Variables and Types

Types are containers for data -- you can think of them as small packing crates. The following code declares a single variable named "a".

Int1.java
1	public class Int1 {
2	public static void main(String[] args) {
3	int a; // Declare a
4	a = 3; // Assign a.
5	int b = 4; // Declare and assign b
6	a = a + b; // Updates a, adding b
7	System.out.print("a=");
8	System.out.print(a);
9	System.out.println();
10	}
11	}
$ javac Int1.java
$ java Int1 a=7

The text "int a" creates a box capable of holding one integer. We use the "=" character to fill it (line 4) or update it (line 6).

We have also shown a variation on the print function. print() simply writes to the screen without appending a newline. If you want the newline, use println().

Int2.java
1	public class Int2 {
2	public static void main(String[] args) {
3	int a = 3; // The type of the variable is "int", the name is "a"
4	a += 4; // Updates a, adding 4
5	System.out.print("a="+a);
6	}
7	}
$ javac Int2.java
$ java Int2 a=7

Here we've introduced a shorthand "a += 4" means the same as "a = a + 4". There are a number of other bits of shorthand that work similarly: /=, -=, *=, and %=.

Int3.java
1	public class Int3 {
2	public static void main(String[] args) {
3	int a = 2, b = 9, c = a;
4	System.out.print("a="+a+", b="+b+", c="+c);
5	}
6	}
$ javac Int3.java
$ java Int3 a=2, b=9, c=2

You can declare and initialize several variables on one line if you like. Int3 shows you how.

Prim.java
1	public class Prim {
2	// The eight primitive types: int, short, float, double, char, long,
3	// boolean, and byte. There is also a type called "void" and it is a way of saying "nothing" or "no value."
4	public static void main(String[] args) {
5	int a = 3; // a signed 4 byte value,
6	short b = 2; // a signed 2 byte value
7	float c = 1.34e2f; // put a f at the end to make this a float, 4 bytes
8	double d = 1.96e1; // like a float, but with 8 bytes for greater precision
9	// and larger possible absolute value.
10	char e = 'E'; // single quotes for character constants, 2 byte value
11	long f = 9; // an 8 byte value
12	boolean g = true; // the other possible value is 'false'
13	byte h = 3; // a signed 1 byte value
14	System.out.println("nothing to do!");
15	}
16	}
$ javac Prim.java
$ java Prim nothing to do!

Definitions:

A signed value means the number can be either positive or negative.
Boolean is a fancy name for a value has only two states: true or false.
A byte consists of eight bits.
A bit is a value that can either be 1 or 0. Using one bit you can have an integer with a maximum value of 1 and a minimum value of zero. Using three bits, for example, you can create a value from zero to seven.

binary decimal

000 0

001 1

010 2

011 3

100 4

101 5

110 6

111 7

4*bit2+2*bit1+bit0 decimal value

For signed numbers we can use one bit to identify whether the number is positive or negative. A Java integer is a signed 32-bit number and can have a value anywhere from -2147483648 to 2147483647.
Primitive types are types that are built-in to Java are handled specially. They are distinguished from Objects, which are types that programmers are free to invent. There are, however, some special objects that are built-in to java. Two good examples are strings and arrays.