📐Variables & Types

Although Java is object-oriented, not all types are objects. It is built on top of basic variable types called primitives.

Primitive Types

• byte (number, 1 byte)

• short (number, 2 bytes)

• int (number, 4 bytes)

• long (number, 8 bytes)

• float (float number, 4 bytes)

• double (float number, 8 bytes)

• char (a character, 2 bytes)

• boolean (true or false, 1 byte)

Java is a strongly typed language, which means variables need to be defined before we use them.

ASSIGNMENT COMPATIBILITIES

A value of one type can be assigned to a variable of any type further to the right

byte --> short --> int --> long --> float --> double 

But not to a variable of any type further to the left. You can assign a value of type char to a variable of type int.

Example : float var = 5;

Numbers

To declare and assign a number use the following syntax:

int myNumber;
myNumber = 5;

Or you can combine them:

int myNumber = 5;

To define a double floating point number, use the following syntax:

double d = 4.5;
d = 3.0;

If you want to use float, you will have to cast:

float f = (float) 4.5;

Or, You can use this:

float f = 4.5f; // (f is a shorter way of casting float)

Characters and Strings

In Java, a character is its own type and it's not simply a number, so it's not common to put an ASCII value in it, there is a special syntax for chars:

char c = 'g';

String is not primitive. It's a real type, but Java has special treatment for String.

Here are some ways to use a string:

// Create a string with a constructor
String s1 = new String("Who let the dogs out?");        // String object stored in heap memory
// Just using "" creates a string, so no need to write it the previous way.
String s2 = "Who who who who!";                         // String literal stored in String pool
// Java defined the operator + on strings to concatenate:
String s3 = s1 + s2;

There is no operator overloading in Java but there is the exception that proves the rule - string is the only class where operator overloading is supported. We can concat two strings using + operator. The operator + is only defined for strings, you will never see it with other objects, only primitives.

You can also concat string to primitives:

int num = 5;
String s = "I have " + num + " cookies"; //Be sure not to use "" with primitives.

boolean

Every comparison operator in Java will return the type boolean. Unlike other languages, it only accepts two special values: true or false.

boolean b = false;
b = true;

boolean toBe = false;
b = toBe || !toBe;
if (b) {
    System.out.println(toBe);
}

int children = 0;
b = children; // Will not work
if (children) { // Will not work
    // Will not work
}

int a;
boolean b = true; 
boolean c = false; 
a = b + c;            //The following line will give an error
System.out.println(a);

Quick Reference:

Data Type	Type	Default Value	Sample Code
byte	Primitive	0	byte myByte = 0;
short	Primitive	0	short myShort = 0;
int	Primitive	0	int myInt = 0;
long	Primitive	0L	long myLong = 0L;
float	Primitive	0.0f	float myFloat = 0.0f;
double	Primitive	0.0d	double myDouble = 0.0d;
char	Primitive	'\u0000'	char myChar = '\\u0000';
boolean	Primitive	false	boolean myBoolean = false;
String	Non-Primitive	null	String myString = null;
Array	Non-Primitive	null	int[] myArray = null;
Class	Non-Primitive	null	Class myClass = null;
Object	Non-Primitive	null	Object myObject = null;
HashMap	Non-Primitive	null	HashMap<String, Integer> myHashMap = null;