btn_rigister.setOnClickListener(new View.OnClickListener() { 
  @Override 
  public void onClick(View view) { 
    // TODO log in 
  } 
}); 

public interface OnClickListener { 
    /** 
     * Called when a view has been clicked. 
     * 
     * @param v The view that was clicked. 
     */ 
    void onClick(View v); 
  } 

public boolean callOnClick() { 
    ListenerInfo li = mListenerInfo; 
    if (li != null && li.mOnClickListener != null) { 
      li.mOnClickListener.onClick(this); 
      return true; 
    } 
    return false; 
  } 

public interface Callback{  
    public abstract void work();  
  }  

private Callback mCallback;  

public void setCallback(Callback callback) {  
    this.mCallback = callback;  
  }  

public void doWork() {  
    mCallback.work();  
  }  

public class Employee {  
  /* 
   * 定义回调接口的成员变量 
   */  
  private Callback mCallback;  
  /* 
   * 声明回调接口 
   */  
  public interface Callback{  
    public abstract void work();  
  }  
  /* 
   * 设置回调接口对象成员变量 
   */  
  public void setCallback(Callback callback) {  
    this.mCallback = callback;  
  }  
  /* 
   * 调用回调接口对象中的方法 
   */  
  public void doWork() {  
    mCallback.work();  
  }  
}  

public class Boss {  
  private Employee employee;  
  /* 
   * 为Employee设置回调函数, 在这里定义具体的回调方法 
   */  
  public void setCallback() {  
    employee.setCallback(new Employee.Callback() {  
      @Override  
      public void work() {  
        System.out.println("work");  
      }  
    });  
  }  
}