public class ClassB {

  public void method_from_classB(){
    
    for(int i=0;i<10;i++)
      System.out.print("..."+i);
  }
}
public class ClassA {

   
  
  public static void main(String args[]){
   
    ClassB classB = new ClassB();
    
    classB.method_from_classB();
  }
}

public class ClassB implements ClassA.ClassAInterface{

  public ClassB(){
    
    new ClassA().RegisterInterface(this);
    System.out.println("...ClassB..."+this);
  }

  @Override
  public void method_from_interface() {

    for(int i=0;i<10;i++)
      System.out.print("..."+i);
  }
  
  /*  public void method_from_classB(){
  
  for(int i=0;i<10;i++)
    System.out.print("..."+i);
  }*/
}

public class ClassA {

  public static ClassAInterface classAInterface;
  
  public interface ClassAInterface{
    
    public void method_from_interface();
  }
  public void RegisterInterface(ClassAInterface a_interface){

    this.classAInterface = a_interface;
    System.out.println("...a_interface..."+a_interface);
  }
 
  public static void main(String args[]){
   
    ClassB classB = new ClassB();// 标记@1，最后面做解释
    //classB.method_from_classB();
    System.out.println("...classAInterface..."+classAInterface);
     if(classAInterface != null){
        classAInterface.method_from_interface();
      }
  }
}

  public void RegisterInterface(ClassAInterface a_interface){

    this.classAInterface = a_interface;
    System.out.println("...a_interface..."+a_interface);
  }

public ClassB(){
    
    new ClassA().RegisterInterface(this);
    System.out.println("...ClassB..."+this);
  }

System.out.println("...classAInterface..."+classAInterface);
     if(classAInterface != null){
        classAInterface.method_from_interface();
      }

    ClassB classB = new ClassB();  
    classB.method_from_classB();

    ClassB classB = new ClassB();  
    classB.method_from_classB();

public ClassB(){
    // 相当于回调事件的注册，初学者出现回调空指针很有可能这边忘记‘注册'了
    new ClassA().RegisterInterface(this);
  }

@Override
  protected void onCreate(Bundle savedInstanceState) {

     // 相当于回调事件的注册，初学者出现回调空指针很有可能这边忘记‘注册'了
    new ClassA().RegisterInterface(this);
}

button.setOnClickListener(new OnClickListener() {
      
      @Override
      public void onClick(View v) {
        // TODO 自动生成的方法存根
        if(classAInterface != null){
        classAInterface.method_from_interface();
      }
    });