Java 8 Default Methods

Java 8 新特性Java 8 New Features

Java 8 introduced default methods for interfaces.

Simply put, default methods mean that interfaces can have implemented methods, and implementing classes do not need to implement them.

We just need to add the default keyword before the method name to implement a default method.

Why have this feature?

First, the previous interface was a double-edged sword. The advantage is that it is oriented towards abstraction rather than specific programming. The disadvantage is that when the interface needs to be modified, all classes implementing that interface must be modified. The current collection framework before Java 8 did not have a foreach method. The usual solution that can be thought of is to add new methods and implementations to the relevant interfaces in the JDK. However, for already released versions, there is no way to add new methods to an interface without affecting existing implementations. So default methods were introduced. Their purpose is to solve the incompatibility between interface modifications and existing implementations.

Syntax

The syntax format of a default method is as follows:

public interface Vehicle { default void print(){ System.out.println("I am a car!"); } }

Multiple Default Methods

If an interface has a default method, consider the situation where a class implements multiple interfaces, and these interfaces have the same default method. The following example illustrates the solution to this situation:

public interface Vehicle { default void print(){ System.out.println("I am a car!"); } } public interface FourWheeler { default void print(){ System.out.println("I am a four-wheeled vehicle!"); } }

The first solution is to create your own default method to override the default method of the interface:

public class Car implements Vehicle, FourWheeler { default void print(){ System.out.println("I am a four-wheeled car!"); } }

The second solution is to use super to call the default method of the specified interface:

public class Car implements Vehicle, FourWheeler { public void print(){ Vehicle.super.print(); } }

Static Default Methods

Another feature of Java 8 is that interfaces can declare (and provide implementations of) static methods. For example:

public interface Vehicle { default void print(){ System.out.println("I am a car!"); } //Static Methods static void blowHorn(){ System.out.println("Honk the horn!!!"); } }

Default Method Example

We can understand the use of default methods through the following code. You can put the code in the Java8Tester.java file:

Java8Tester.java file

public class Java8Tester { public static void main(String args[]){ Vehicle vehicle = new Car(); vehicle.print(); } } interface Vehicle { default void print(){ System.out.println("I am a car!"); } static void blowHorn(){ System.out.println("Honk the horn!!!"); } } interface FourWheeler { default void print(){ System.out.println("I am a four-wheeled vehicle!"); } } class Car implements Vehicle, FourWheeler { public void print(){ Vehicle.super.print(); FourWheeler.super.print(); Vehicle.blowHorn(); System.out.println("I am an automobile!"); } }

Executing the above script, the output result is:

$ javac Java8Tester.java 
$ java Java8Tester
我是一辆车!
我是一辆四轮车!
按喇叭!!!
我是一辆汽车!

Java 8 新特性Java 8 New Features

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