Java 中的接口 - 它们是做什么用的? [英] Interfaces in Java - what are they for?

问题描述

可能的重复:
接口的目的继续

我不久前才开始学习 Java.
我遇到过 Interfaces，我知道如何使用它，但仍然不能完全理解它的概念.
据我了解，interfaces 通常由类实现，然后类必须实现接口中声明的方法.
问题是——这到底是什么意思?将接口中的方法作为普通类方法实现不是更容易吗?使用接口到底有什么好处?

I just started learning Java not so long ago.
I've come across Interfaces which I know how to use but still can't quite grasp the idea of it.
As I understand it, interfaces are usually implemented by classes, which then have to implement the methods declared in the interface.
The problem is - what exactly is the point? Wouldn't it be easier to just implement the methods from the interface as normal class methods? What exactly are the advantages of using interfaces?

我试过在谷歌上寻找答案.有很多，但我仍然无法理解它的意义.我还阅读了这个问题及其答案，但整个合同的事情只会让它变得更加复杂......

I've tried looking for an answer on google. There were many but I still couldn't understand the point of it. I also read this question and its answers, but the whole contract thing just makes it more complicated...

希望有人能把它简化得足够好！:)
提前致谢！

Hopefully someone could simplify it good enough! :)
Thanks in advance!

推荐答案

一个接口允许你在运行时提供不同的实现，注入依赖，分离关注点，使用不同的实现进行测试.

An interface allows you to provide a different implementation at runtime, inject dependencies, separate concerns, use different implementations for testing.

只需将接口视为类保证实现的契约即可.实现接口的具体类是无关紧要的.不知道这是否有帮助.

Just think of an interface as a contract that a class guarantees to implement. The concrete class that implements the interface is irrelevant. Don't know if this helps.

想一些代码可能会有所帮助.这并不能解释接口的所有内容，请继续阅读，但我希望这可以帮助您入门.这里的要点是你可以改变实现......

Figured some code might help. This doesn't explain everything there is more to interfaces, keep reading but I hope this gets you started. Point here is that you can vary the implementation...

package stack.overflow.example;

public interface IExampleService {
void callExpensiveService();
}

public class TestService implements IExampleService {

@Override
public void callExpensiveService() {
    // This is a mock service, we run this as many 
    // times as we like for free to test our software

}
}

public class ExpensiveService implements IExampleService {
@Override
public void callExpensiveService() {
    // This performs some really expensive service,
    // Ideally this will only happen in the field
    // We'd rather test as much of our software for
    // free if possible.
}
}


public class Main {

/**
 * @param args
 */
public static void main(String[] args) {

    // In a test program I might write
    IExampleService testService = new TestService();
    testService.callExpensiveService();

    // Alternatively, in a real program I might write
    IExampleService testService = new ExpensiveService();
    testService.callExpensiveService();

    // The difference above, is that we can vary the concrete 
    // class which is instantiated. In reality we can use a 
    // service locator, or use dependency injection to determine 
    // at runtime which class to implement.

    // So in the above example my testing can be done for free, but 
    // real world users would still be charged. Point is that the interface
    // provides a contract that we know will always be fulfilled, regardless 
    // of the implementation. 

}

}

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