回调方法是什么及其理解

回调的思想

1. 类A的a()方法调用类B的b()方法
2. 类B的b()方法执行完毕主动调用类A的callback()方法

代码分析

public interface Callback {
    public void tellAnswer(int answer);
}

public class Teacher implements Callback {
    private Student student;
    
    public Teacher(Student student) {
        this.student = student;
    }
    
    public void askQuestion() {
        student.resolveQuestion(this);
    }
    
    @Override
    public void tellAnswer(int answer) {
        System.out.println("知道了，你的答案是" + answer);
    }
}

public interface Student {
    public void resolveQuestion(Callback callback);
}

public class Ricky implements Student {
    @Override
    public voidresolveQuestion(Callback callback) {
        // 模拟解决问题
        try {
            Thread.sleep(3000);
        } catch (InterruptedException e) {
            
        }
        // 回调，告诉老师作业写了多久
        callback.tellAnswer(3);
    }
}

测试

@Test
public void testCallBack() {
    Student student = new Ricky();
    Teacher teacher = new Teacher(student);
    
    teacher.askQuestion();
}

Student也可以使用匿名类定义，更加简洁：

@Test
public void testCallBack2() {
    Teacher teacher = new Teacher(new Student() {
        
        @Override
        public void resolveQuestion(Callback callback) {
            callback.tellAnswer(3);
        }
    });
    
    teacher.askQuestion();
}

分析

　　Teacher 中，有一个解决问题的对象：Student，在Student中解决问题之后，再通过引用调用Teacher中的tellAnswer接口，所以叫回调。

同步、异步调用

上面的例子是同步回调，下面介绍异步调用

public interface Callback {
    void tellAnswer(int answer);
}

public class Teacher implements Callback{
    private Student student;
    public Teacher (Student student) {
        this.student = student;
    }

    public void askQuestion() {
        //student.resolveQuestion(this); //此处是同步回调。
        new Thread(()-> student.resolveQuestion(this)).start();  //这里实现了异步，此处的this也可以用Teacher.this代替，
                                                                // 如果不用lambda表达式，用匿名内部类创建new Runnable()则一定要用Teacher.this
    }


    @Override
    public void tellAnswer(int answer) {
        System.out.println("你的答案是:" + answer + "正确");
    }
}

public interface Student {
    public void resolveQuestion(Callback callback);
}

public class Ricky implements Student {

    @Override
    public void resolveQuestion(Callback callback) {
        try {
            Thread.sleep(3000);
        } catch (Exception e) {
            e.printStackTrace();
        }
        //回调，告诉老师问题的答案
        callback.tellAnswer(3);
    }
}

测试

public class CallbackTest {
    @Test
    public void testCallback() {
        Student student = new Ricky();
        Teacher teacher = new Teacher(student);

        teacher.askQuestion();
        System.out.println("end");
        try {
            Thread.sleep(5000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

}

出处：　　

　　https://cloud.tencent.com/developer/article/1351239

　　https://blog.csdn.net/qq_31617121/article/details/80861692