int month) {...}
public int getYear() {...}
for first day
public int getMonth() {...}
of month
public int dayOfWeek() {...}
(1 to 7)
public int daysInMonth() {...}
public String monthName() {...}
public String toString() {...}
}
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104
Class implementation (1)
import java.util.Calendar;
import java.util.SimpleDateFormat
public class CalendarMonth
adapt this class
{
private Calendar calendar;
Constants for month
and day names
public static final int JANUARY
= Calendar.JANUARY;
...
public static final int SATURDAY
= Calendar.SATURDAY.
BGA
105
Class implementation (2)
public CalendarMonth()
{ calendar = Calendar.getInstance();
calendar.set(Calendar.DAY_OF_MONTH,
1);
set calendar to
}
first day of month
public CalendarMonth(int year,
int month)
{
calendar = Calendar.getInstance();
calendar.set(year, month, 1);
}
BGA
106
Class implementation (3)
public int getYear()
{
return calendar.get(Calendar.YEAR);
}
public int getMonth()
{
return calendar.get(Calendar.MONTH);
}
BGA
107
Class implementation (4)
public int dayOfWeek()
{ return calendar.get(
Calendar.DAY_OF_WEEK);
}
1 = Sunday
calendar is already
set to first day of
month
public int daysInMonth()
{
return calendar.getActualMaximim(
Calendar.DAY_OF_MONTH);
}
BGA
108
Class implementation (5)
public String monthName()
{ SimpleDateFormat f =
new SimpleDateFormat("MMMM");
return f.format(calendar.getTime());
}
public String toString()
{ SimpleDateFormat f =
new SimpleDateFormat("MMMM yyyy");
return f.format(calendar.getTime());
}
} // end of class
BGA
109
Using CalendarMonth class (1)
addClassPath("c:/book-projects/chapter4/calendar");
CalendarMonth thisMonth = new CalendarMonth();
print(thisMonth.dayOfWeek());
2
Monday
print(thisMonth.daysInMonth());
30
print(thisMonth.monthName());
September
print(thisMonth);
September 2003
BGA
110
Using CalendarMonth class (2)
// continued from previous slide
our constants
CalendarMonth feb2004 =
new CalendarMonth(2004, CalendarMonth.FEBRUARY);
print(feb2004.dayOfWeek());
1
Sunday
print(feb2004.daysInMonth());
29
leap year
print(feb2004.monthName());
February
print(feb2004);
February 2004
BGA
111
CalendarMonthTester (1)
public class CalendarMonthTester
{
public CalendarMonthTester() {}
public void doTest()
{ CalendarMonth thisMonth = new CalendarMonth();
System.out.println("First day of month is "
+ thisMonth.dayOfWeek());
System.out.println("Number of days in month is "
+ thisMonth.daysInMonth());
System.out.println("Month name is "
+ thisMonth.monthName());
System.out.println("Calendar name is "
+ thisMonth);
BGA
112
CalendarMonthTester (2)
System.out.println();
CalendarMonth feb2004 = new
CalendarMonth(2004, CalendarMonth.FEBRUARY);
System.out.println("First day of month is "
+ thisMonth.dayOfWeek());
System.out.println("Number of days in month is "
+ thisMonth.daysInMonth());
System.out.println("Month name is "
+ thisMonth.monthName());
System.out.println("Calendar name is "
+ thisMonth);
} // end of doTest
} // end of class
BGA
113
CalendarMonth in Bluej
BGA
114
Currency formatting

Depends on locale



English: $100,000.56
French: 100000,56 $
NumberFormat is in java.text package
Construct a NumberFormat object
NumberFormat currency =
NumberFormat.getCurrencyInstance();

Format it as a string
double salary = 4000.56
String value = currency.format(salary);
BGA
115
BeanShell example
import java.text.NumberFormat;
double salary = 100000.555;
NumberFormat currency =
NumberFormat.getCurrencyInstance();
print(currency.format(salary));
$100,000.56
NumberFormat currencyCF =
NumberFormat.getCurrencyInstance(
Locale.CANADA_FRENCH);
print(currencyCF.format(salary));
100000,56 $
BGA
116
Formatting fixed numbers

Formatting numbers with a fixed number of
digits after decimal point
import java.text.DecimalFormat;
DecimalFormat fix =
new DecimalFormat(" 0.00000;-0.00000);
print(fix.format(Math.PI));
3.14159
print(fix.format(-Math.PI));
-3.14159
BGA
format for
negative numbers
format for
positive numbers
117
Formatting scientific numbers

Formatting numbers with a an exponent and
five digits after decimal point
import java.text.DecimalFormat;
DecimalFormat fix =
new DecimalFormat(" 0.00000E000;-0.00000E000);
double d = 1.2345678E-23;
print(sci.format(d));
format for
1.23457E-023
negative numbers
print(fix.format(d));
-1.23457E-023
format for
positive numbers
BGA
118
Review of OOP concepts
constructing objects
object references
using references
data encapsulation and integrity
instance variables and methods
static variables,constants,methods
119
Constructing objects

Objects are constructed in two ways

Using a constructor


or using a static factory method which returns
an object of the class using a static method call
expression

BGA
Circle c1 = new Circle(3,4,5);
Calendar now = Calendar.getInstance();
120
Constructor Examples
Constructor call expressions (underlined)
Circle c1 = new Circle(new Point(3,4), 5);
Point p = new Point(3,4);
Point q = new Point();
Circle c1 = new Circle(3, 4, 5);
BankAccount a = new BankAccount(123, "Fred", 4000);
SimpleDateFormat f = new SimpleDateFormat("MMMM yyyy");
Constructor prototypes
public Circle(Point p, double radius)
public Point(double x, double y)
public Circle(double x, double y, double radius)
public BankAccount(int number, String name,
double balance)
public SimpleDateFormat(String pattern)
BGA
121
Static Factory Examples
Static method call expressions (underlined)
Calendar now = Calendar.getInstance();
NumberFormat currency =
NumberFormat.getCurrencyInstance();
Static method prototypes
public static Calendar getInstance()
public static NumberFormat getCurrencyInstance()
BGA
122
Using this as a constructor call
public Circle()
{ center = new Point();
radius = 1;
}
this(new Point(),1);
public Circle(double x, double y
double r)
{ center = new Point(x,y);
this(new Point(x,y),r);
radius = r;
}
public Circle(Point p, double r)
{ center = p;
radius = r;
}
WARNING
BGA
"this" will call
this constructor
123
The Default constructor

If you don't put any constructors in a class
the default no-arg constructor
public ClassName()
{
}

BGA
is automatically provided by the compiler.
For example in CircleTester we could
have omitted the no-arg constructor that
was included in the class.
124
Miranda convention

BGA
You have a right to a constructor. If you do
not have one, a default one will be provided
to you by the compiler.
125
What does it do?

The default constructor simply supplies
default initialization for any uninitialized
instance data fields:



BGA
A value of zero is assigned to all uniinitialized
numeric data fields.
A reference value of null is assigned to all
uninitialized data fields of object type (explained
later)
Try it: see page 142 of text book
126
Object references

Constructing an object is a three step
process




BGA
Memory space is allocated for the object and its
instance data fields.
A reference (address) to the object is returned
so that it can be located
This reference is assigned as the value of an
object reference variable
This variable is the object reference variable
127
Pictorial representation
objectName
ClassName
ClassName objectName =
new ClassName(actualArguments)
objectName
Object
Data
null
ClassName objectName;
ClassName objectName = null;
BGA
128
Circle Example
BGA
Circle c;
Uninitialized
reference
Circle c = null;
null reference
Circle c =
new Circle(3,4,5);
Initialized
Reference
129
Primitive and reference types

Primitive types

numeric types such as int, long, float, double
area

17.902
Reference types

object types such as String, Circle, BankAccount
greeting
String
Hello World
BGA
130
Why do we need both types?



BGA
Types such as int and double are primitive
types for efficiency reasons. There is a
certain amount of overhead in following a
reference to find an object.
There are wrapper classes that let us convert
the primitive types to object types
Example: Integer class encapsulates an
int value
131
Assignment for reference types



BGA
Primitive types: the assignment statement
a = b; means to assign the value of b as
the value of a.
Reference types: the assignment
statement a = b; means to assign the
reference b as the value of the reference a
and this means that a and b now both
reference the same object
assignment does not copy objects.
132
a=b for primitive types
a
17
a
19
b
19
b
19
Before a = b
BGA
After a = b
133
a=b for reference types
a
object
A
a
object
A
b
object
B
b
object
B
Before a = b
BGA
After a = b
134
BankAccount example
BankAccount fred =
new BankAccount(123, "Fred", 150.0);
BankAccount mary =
new BankAccount(345, "Mary", 250.0);
mary = fred; // both reference fred
mary.withdraw(100.0);
print(mary);
BankAccount[number=123, name=Fred, 50.0]
print(fred);
BankAccount[number=123, name=Fred, 50.0]
BGA
135
Where are references used?

Local variables in the constructor or method


Constructor or method arguments


Example: return center;
instance data fields (aggregation)

BGA
Example: public Circle(Point p, double r)
method return values


Example: TriangleCalculator tri =
new TriangleCalculator(a,b,g);
Example: private Calendar calendar;
136
Data encapsulation (1)




BGA
In OOP the data is spread out over the
instance data fields of one or more objects
Each object is only responsible for its own
data
This data is normally private so that it
cannot be directly changed outside the class
This is called data encapsulation.
137
Data encapsulation (2)



BGA
Classes can be mutable or immutable
For mutable classes the constructors and
methods that change the values of one or
more instance data fields can be written to
check that objects are not in an inconsistent
state.
Example: See page 146 for discussion of
public data fields and set methods.
138
Side effects




BGA
Changing the state of an object indirectly
from outside the class
Sometimes this is desirable
Sometimes side-effects are undesirable
Side-effects are normally created by
returning references from methods that can
be used outside the class to change the
state of an object.
139
The MPoint class


Our Point class is immutable: it is not
possible to change the state of a Point
object after it has been constructed
Consider a mutable version called MPoint
like Point but with following set methods
public void
{ this.x =
}
public void
{ this.y =
}
BGA
setX(double x)
x;
setY(double y)
y;
140
The MCircle class


Identical to Circle except that it uses
aggregation with the mutable MPoint class
instead of the immutable Point class
The following constructor has side-effects
public MCircle(MPoint p, double r)
{
center can be changed
center = p;
through p
radius = r;
}
BGA
141
Example of side-effect (1)
addClassPath("c:/book-projects/chapter4/side-effects");
MPoint p = new MPoint(3,4);
MCircle c = new MCircle(p, 5);
Also try it
print(c);
in BlueJ
MCircle[MPoint[3.0, 4.0], 5.0]
p.setX(999);
print(c);
MCircle[MPoint[999.0, 4.0], 5.0]
The change in the x coordinate of point p outside the MCircle class has
the side-effect of changing the x coordinate of the center of the circle.
This is normally an undesirable side-effect since it violates the data
encapsulation.
BGA
142
Example of side-effect (2)
addClassPath("c:/book-projects/chapter4/side-effects");
MCircle c = new MCircle(3, 4, 5);
print(c);
MCircle[MPoint[3.0, 4.0], 5.0]
reference to
MPoint p = c.getCenter();
center is
returned
p.setX(999);
print(c);
MCircle[MPoint[999.0, 4.0], 5.0]
Also try it
in BlueJ
BGA
143
Example of side-effect (3)

The problems occurs with the assignment
statement
center = p;
and with the statement
return center;

BGA
This gives two references to the same point
object. One reference inside the class
(center) and one outside the class (p).
144
Example of side-effect (4)
MPoint
p
c
x
999
y
4
MCircle
center
radius
5
There are two references to
one MPoint object
BGA
145
No side-effect in original classes
addClassPath("c:/book-projects/chapter4/geometry");
import Point;
Point p = new Point(3,4);
Circle c = new Circle(p, 5);
print(c);
Circle[Point[3.0, 4.0], 5.0]
MPoint p = c.getCenter();
p = new Point(999,4);
print(c);
Circle[Point[3.0, 4.0], 5.0]
BGA
146
Fixing the side-effect (1)

Include a copy constructor in MPoint class
public MPoint(MPoint p)
{ x = p.x;
y = p.y;
}

General form of copy constructor
public ClassName ( ClassName objectName )
BGA
147
Fixing the side-effect (1)

Revise the constructor that takes an MPoint
object as an argument
public MCircle(MPoint p, double r)
{ center = new MPoint(p);
radius = r;
}

Revise getCenter method
public MPoint getCenter()
{ return new MPoint(center);
}
BGA
Now center
references its own
private copy of the
MPoint object
Now we return a
copy of the private
MPoint object
148
Side-effects are gone
addClassPath(
"c:/book-projects/chapter4/no-side-effects");
MPoint p = new MPoint(3,4);
MCircle c = new MCircle(p, 5);
print(c);
This has no
MCircle[MPoint[3.0, 4.0], 5.0]
effect on
p.setX(999);
MCircle
print(c);
MCircle[MPoint[3.0, 4.0], 5.0]
BGA
149
Pictorial Representation
MPoint
p
c
MPoint
x
999
x
3
y
4
y
4
MCircle
center
Now p and c are
references to different
objects
BGA
radius
5
150
BankAccount transfer (1)
public class TransferAgent
{
public TransferAgent(BankAccount from,
BankAccount to) {...}
public void transfer(double amount) {...}
}
The constructor takes references to the two BankAccount objects
that participate in the transfer.
The transfer method does the transfer of a given amount from
one account to the other
BGA
151
BankAccount transfer (2)
public class TransferAgent
{
private BankAccount from;
private BankAccount to;
Aggregation
public TransferAgent(BankAccount from,
BankAccount to)
{ this.from = from;
this.to = to;
}
public void transfer(double amount)
{ from.withdraw(amount);
to.deposit(amount);
}
}
BGA
bank-account project
152
Side-effects (1)


BGA
The BankAccount class is mutable and
there will be side-effects in the
TransferAgent class
The side-effects are desired here since the
purpose of the transfer method is to
change the balances in the two accounts
referenced by from and to suplied by the
user.
153
Side-effects (2)
addClassPath("c:/book-projects/chapter4/bank-account");
BankAccount fred = new BankAccount(123,"Fred",1000);
BankAccount mary = new BankAccount(345,"Mary",1000);
TransferAgent agent = new TransferAgent(fred, mary);
agent.transfer(500);
print(fred);
BankAccount[number=123, name=Fred, balance=500]
print(mary);
try it in
BankAccount[number=345, name=Mary, balance=1500] BlueJ
BGA
154
Instance variables




BGA
Instance variables are also called instance
data fields
They are declared in the class but outside
any method or constructor
Only instance methods have access to these
variables
Each object has its own set of instance
variables
155
Instance methods


BGA
Instance methods are associated with
objects of the class
Instance methods can access the instance
data fields
156
Using an instance method


BGA
To use or call an instance method means to
invoke it on some object using an instance
method call expression
We also say that instance methods are used
to send messages to objects.
157
Examples
int length = name.length();
char first = test.toUpperCase.charAt(0);
password = password + LETTERS.substring(index,index+1);
String upper = test.toUpperCase();
account.deposit(100));
message composition
String firstInitial =
firstName.substring(0,1).toUpperCase();
double x = c2.getCenter().getX();
doCalculations(); // this is the implied object
long t = now.getTime();
String n3 = f3.format(now);
Method call expressions are underlined in red
BGA
objectName.methodName(actualArguments)
this.methodName(actualArguments)
158
prototypes and call expressions
String letter = LETTERS.substring(index, index + 1);
public String substring(int first, int lastPlusOne)
BGA
159
Method composition

Also called message composition
objectName.methodName(args1) . methodName(args2) ...
methodName(argsN)
this.methodName(args1) . methodName(args2) ...
methodName(argsN)
BGA
160
Static variables (1)




Also called class variables
Declared using the static modifier inside a
class but outside any method or constructor
declaration.
We have not used them yet
Example

BGA
private static int count = 0;
161
Static variables (2)




They are not associated with any objects.
They belong to the class.
Each static variable is shared by all the
objects of the class (shared variable)
This is very different from instance variables
1
2
count
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3
4
0
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Static constants


Constants are declared to be static since
there is no need to have separate copies of a
constant, one for each object.
Examples
public static final String LETTERS =
"abcdefghijklmnopqrstuvwxyz";
public static final int JANAURY = Calendar.JANUARY;
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Static methods (1)
They are methods that are not associated
with any objects of a class.
To use them in another class prefix the name
of the method with the class name


beta = Math.toDegrees(beta);
System.out.println("Area: " + area);
Calendar now = Calendar.getInstance();
NumberFormat currency =
NumberFormat.getCurrencyInstance();
Static method call expressions are underlined in red
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Static Methods (2)

General syntax instance method call
expressions
(1) objectName.methodName(actualArguments)
(2) this.methodName(actualArguments)

General syntax for static method call
expressions
(1) className.methodName(actualArguments)
(2) methodName(actualArguments)
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Counting objects (1)


We can illustrate static variables (shared
varibles) and static methods by writing a
version of the Point class that keeps track of
the number of Point objects that have been
counted so far.
Add following static data field to the Point
class
public static int count = 0;
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166
Counting objects (2)

Add the following statement to the body of
each different constructor (but not if "this" is
used)
count++;

Add the following static method to the class
public static int getCount()
{
return count;
}
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Testing Point (BeanShell)
addClassPath(
"c:/book-projects/chapter4/static-variable");
import Point;
Point p1 = new Point(1,2);
Point p2 = new Point(3,4);
this does not
Point p3 = new Point(4.5);
create a new object
Point p4 = p1;
int objectCount = Point.getCount();
print(objectName);
3
Also try it with BlueJ
Note that static methods appear on the Constructor menu
not the object menu.
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Classifying variables and args

Instance data fields


Static (class) data fields


declared in the body a constructor or method and are
only accessible there
Formal arguments

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static variables not associated with objects
Local variables


instance variables associated with objects
declared in method or constructor prototype and are
local variables
169
Call by value

Argument passing mechanism is always call
by value in Java:



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if an actual argument is a variable then a copy of
its value is supplied as the value of the formal
argument.
If an actual argument is a literal (literal string or
number for example) then this value is supplied
as the value of the formal argument
If an actual argument is an expression then it is
evaluated and its value is supplied as the value
of the formal argument.
170
Call by value (primitive type)

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If a constructor or method argument is a
primitive type and a variable is used as an
actual argument, then the value of the
variable can never be changed by the
method or constructor since only a copy of
the variable's value, not its location, is
passed to the method.
171
Example (BeanShell)
void addOne(int k) { k = k + 1; }
int m = 5;
addOne(m);
static methods can
print(m);
be defined directly
5
in BeanShell
print(k);
// Error: indefined variable
The value of m is not changed by the addOne method
since only the value 5 is passed to the method so the
method simply adds 1 to the local variable k which
disappears anyway when the method exits.
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Example (BlueJ)

Write the following simple class
public class ArgumentTester1
{
public void doTest()
{ int m = 5;
addOne(m);
System.out.println(m);
}
private static void addOne(int k)
{ k = k + 1;
}
}
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arguments project in Chapter 4
173
Call by value (reference type)



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For reference type (object type) the situation
is quite different
We still use "call by value" but now the value
passed is a copy of a reference to the caller's
object so there will now be two references to
the caller's object
In the case of mutable classes this means
that the caller's object can be changed (sideeffect).
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Example (BeanShell)
addClassPath("c:/book-projects/chapter4/arguments");
void addOne(BankAccount b) { b.deposit(1) }
BankAccount a = new BankAccount(123,"Fred",1000);
addOneDollar(a);
print(a);
BankAccount[number=123, name=Fred, balance=1001.0]
The value of a is not changed by the addOneDollar method
but since a is a reference to a BankAccount then the deposit
statement does change the balance of the caller's account
as referenced by a
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Example (BlueJ)

Write the following simple class
public class ArgumentTester2
{
public void doTest()
{ BankAccount a =
new BankAccount(123, "Fred", 1000);
addOneDollar(a);
System.out.println(a);
}
}
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private static void addOneDollar(BankAccount b)
{ b.deposit(1);
}
arguments project in Chapter 4
176
Pictorial Representation
a
BankAccount
balance
Before
method
call
1000
a
b
a
BankAccount
balance
1001
BankAccount
balance
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Reference b is local to method
so it has disappeared
During
method
call
1001
After
method
call
177
The main method (1)



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Executing a class outside the BlueJ
environment
The JRE (java run time environment) has
an interpreter that can be run from the
command line using a command prompt
For this to work the class must have a
special static method called the main
method
178
Main method (2)

Form of the main method must be
public static void main(String[] args)
{
...
}
The java interpreter will begin
executing your program at the
first statement in the main method.
This method is normally used only
to create an object of the class that
starts your program
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We won't use
this argument
until Chapter 8
but it must be
there
179
Adding a main method

Add a main method to the CircleTester class
public class CircleTester
{
...
public void doTest()
{ ...
}
}
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main will construct
a CircleTester object
and run the test
public static void main(String[] args)
{
CircleTester tester = new CircleTester();
tester.doTest();
}
see main-method project
Can also execute main method from inside BlueJ
180
Another approach


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If you don't want to add a main method to
the class you want to run you can write a
special class that only has a main method
This main method can construct an object of
the other class and execute its methods.
181
BankAccountRunner
public class BankAccountRunner
{
public void doTest()
{ BankAccount account =
new BankAccount(123, "Fred", 125.50);
System.out.println("Initial balance is " +
account.getBalance();
account.withdraw(100);
System.out.println("Balance is " +
account.getBalance();
...
}
uses default
constructor
public static void main(String[] args)
{
BankAccountRunner runner = new BankAccountRunner();
runner.doTest();
}
}
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Run from command line

cd to the directory containing the
BankAccountRunner class


Compile the class (if you haven't used BlueJ
to do it)


javac BankAccountRunner.java
Now run the class using its main method

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c:\book-projects\chapter4\main-method
java BankAccountRunner
183
BankAccountRunner output
java BankAccountRunner
interpreter
command
Initial balance is 125.5
Balance is 25.5
Balance is 125.5
BankAccount[number=123, name=Fred, balance=125.5]
output produced by
System.out.println
statements
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