College Board
A.P. Computer Science A Topics
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Program Design - Read and understand a
problem's description, purpose, and goals.
Procedural Constructs - Constant
declarations ; Variable declarations
Standard Data Stuctures - Simple data types
(int, boolean, double)
Vocabulary & Terms
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Reference
Variable
Keyword
Identifier
Data type
int
char
double
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boolean
Object
Initializing a variable
Assignment operator
Basic Output Commands
print()
println()
object / reference
command / method
System.out.print("Hello"
System );
is the class name.out is an object that prints characters on
the console window (black window on the screen).
• println is a method, or behavior, that the System.out object
carries out.
• The characters enclosed in double quotation marks and are called
a string. The string is printed in the console window.
• Each statement in a program ends with a semicolon (;).
OUTPUT
Hello
Try this in the interactions
pane of Dr. Java. You
should get the output to the
left.
Type the following into java:
public class TestOutputs
{
public static void main(String [] args)
{
System.out.print("Hello");
System.out.print("Hello");
}
}
OUTPUT
HelloHello
public class TestOutputs
{
public static void main(String [] args)
{
System.out.println("Hello");
System.out.print("Hello");
}
}
OUTPUT
Hello
Hello
Try the statements below in your TestOutputs program. Be sure
to run each one and not any differences. Pay close attention to
the difference between print and println.
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Java provides a facility for displaying special characters in a string. These special
characters are indicated by placing a backslash (\) in the string. For example, since double
quotation marks are used to indicate the start and end of a string, there is no obvious way
to include quotation marks within a string.
Therefore, the backslash must be used as in the statement:
System.out.println(“You say \”Goodbye,\” and I say \“Hello.\””);
Try this in the TestOutputs program. It should display:
You say "Goodbye," and I say "Hello.“
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The double quotation marks that follow the backslashes are displayed rather
than interpreted as ending the string. The combination of symbols is called an
escape character. The idea is that it escapes from the normal interpretation of
characters in a string.
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Another special escape character is the end-of-line character.
This is indicated by \n and can replace the use of –ln in println.
For example:
System.out.print(“This is the first line,\n”);
System.out.print(“and this is the second line.”);
Try this in the TestOutputs program.
Displays:
This is the first line,
and this is the second line.
\\
\“
\’
\n
\t
Prints out \
Prints out “
Prints outs ’
Goes to next line
Tab(moves over 8 spaces
System.out.println("\\hello\"/");
OUTPUT
\hello"/
What is the output?
(Attempt to answer the questions first, then type each into the
interactions pane in Dr. Java to confirm your answers).
System.out.println( “h\tello”);
System.out.println( “hel\\lo\””);
System.out.println( “hel\nlo”);
What is a data type?
byte
long
char
short
float
boolean
int
double
int num = 9;
double total = 3.4;
We will use int, double and boolean in this course.
TYPE
SIZE
RANGE
byte
8 bits
-128 to 127
short
16 bits
-32768 to 32767
int
32 bits
-2 billion to 2 billion
long
64 bits
-big to +big
Think about a light bulb. It has 2 states ON or
OFF. The same principals can be applied to a
computer. The electronic circuits are either On
or Off. Different combinations of On and Off
mean different things for the computer.
For the computer On is 1, and Off is 0. A
number system made up of 1’s and 0’s is
known as the Binary Number System. We use
the Decimal Number System: digits 0 – 9.
What is a BIT?
Each 1 or 0 is called a BIT. It is short hand… take the b at the
start of binary and the it at the end of digit… BIT = Binary
digIT.
Memory consists of bits and bytes.
1 byte = 8 bits
Every character on the keyboard has a binary digit associated
with it in accordance with the UNICODES.
Look at the following site:
http://www.neurophys.wisc.edu/comp/docs/ascii/#table
What is the Binary Number for the letter A? What about a? 7?
The more bits you have the more you can store.
int one = 120;
int two = 987123;
System.out.println(one);
System.out.println(two);
OUTPUT
120
987123
(Type the code above into the interactions
pane in Dr. Java and check your output.)
Type the following code into Dr. Java exactly as it is written. Compile the
code and run the program. Are the results what you expected?
//integer example
public class Integers
{
public static
{
int
int
int
void main(String args[])
one = 120; //legal assignment
two = 987123;
three = 999999999;
System.out.println(one);
System.out.println(two);
three = three * 3;
//creates an overflow error at runtime
System.out.println(three);
}
}
TYPE
SIZE
RANGE
float
32 bits
-big to +big
double
64 bits
-big to +big
Float has 7 digits of precision and double
has 15 digits of precision.
We are talking about decimal numbers now!
int can NOT store decimal numbers.
double one = 99.57;
double two = 3217;
double three = 23.32;
System.out.println(one);
System.out.println(two);
System.out.println(three);
OUTPUT
99.57
3217.0
23.32
(type the code above into the interactions
pane in Dr. Java and check your output.)
Type the following program into Dr. Java exactly as it is written. Compile
the code and then run the program. Are the results what you would have
expected?
//real number example
public class Reals
{
public static void main(String args[])
{
double one = 99.57;
double two = 3217;
System.out.println(one);
System.out.println(two);
}
}
Characters store letters.
char let = ‘A’;
char is a 16-bit unsigned integer data type.
We use single quotes for character literals.
We use double quotes for String literals… more
than 1 character.
char is a 16-bit unsigned int data type.
For example, here is a 16 bit pattern:
000000000110011
char let = 65;
ASCII VALUES YOU SHOULD KNOW!!!
‘A’ – 65
‘a’ – 97
‘0’ - 48
char alpha = 'A';
char ascii = 65;
char sum = 'B' + 1;
OUTPUT
A
A
67
C
System.out.println(alpha);
System.out.println(ascii);
System.out.println('B'+1);
//’B’ gets converted to its ASCII value
//66 and then 1 is added to it.
System.out.println(sum);
//since sum holds 67 and it is a char, we
//get C.
Type the following program in Dr. Java exactly as it is written. Compile the code and run the program. Are
the results what you expected?
public class Char
{
public static void main(String args[])
{
char alpha = 'A';
char ascii = 65;
char sum = 'B' + 1;
System.out.println(alpha);
System.out.println(ascii);
System.out.println('B'+1); //char is an integer type
System.out.println(sum);
}
}
The boolean data type can store true
or false only.
boolean heads = true;
boolean tails= false;
Type the following program into Dr. Java exactly as it is written. Compile the code and run the
program. Are the results what you expected?
public class BooleanTest
{
public static void main(String args[])
{
boolean stop = true;
boolean go = false;
System.out.println( stop );
stop = go;
System.out.println( go );
System.out.println( stop );
}
}
In JAVA, you have 8 primitive data types
that take up very little memory.
Everything else in Java in an Object.
Strings are objects.
String temp = "abc";
When we have values that should not change during
the program, we make them constants.
Use the keyword final to create a constant.
Constants should be assigned a value upon creation.
final int birthYear = 1976;
For this program, birthYear will always be set to
1976.
Strings are collections of characters. They are
words, sentences, combinations of letters and
numbers, etc.
When declaring and creating an instance of
any class, we follow this pattern:
ClassName objectName = new ClassName(parameters);
String name = new String (“Furman”);
String literals: Actual Strings enclosed in double quotes.
The String class is a special class. It is used so much, that
we have some exceptions for how they can be created.
String name = new String (“Furman”);
OR
String name = “Furman”;
//whenever Java comes across a String literal, it
automatically creates a String object.
The first part String name declares name as a variable to
hold a String object.
The second part = “Furman”; or = new (“Furman”);
creates a String object, and gives name the memory
address.
receiver = 57;
receiver is assigned( = ) the value 57.
In an assignment statement, the receiver
is always on the left of the assignment
symbol ( = ).
Assignment statements examples :
total = 10;
amount = 100.34;
You can initialize more than one variable in
one statement if they have the same type.
Just separate them using a comma.
int number = 75, it=99;
float taxrate = 7.75;
char letter = ‘A’, newlet = ‘a’;
boolean isprime = false;
String sone = "abc";
Java is a strong typed language. As a result,
you must be very careful to look at data
types when assigning values.
int one=90;
char letter= ‘A’;
char let= 65;
one=letter;
letter=let;
one=let;
int one = 90;
double dec = 234.5;
char letter = 'A';
System.out.println( one );
one = letter; //char to int
System.out.println( one );
one = 'A';
//char to int
System.out.println( one );
System.out.println( dec );
dec = one;
//int to double
System.out.println( dec );
System.out.println (one);
one = dec;
//double to int… Illegal
System.out.println (one);
OUTPUT
90
65
65
234.5
65.0
65
Error
Type the following program into Dr. Java. Compile the code
and run the program. Are the results what you expected?
//strong typed language example
public class MixingData
{
public static void main(String args[])
{
int one = 95;
double dec = 12.4;
char letter = ‘B';
System.out.println( one );
one = letter;
System.out.println( one );
//char to int
one = 'A'; //char to int
System.out.println( one );
System.out.println( dec );
dec = one;
System.out.println( dec );
//int to double
System.out.println( letter );
//letter = dec;
//double to int - not legal
//System.out.println( letter );
}
}