As explained in the previous chapter, a variable in Java must be a specified data type:
int myNum = 5; // Integer (whole number)
float myFloatNum = 5.99f; // Floating point number
char myLetter = 'D'; // Character
boolean myBool = true; // Boolean
String myText = "Hello"; // StringData types are divided into two groups:
- Primitive data types - includes byte , short , int , long , float , double , boolean and char
- Non-primitive data types - such as String , Arrays and Classes (you will learn more about these in a later chapter)
Primitive Data Types
A primitive data type specifies the type of a variable and the kind of values it can hold.
There are eight primitive data types in Java:
| Data Type | Description |
|---|---|
| byte | Stores whole numbers from -128 to 127 |
| short | Stores whole numbers from -32,768 to 32,767 |
| int | Stores whole numbers from -2,147,483,648 to 2,147,483,647 |
| long | Stores whole numbers from -9,223,372,036,854,775,808 to 9,223,372,036,854,775,807 |
| float | Stores fractional numbers. Sufficient for storing 6 to 7 decimal digits |
| double | Stores fractional numbers. Sufficient for storing 15 to 16 decimal digits |
| boolean | Stores true or false values |
| char | Stores a single character/letter or ASCII values |
You Cannot Change the Type
Once a variable is declared with a type, it cannot change to another type later in the program:
int myNum = 5; // myNum is an int
// myNum = "Hello"; // Error: cannot assign a String to an int
String myText = "Hi"; // myText is a String
// myText = 123; // Error: cannot assign a number to a StringNote
This rule makes Java safer, because the compiler will stop you if you try to mix up types by mistake.
If you really need to change between types, you must use type casting or conversion methods (for example, turning an int into a double ).