Interface vs Abstract Class in Java — When to Use Each

An interface defines a contract of methods a class agrees to implement, and a class can implement multiple interfaces. An abstract class can hold shared fields, constructors, and partially implemented methods, but a class can only extend one abstract class.

Common causes

• Java doesn't support multiple inheritance of state (to avoid the ambiguity problems it can cause), so interfaces exist to let a type participate in multiple unrelated contracts without that ambiguity

How to fix it

• Use an interface when you're defining a capability unrelated classes might share, like Comparable or Serializable, especially when multiple inheritance of the contract is needed
• Use an abstract class when subclasses share common state or behavior that shouldn't be duplicated, and the relationship between them is a genuine 'is-a' hierarchy
• Since Java 8, interfaces can include default methods with implementations, blurring the line somewhat — but interfaces still cannot hold instance state (fields) the way abstract classes can

Example

interface Shape {
    double area();
}

abstract class Animal {
    protected String name;
    abstract void makeSound();
}

FAQ

Can an abstract class implement an interface?

Yes — and it can leave some of the interface's methods unimplemented, deferring them to its own concrete subclasses, since the abstract class itself is never instantiated directly.