Java中的匿名内部类及其语法

Question

I'm very new to Java and I'm attempting to write an android app and going through the facebook api. I'm looking at this page , specifically, this

    LoginManager.getInstance().registerCallback(callbackManager,
        new FacebookCallback<LoginResult>() {
            @Override
            public void onSuccess(LoginResult loginResult) {
                // App code
            }

            @Override
            public void onCancel() {
                 // App code
            }

            @Override
            public void onError(FacebookException exception) {
                 // App code   
            }
});

This work just fine, I'm just trying to understand how. I'm not understanding the method in a method and more specifically, how would I extract this if I wanted to write it differently?

I tried creating another class

public class FaceBookCallbackLogin implements FacebookCallback<LoginResult> {



@Override
public void onSuccess(LoginResult loginResult) {
    ....
}

@Override
public void onCancel() {

}

and tried to initiate the login from my MainActivity

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    callbackManager = CallbackManager.Factory.create();
    setContentView(R.layout.activity_main);
    info = (TextView)findViewById(R.id.info);
    loginButton = (LoginButton)findViewById(R.id.login_button);
    loginButton.setReadPermissions("email");

    loginButton.registerCallback(callbackManager, new FaceBookCallbackLogin());
}

but, this never calls the onSuccess method in FaceBookCallbackLogin, and I can't call it explicitly because I don't have the param LoginResult.

So, how would i write this so I can have the login in a different class? In the original code, where is the param for onSuccess - LoginResult - coming from?

Answer 1

The way you have changed from anonymous inner class to separate class looks just fine.

However, in first example you have done this:

LoginManager.getInstance().registerCallback

In second example you have done this:

loginButton.registerCallback

Maybe that is the problem..

Answer 2

Okay, So the Javadocs say

"Anonymous classes enable you to make your code more concise. They enable you to declare and instantiate a class at the same time. They are like local classes except that they do not have a name. Use them if you need to use a local class only once".

A benefit of the above is it allows you to be precise about your definition for a particular interface implementation without having to go through the hassle of creating new classes for each implementation. Look at the example below.

interface Foo {
    String saySomething(String something);
}

class Bar {

  public static void main(String [] args) {
      //I want this anonymous inner classes implementation to **uppercase** the strings
      Foo sayUppercase = new Foo() {
          @Override
          public String saySomething(String something) {
              return something.toUpperCase();
          }
      };
      sayUppercase.saySomething("Java");
      //output: JAVA

      Foo sayLowerCase = new Foo() {
          //I want this anonymous inner classes implementation to **lowercase** the strings
          @Override
          public String saySomething(String something) {
              return something.toLowerCase();
          }
       };
       sayLowerCase.saySomething("Java");
      //output: java
  }  
}

Notice how I am implementing the same saySomething method differently without having to create two separate classes (class files) for each implementation. I am almost inlining the implementation of the class.

From an Android perspective this is very useful, imagine having to create a separate class for your click listeners. It is still possible, however it is much more simple and readable to create a simple anonymous inner-class for your implementation of your click-listener

mLoginButton.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            //Perform Action
        }
    });

Is much simpler to read and understand than creating a new class that implements View.OnClickListener like below

public class LoginButtonOnClickListener implements View.OnClickListener {

    @Override
    public void onClick(View view) {
        //Perform action
    }
}