Java Interfaces vs. Abstract Classes: Differences and Implementation, A Must-Know for Beginners

This article explains the differences and core usages between Java interfaces and abstract classes. An interface is a special reference type declared with the `interface` keyword, containing only abstract methods (before Java 8) and constants. It specifies class behavior, implemented by classes using `implements`, supports multiple implementations, cannot be instantiated, and is used to define "what can be done" (e.g., `Flyable` specifies flying behavior). An abstract class is declared with `abstract`, containing abstract methods, concrete methods, and member variables. It serves as a class template, extended by subclasses via `extends` (single inheritance required), and can be instantiated through subclass implementation of abstract methods. It defines "what something is" (e.g., `Animal` defines animal attributes and common methods). Core differences: Interfaces specify behavior, support multiple implementations, and only contain abstract methods/constants; abstract classes define templates, use single inheritance, and can include concrete implementations. Selection suggestions: Use interfaces for behavior specification or multi-implementation scenarios, and abstract classes for class templates or single-inheritance scenarios. Neither can be directly instantiated; abstract class abstract methods must be implemented by subclasses, while interface methods are implicitly `public abstract`. Summary: Interfaces define "what can be done" focusing on behavior, abstract classes define "what something is" focusing on templates. Choose based on specific scenarios.