Variables in Java

⏱️ 6 min read • Beginner Level • Lesson 10

Variables in Java are named memory locations used to store data. They allow programs to save values and change them during execution. Understanding variables is essential before learning data types, operators, and control statements.

Syntax for Variable Declaration

DataType identifier = value;

Types of Variables in Java

1. Local Variables

   A local variable is declared inside a method or block and its scope is limited to that block.

   They don’t get default values and must be initialized before use.

2. Instance Variables (Non-Static Fields)

   A non-static variable declared inside a class but outside methods is called an instance variable. Each object gets its own copy of instance variables.

   They are automatically initialized with default values (e.g., 0 for int, null for objects).

3. Class Variables (Static Fields)

   A static variable is declared with the static keyword inside a class. It belongs to the class rather than any object.

   Static variables cannot be local.

   Since only one copy exists per class, changing it affects all objects.

Example:

public class Test {
  int x = 50;        // instance variable
  static int y = 100; // static variable

  void myMethod() {
    int z = 150;     // local variable
  }
}
  

Here: x is tied to an object, y is tied to the class, and z exists only during method execution.

Local variables must be initialized before use, while instance and static variables get default values.

Default Values of Instance and Class Variables

In Java, instance and class (static) variables are automatically initialized with default values if not explicitly assigned. Local variables do not have default values and must be initialized before use.

This table applies to both instance and class (static) variables. Local variables must be explicitly initialized, or the compiler will throw an error.

Interview Questions ⭐

🧠 Test your understanding with a quick quiz

public class Counter {
    static int count = 0;
    void increment() {
        count++;
    }
}

public class Test {
    int x;             // instance variable
    static int y;      // static variable
    public static void main(String[] args) {
        Test obj = new Test();
        System.out.println(obj.x);
        System.out.println(y);
    }
}

public class Test {
    int num = 10;  // instance variable
    public static void main(String[] args) {
        Test t1 = new Test();
        Test t2 = new Test();
        t1.num = 20;
        System.out.println(t1.num + " " + t2.num);
    }
}

public class Example {
    public static void main(String[] args) {
        int a;  // local variable
        System.out.println(a);
    }
}

🎉 Great job! Continue learning Java step by step.