Java Interfaces and Abstract Classes: A Beginner's Guide

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interface

An interface is a special class that contains only abstract methods (unimplemented methods). It defines a contract that any class that implements the interface must implement these abstract methods. Interfaces are used to establish a unified interface between different classes to promote code portability.

Abstract method

Abstract method is a method in an interface or abstract class, which has no implementation body. When a class implements an abstract method, it must provide an implementation of the method.

Features of interface

• Contains only abstract methods.
• Cannot be instantiated.
• Can inherit multiple interfaces.
• Support multiple inheritance.

Abstract class

Abstract class is a special class that can contain abstract methods and concrete methods (implemented methods). It is used to define an incomplete class from which other classes can extend and provide the missing implementation. Abstract classes are often used to represent common functionality that can be customized by subclasses.

Abstract class and concrete class

The main difference between abstract classes and concrete classes is that abstract classes cannot be instantiated, while concrete classes can. Abstract classes must be extended by subclasses in order to be used.

Abstract classes and interfaces

• Similarities:Both can define abstract methods.
• the difference:
  • Abstract classes can contain concrete methods, but interfaces cannot.
  • Only one abstract class can be inherited, but multiple interfaces can be inherited.
  • Abstract classes do not support multiple inheritance, but interfaces do.

Best Practices

Choosing to use an interface or an abstract class depends on the specific needs:

• Using interface:
  • When it is necessary to define a common contract that is implemented by multiple classes.
  • When you need to support multiple inheritance.
• Use abstract class:
  • When you need to define an incomplete class and customize it by a subclass.
  • When it is necessary to provide default implementations of certain methods.

Example

The following are examples of interfaces and abstract classes:

interface:

public interface Animal {
void eat();
void sleep();
}

Abstract class:

public abstract class Mammal {
public abstract void eat();
public abstract void sleep();
public void move() {
// 默认实现
}
}

Application scenarios

Interfaces and abstract classes are widely used in Java programming, including:

• Create a plug-in architecture: The interface is used to define the contract of the plug-in , and the abstract class is used to provide the basic implementation of the plug-in.
• Implement polymorphism: Define a common type through an interface or abstract class, and objects of different types can have the same behavior.
• Define Design Patterns: Interfaces and abstract classes play a key role in Design Patterns, such as Strategy Pattern and Template Method Pattern.

Summarize

Interfaces and abstract classes are powerful tools in Java that can be used to represent abstract types. The main difference between them is abstract methods, inheritance and multiple inheritance. By understanding their similarities and differences, you can use them effectively to create reusable, extensible, and maintainable code.

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