Java Variable

Source: Dev.to

Why use variables

• Store data temporarily in memory
• Reuse values multiple times
• Perform calculations
• Make programs dynamic and flexible
• Improve code readability and maintenance

How a variable is created in Java

1. Declaration (Memory allocation)

int age;

• Tells Java what type of data to store
• Reserves memory for the variable

2. Initialization (Assign value)

int age = 25;

• Stores the value in the allocated memory

When is a variable created?

Local Variable

• Where created? Inside a method or block
• When created? When the method/block is entered
• When destroyed? When the method/block exits
• Memory location: Stack

void show() {
    int x = 10; // local variable
}

• Must be initialized before use
• No default value

Instance Variable (Non‑static)

• Where created? As a member of a class instance
• When created? When an object is instantiated (new)
• When destroyed? When the object is garbage‑collected
• Memory location: Heap

class Student {
    int id; // instance variable
}

Student s = new Student(); // instance variable created here

• Receives a default value if not explicitly initialized

Static Variable (Class Variable)

• Where created? Inside a class with the static keyword
• When created? When the class is loaded by the JVM
• When destroyed? When the JVM shuts down
• Memory location: Method Area (Class Area)

class College {
    static String collegeName = "ABC College";
}

• A single copy is shared by all objects of the class

Parameter Variable

• Where created? In a method’s parameter list
• When created? When the method is invoked
• Memory location: Stack

void add(int a, int b) {
    int sum = a + b; // 'a' and 'b' are parameter variables
}