what is interface in java

Interface in Java is a reference type and a collection of methods. Interfaces provide a way to specify the methods that a class must implement, but do not specify the specific implementation of these methods. When a class implements an interface, the class must provide concrete implementations of all abstract methods in the interface. An interface can inherit from another interface, which means that an interface can inherit methods from other interfaces. All methods in an interface are implicitly abstract, all methods in an interface are public, and an interface cannot contain instance fields. A class can implement multiple interfaces, which can be implemented by the class or inherited by other interfaces.

# Operating system for this tutorial: Windows 10 system, Dell G3 computer.

In Java, an interface is a reference type and a collection of methods. Specifically, an interface is a completely abstract class that only contains the declaration of abstract methods but no implementation of the methods. Interfaces provide a way to specify the methods that a class must implement, but do not specify the specific implementation of these methods.

Definition of interface:

In Java, you can use the interface keyword to define an interface. For example:

public interface MyInterface {  
    void myMethod(); // 这是一个抽象方法，没有方法体  
}

Implementation of interface:

When a class implements an interface, the class must provide concrete implementations of all abstract methods in the interface. Use the implements keyword to indicate that a class implements one or more interfaces. For example:

public class MyClass implements MyInterface {  
    @Override  
    public void myMethod() {  
        // 具体实现  
    }  
}

Inheritance of interface:

An interface can inherit another interface, which means that an interface can inherit methods of other interfaces. Use the extends keyword to indicate the inheritance relationship between interfaces. For example:

public interface AnotherInterface extends MyInterface {  
    void anotherMethod();  
}

Characteristics of the interface:

All methods in the interface are implicitly abstract, even if they are not declared using the abstract keyword.

All methods in an interface are public, even if they are not declared using the public keyword.

Interfaces cannot contain instance fields (i.e. non-static fields). They can only contain static constant fields (implicitly public, static, and final).

Starting from Java 8, interfaces can contain default methods and static methods. Default methods provide a default implementation of a method that can be selectively overridden by the implementation class. Static methods can only be called through interfaces, not through implementation classes.

The relationship between interfaces and classes:

A class can implement multiple interfaces, which provides a way to implement certain features of multiple inheritance without the complexity of multiple inheritance.

Interfaces can be implemented by classes and can also be inherited by other interfaces.

Purpose of interface:

Interface is usually used to define a set of related methods that can be implemented by unrelated classes. This allows developers to create pluggable code because implementation classes can be dynamically replaced at runtime.

Interfaces are widely used in Java APIs and frameworks, such as collection frameworks, event listeners, etc.

Comparison with abstract classes:

Interfaces and abstract classes are both used to define abstract behavior, but they have some key differences. An abstract class can contain concrete implementations of abstract and non-abstract methods, while an interface can only contain declarations of abstract methods (and starting from Java 8 can contain default and static methods). A class can only inherit from one abstract class, but it can implement multiple interfaces.

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