Variables & Data Types

Variables are containers for storing data values. In Java, because it is statically typed, you must define the type of the variable before using it.

Declaring Variables

Syntax: type variableName = value;

int myAge = 25;
String myName = "John";

Type Inference (Java 10+)

You can use the var keyword to let the compiler infer the type.

var city = "New York"; // Inferred as String
var population = 8000000; // Inferred as int

Constants

Use the final keyword to create variables that cannot be changed (constants).

final double PI = 3.14159;
// PI = 3.14; // Error: cannot assign a value to final variable

Data Types in Java

Java has two main categories of data types:

1. Primitive Data Types

There are 8 primitive types. They store simple values.

Type	Size	Description	Example
`byte`	1 byte	Stores whole numbers from -128 to 127	`byte b = 100;`
`short`	2 bytes	Stores whole numbers from -32,768 to 32,767	`short s = 5000;`
`int`	4 bytes	Stores whole numbers (most common)	`int i = 100000;`
`long`	8 bytes	Stores large whole numbers	`long l = 15000000000L;`
`float`	4 bytes	Stores fractional numbers	`float f = 5.75f;`
`double`	8 bytes	Stores fractional numbers (more precise)	`double d = 19.99;`
`boolean`	1 bit	Stores `true` or `false`	`boolean isFun = true;`
`char`	2 bytes	Stores a single character/letter	`char grade = 'A';`

[!NOTE] long literals end with L and float literals end with f.

2. Non-Primitive (Reference) Data Types

These refer to objects.

String (Sequence of characters)
Arrays
Classes
Interfaces

Type Casting

Converting a value from one data type to another.

Widening Casting (Automatic): Smaller type to larger type. byte -> short -> char -> int -> long -> float -> double
```
int myInt = 9;
double myDouble = myInt; // Automatic casting: 9.0
```

Narrowing Casting (Manual): Larger type to smaller type.

double myDouble = 9.78d;
int myInt = (int) myDouble; // Manual casting: 9

Interactive Example

Play with different data types!

JAVA PLAYGROUND

⏳ Loading editor…

AI Mentor

Confused about "Java primitive types, reference types and type casting"? Ask our AI mentor for a simplified explanation.

Quiz

Question 1 of 4

How many primitive data types are there in Java?

Unlimited

Next Steps

Now that we have variables, let's learn how to manipulate them using Operators.