Unit 4 · The Four Pillars in Depth · 9 min

Interfaces

An interface defines a contract a class promises to follow. It can declare abstract methods and may also provide default or static methods. A class can implement many interfaces.

Why this matters

Java does not allow a class to extend two classes, but a class can implement many interfaces — that is how Java achieves multiple inheritance of type.

Real-life analogy

A driving licence is a contract: it says you can drive. It does not say how you learned. Anyone holding it can be handed a car.

Syntax

interface Name {
    void doSomething();     // public abstract by default
}
class X implements Name { public void doSomething() { } }

Example

Java

interface Printable {
    void print();
}

interface Showable {
    void show();
}

class Report implements Printable, Showable {
    String title = "Semester Report";

    @Override
    public void print() {
        System.out.println("Printing: " + title);
    }

    @Override
    public void show() {
        System.out.println("Showing: " + title);
    }
}

public class Main {
    public static void main(String[] args) {
        Report r = new Report();
        r.print();
        r.show();

        Printable p = r;   // interface reference
        p.print();
    }
}

Output

Printing: Semester Report
Showing: Semester Report
Printing: Semester Report

Step by step

1A method declared without a body in an interface is public and abstract by default.
2`implements Printable, Showable` promises to define both methods.
3Overriding methods must be declared public, because interface methods are public.
4`Printable p = r;` works because Report IS-A Printable. This enables loose coupling.

Key points

Use implements for interfaces, extends for classes
A class can implement many interfaces
Interface fields are public static final by default
Since Java 8 interfaces may also have default and static methods

Common mistakes

Forgetting the `public` modifier on the implementing method
Trying `new Printable()` — interfaces cannot be instantiated
Using extends when you meant implements

Try it yourself

Write a Drawable interface and implement it in Circle and Square.
Make a class implement two interfaces and call both methods.

Open the practice notebook