What is the difference between an abstract class and an interface?

Abstract class: a shared base

An abstract class is meant to be a common parent. It can provide some finished methods and leave others for children to complete. You cannot create an object of an abstract class directly.

abstract class Shape {
    abstract double area();          // no body, children must fill in
    void describe() {                // shared, finished method
        System.out.println("I am a shape");
    }
}

Interface: a pure contract

An interface says what a class must be able to do, without saying how. Many unrelated classes can implement the same interface to promise the same capability.

interface Drawable {
    void draw();   // any Drawable must provide draw()
}
class Circle implements Drawable {
    public void draw() { System.out.println("circle"); }
}

Quick comparison

• Methods: an abstract class can have finished and unfinished methods, an interface is traditionally only a contract.
• State: an abstract class can hold fields and state, an interface mainly holds constants.
• Inheritance: a class extends one abstract class but can implement many interfaces.
• Use: abstract class for a shared base type, interface for a capability shared across unrelated types.