Interface Methods and Interface variables

Interface Method

Every method present inside an interface is always public and abstract, whether we declare it or not.

interface Interf {
    void m1();
}

Why public?
Because interface methods must be accessible to every implementing class.

Why abstract?
Because interfaces only define behavior, and the implementation is provided by the implementing class.

Important Point: Interface acts like a contract. Any class implementing it must provide implementation for all abstract methods.

Equivalent Declarations:

void m1();
public void m1();
abstract void m1();
public abstract void m1();

Extra Explanation: Even if you don’t write public and abstract, the compiler automatically adds them.

Invalid Method Declarations:

public void m1() {}        // ❌ Method body not allowed
private void m1();        // ❌ Not allowed
protected void m1();      // ❌ Not allowed
static void m1();         // ❌ (in older versions)
public abstract native void m1(); // ❌ Invalid combination

abstract public void m1(); // ✔ Valid

Extra Note: Interface methods cannot be private or protected because they must be visible to implementing classes.

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Interface Variable

An interface can contain variables. These variables are used to define constants.

Every interface variable is always public static final, whether we declare it or not.

interface Interf {
    int x = 10;
}

Explanation:

• public → Accessible everywhere
• static → Belongs to interface, not object
• final → Value cannot be changed

Equivalent Declarations:

int x = 10;
public int x = 10;
static int x = 10;
public static int x = 10;
public final int x = 10;
static final int x = 10;
public static final int x = 10;

Important Rule: Interface variables must be initialized at the time of declaration.

interface Interf {
    int x;   // ❌ Compile-time error
}

Error: '=' expected

Extra Explanation: Since variables are final, we cannot modify them after initialization.

interface Interf {
    int x = 10;
}

class Test implements Interf {
    public static void main(String[] args) {
        x = 333;   // ❌ Error
    }
}

Error: Cannot assign value to final variable

Correct Approach:

class Test implements Interf {
    public static void main(String[] args) {
        int x = 333;
        System.out.println(x); // ✔ Output: 333
    }
}