Classes, Types and Objects
Data Structures
Chapter 1
Prof. Vidya Manian
Dept. of Electrical and Comptuer Engineering
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Java Programming Basics
Overview
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Objects, classes
expressions
Control flow
Arrays
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Objects
• Objects –store data and provide methods for
accessing and modifying data
• An object is an instance (member) of a class
• Class – specifies type of objects, operations
that it performs
• Data of objects are stored as instance
variables (fields) (of types e.g. integers,
floating point numbers, Booleans)
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Operations
• Methods :constructors, procedures, and
functions
• Example: class declaration
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• Counter class is public-other class can create
and use a counter object
• Instance variable –count
• Constructor method initializes it to 0
• Accessor method, get Count
• Update methods, incrementCount,
decrementCount
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Class modifiers
• Abstract-empty methods with abstract
keyword
• Final – cannot have subclasses
• Public – can be instantiated by anything that
imports the class
• Base types (primitve types (boolean, char,
byte, short, int, long, float, double)
• Int x, x=14 or x=195
• Float, y=3.1415
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Example of base types
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• Comments: /** **/ or //
• ‘new’ creates a new object and returns
reference to that object
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new operator
• A new object is dynamically allocated in
memory, initialized to default values ‘null’
• Constructor is called
• The new operator returns a reference (a
memory address) to the newly created object
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Number classes
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Byte
Short
Integer
Long
Float
Double
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n.byteValue()
n.shortValue()
n.intValue()
n.longValue()
n.floatValue()
n.doubleValue()
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String objects
• String is a sequence of characters that comes
form some alphabet (set of possible
characters)
• P = “barney the bear”
• Length 15
• Concatenation
• String s = “kilo” + “meters”
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Object reference
• Reference variable is a pointer to some object
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Dot operator
• Can have many references to the same object
• Dot operator”.”-access methods and instance
variables associated with an object
• Examples of signatures (methods name with
parameters)
• oven.cookDinner();
• oven.cookDinner(food);
• oven.cookDinner(food,seasoning);
• Note: Java doesn not allow 2 methods with
the same signature to return different types
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Instance Variables
• Instance variables must have a type (int, float,
double) or a reference type : class such as
String, an array
• A referrence variable v, points to object o, we
can access any of the instance variables fo o
that the access rules allow
• Public instance variables are accessible by
everyone
• Dot operator can get and set the value of any
instance variable (v.i)
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• gnome.name =“professor smith”;
• gnome.age=35;
• gnome –reference to a Gnome object, with
public instance variables name and age
• Variable modifiers
• Public: anyone can access
• Protected: only methods of the same package
or of its subclasses can access protected
instance variables
• Private: only methods of the same class can
access it
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• Static: variables are used to store “global”
information about a class
• Final: a final instance variable is one that must
be assigned an initial value, and can never be
assigned a new value after that
(MAX_HEIGHT)
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What are the base types ?
What is variable gnomeBuddy ?
What is variable name ?
Note: Constant values associated with a class
should be declared to be both static and final
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Enum Types
• Take on values from a specified set of names
• Modifier enum name {value_name0,
value_name1,….};
• Modifer – black, public, protected or private
• Name – any legal Java identifier
• public enum Day
{MON,TUE,WERD,THU,FRI,SAT,SUN}
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Output:
Initially d is MON
Then it is WED
I say d and t are the same: true
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Methods
• Similar to functions and procedures
• Method has 2 parts: body and signature
modifiers type name(type0 parameter0, …typen-1 parametern-1) {
// method body…
}
• type is the return type of the method, name –
name of the method
• When a method of a class is called, it is
invoked on a specific instance of the class and
can change the state of the object
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• Similar to instance variables, Method
modifiers can be public, protected, private or
abstract
• Note: All parameters are passed by value,
changing the internal reference inside a
method will not change the reference that
was passed in
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Constructors
• A constructor is a special kind of method that
is used to initialize newly created objects
• Note: name of constructor, name, must be
same as the name of the class it constructs
• Return type not specified for constructor
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• Constructor definition and invocation
• Return statements are not allowed in a
constructor body
• A new instance of this class is created and its
constructor is used to initialize its isntance
variables
• Must be called using new operator
• Fish myFish = new Fish(y, “Wally”);
• Classes that define Stand – alone Java
program has the main method
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• Java Aquarium //system looks for compiled
version of Aquarium class and invokes the
special main method in that class
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Statement blocks and local
variables
• Two ways of declaring local variables
• type name;
• type name = initial_value;
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Expressions
• Expressions involve the use of literals,
variables and operators
• Variables and constants are used expressions
to define new values/modify variables
• Literal: is any “constant” value
– Null object reference
– Boolean: true and false
– Integer: type int, (32-bit integer), long iteger: 176L
or -52l (64-bit integer)
– Floating point: default for floating point numbers
is double, 3.14E2 or .19e10
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– Character, ‘a’, ‘?’ are character constants
– Special characters
– String literal: sequence of characters “have a nice
day”
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Operators
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Assignment operator : variable=expression
i = j = 25;
Arithmetic operators: +, -, *, /, %
Increment and decrement operators
– int i=8;
– int j=i++;
– int k=++i;
– int m=i--;
– int n = 9+i++;
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Logical operators
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< , <=, ==, !=, >=, >
Boolean: ! Not
&& conditional and, || conditional or
Bitwise operators: ~ bitwise complement, &
bitwise and, | bitwise or, ^ bitwise exclusive
or, <<, >>, >>>
• Operational assignment operators: variable op
= expression
• Variable = variable op expression
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Arrays
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Declaring arrays
• Element_type[] array_name = {init_val_0,
init_value_1,.., init_val_N-1}
• int[] primes = {2,3,5,7,11}
• element_type[] array_name;
• new element_type[length]
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Arrays are objects
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Cloning an array
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// Example 1 // --------- // First declare
//reference and then construct it.
int[] ExampleArray1;
ExampleArray1 = new int[24];
// Example 2 // --------- // This can be considered
//the short form for declaring and construction.
int[] ExampleArray2 = new int[24];
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// Example 3 // --------- // Construct and assign
//an array using a single command.
String[] ExampleArray3 = { "Sun Solaris" , "HPUX" , "Linux" , "MS Windows" , "Macintosh" };
int[] array = null;
int[] arr = new int[] {1,2,3};
int[][] twoDimArray = { {1,2,3}, {4,5,6}, {7,8,9} };
int[][] myArray = new int[5][];
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Testing 2D array
class TestTwoDimArrays { // initialize # of rows
static int [][] myArray = new int[3][];
public static void main(String[] args) {
myArray[0] = new int[3]; // initialize # of cols
myArray[1] = new int[4]; // in each row
myArray[2] = new int[5];
for(int i=0; i<3; i++)
// fill and print the array
fillArray(i, i+3); System.out.println(); }
// end main()
private static void fillArray(int row, int col) {
for( int i=0; i<col; i++)
myArray[row][i] = i;
for( int i=0; i<col; i++)
System.out.print(myArray[row][i]);
System.out.println();
}
}
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Exercises in group
• Suppose that we create an array A of
GameEntry objects, with integer scores field,
we cloe A and store the result in an array B. If
we set A[4].score =550, what is the score
value of GameEntry object referenced by
B[4]?
• Write a Java method that takes an array of int
values and determines if all the numbers are
different from each other.
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