Variables and Types


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

Here is a list of all primitives in Java:

  • 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 strong typed language, which means variables need to be defined before we use them.

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 it's 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 a 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?");
// Just using "" creates a string, so no need to write it the previous way.
String s2 = "Who who who who!";
// Java defined the operator + on strings to concatenate:
String s3 = s1 + s2;

There is no operator overloading in Java! 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 that not like other languages can only accept 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
}

Exercise

Create all of the primitives (except long and double) with different values. Concatenate them into a string and print it to the screen so it will print: H3110 w0r1d 2.0 true