What is the difference between abstract class and interface in Java?

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Accepted Answer

Both define a contract but differ in capability and intent. Abstract class — can have abstract and concrete methods , instance state, constructors, and any access modifier. A class can extend only one . Best for "is-a" with shared base behavior. Interface — defines a contract; methods are public abstract by default. Java 8+ allows default and static methods ; Java 9+ allows private methods . A class can implement many . Best for capability/"can-do" types. Rule of thumb — pick interfaces by default; reach for abstract classes when you need shared state or partial implementation. Since Java 8, know the new capabilities of interfaces (default methods, static methods, private methods in Java 9). The key remaining difference: interfaces cannot have state or constructors. Use the capability (interface) vs shared implementation (abstract class) design guideline.