Interfaces and Abstract Classes — Free Java Tutorial

TL;DR: Learn Interfaces and Abstract Classes in Java with a free, beginner-friendly tutorial, examples and practice for Indian students on Syllab.in.

Interfaces and Abstract Classes in Java

An interface in Java is a contract — it defines what methods a class MUST implement, without specifying how. It's Java's way of achieving full abstraction. A class can implement multiple interfaces (unlike inheritance, which is single-parent).

Abstract class: A partially implemented class that can't be instantiated directly. Contains both abstract methods (must be implemented by subclasses) and concrete methods (already implemented). Use when you want to share code among related classes.

Interface vs Abstract class: Interface for "can-do" relationships (a Car can drive, a Boat can navigate — different hierarchies implementing the same capability). Abstract class for "is-a" relationships with shared code (Dog and Cat are both Animals with shared behaviour).

// Interface definition
interface Printable {
    void print();           // abstract — no body
    default void save() {  // default method — has body
        System.out.println("Saving...");
    }
}

// Implementing an interface
class Report implements Printable {
    public void print() { System.out.println("Printing report"); }
}

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