Java中的接口 - 它们用于什么？

Question

可能重复:
接口的目的仍在继续

我刚刚开始学习Java.
我遇到过Interfaces哪些我知道如何使用,但仍然无法完全掌握它的想法.
据我所知,它interfaces通常由类实现,然后必须实现接口中声明的方法.
问题是 - 究竟是什么意思？从接口实现方法作为普通类方法不是更容易吗？使用接口的优势究竟是什么？

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.我也读过这个问题及其答案,但整个合同的事情让它变得更加复杂......

希望有人可以简化它!:)
提前谢谢!

Answer 1

接口允许您在运行时提供不同的实现,注入依赖关系,单独关注,使用不同的实现进行测试.

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

想一些代码可能有所帮助.这并没有解释接口上有更多的东西,继续阅读,但我希望这能让你开始.这里要点是你可以改变实施......

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. 

}

}

Answer 2

接口可用于许多事情.大多数公共使用的是多态和依赖注入,您可以在运行时更改依赖项.例如,假设您有一个名为Database的接口,它有一个名为的方法getField(...).

public interface Database 
{
    public void getField(String field);
}

现在假设您在应用程序中使用了两个数据库,具体取决于您的客户端:MySQL和PostgreSQL.你将有两个具体的课程:

public class MySQL implements Database
{
    // mysql connection specific code
    @Override
    public void getField(String field)
    {
        // retrieve value from MySQL
    }
}

和...

public class PostgreSQL implements Database
{
    // postgre connection specific code
    @Override
    public String getField(String field)
    {
        // retrieve value from Postgre
    }
}

现在,根据您的客户端偏好,您可以在main中实例化MySQL或PostgreSQL类

 public static void main(String args[])
 {
     if (args[2].equals("mysql"))
         Database db = new MySQL();
     else
         Database db = new PostgreSQL();
 }

 //now get the field
 String foo = db.getField();

或者您可以使用JS或Ruby的Factory/Abstract Factory或脚本引擎.