Java Variables – Definition, Types & Examples

In Java, variables are containers used to store data values. Every variable has a data type, a name, and a value. They help programs store, update, and reuse information efficiently.

In this section of our Core Java Tutorial, you’ll learn everything about Java variables — their types, scope, memory allocation, best practices, and even advanced concepts like shadowing and type inference.

What is a Variable?

A variable is a named memory location that holds a value which can change during program execution.

int age = 25;
String name = "John";

Here,

• int → data type
• age → variable name
• 25 → value

Variable Declaration and Initialization

Declaration:

int number; // variable is declared but not initialized

Initialization:

number = 10; // value assigned separately

Combined Declaration & Initialization:

int number = 10; // declare and initialize in a single line

Types of Variables in Java

Java variables are mainly categorized into three types based on their position and scope:

1. Local Variables

• Declared inside methods or blocks
• Must be initialized before use
• Not accessible outside that method

public class Example {
    public void display() {
        int number = 10;  // local variable
        System.out.println("Local Variable: " + number);
    }
}

2. Instance Variables

• Declared inside the class but outside methods
• Each object gets its own copy
• Default values are automatically assigned

public class Student {
    String name;   // instance variable
    int age;       // instance variable
}

3. Static Variables

• Declared with the static keyword
• Shared by all objects of the class
• Useful for constants or counters

public class Counter {
    static int count = 0; // static variable

    Counter() {
        count++;
    }
}

Example:

public class Student {
    static String schoolName = "ABC School"; // static variable
    int rollNo;                              // instance variable

    void setRollNo(int rollNo) {
        int marks = 90;                      // local variable
        this.rollNo = rollNo;
        System.out.println("Roll No: " + rollNo);
        System.out.println("Marks: " + marks);
        System.out.println("School: " + schoolName);
    }
}

Final Variables (Constant Variables)

Use the final keyword to make a variable constant (value cannot change).

final double PI = 3.14159; // cannot be changed

If you try to reassign it:

PI = 3.14; // Error – cannot assign value to final variable

Default Values of Variables

Note: Local variables don’t have default values — they must be initialized before use.

Variable Shadowing

When a local variable has the same name as an instance variable, it hides (or shadows) the instance variable.

public class Test {
    int value = 10; // instance variable

    void display() {
        int value = 20; // local variable shadows instance variable
        System.out.println("Local Value: " + value);        // 20
        System.out.println("Instance Value: " + this.value); // 10
    }
}

Use this.variableName to access the instance variable.

Dynamic Initialization

Variables can also be initialized at runtime, not just during declaration.

int a = 10;
int b = 20;
int sum = a + b; // dynamically initialized

Type Inference with var (Java 10+)

Modern Java supports type inference using var — the compiler automatically infers the type.

var message = "Hello Java"; // inferred as String
var count = 10;             // inferred as int

Rules for var:

• Can only be used inside methods (local scope).
• Must be initialized at the time of declaration.
• Cannot be used for class or instance variables.

Memory Allocation of Variables

Scope of Variables (Example)

public class ScopeExample {
    static int staticVar = 100; // class-level
    int instanceVar = 50;       // object-level

    void display() {
        int localVar = 10;      // method-level
        System.out.println(localVar);
        System.out.println(instanceVar);
        System.out.println(staticVar);
    }
}

Each variable has its own visibility and lifetime, depending on where it is declared.

Declaring Multiple Variables

You can declare multiple variables of the same data type in one line:

int x = 10, y = 20, z = 30;
System.out.println(x + ", " + y + ", " + z);

Printing Variables in Java

Use System.out.println() to display variable values.

int age = 25;
String name = "John";
double salary = 50000.50;

System.out.println("Name: " + name);
System.out.println("Age: " + age);
System.out.println("Salary: " + salary);

Output:

Name: John
Age: 25
Salary: 50000.5