Is a function callback in Java also an anonymous class?

Question

For example in this code snippet:

c.addMouseListener(new MouseAdapter() {
public void mouseEntered(MouseEvent e) {
    System.out.println("Mouse entry event: " + e
                       + " on button: " + buttonLabel);
  }
});

Is the code block inside the braces of addMouseListener declaring an anonymous class of MouseAdapter() and then the whole thing is called a 'function callback', or am I mixing up the terms? What is the difference?

Answer 1

No.

You can implement callback in several ways in Java, anonymous class is a common way, especially before Java 8.

In OO , callback is relative to a principle called Hollywood Principle: "Don't call us, we'll call you". Check here for more information about callback in java.

In Java, we only have class/object, so, callbacks will always be implemented using objects in Java. Thus, I don't think we should call it "function callback" in Java, instead, callback object may be more appropriate.

We pass a callback object to another object, so that that object can call some specific method on the callback object when that object thinks it is time. In your case, you pass a MouseAdapter instance to your c. And c will call the method mouseEntered of the passed MouseAdapter instance when mouse is entered(c thinks it is time). So, we don't explicitly call c'method relative to mouse entered like:

while(true){
    if(mouce_entered){
         mouseEntered();
     }
    wait_for_mouseentered_event();
}

Instead, c does all this job, we only need to pass a callback object to him and let c control the logic flow.

1.So, actually, the first way to implement this callback is you declare a class extends MouseAdapter

public class YourMouseListener extends MouseAdapter() 
{
    public void mouseEntered(MouseEvent e) { ... } 
} 

then create an instance and pass to it:

  c.addMouseListener(new YourMouseListener());

2.To minimize code and class declared, Java enable you to use anonymous class to achieve callback like:

c.addMouseListener(new MouseAdapter() {
public void mouseEntered(MouseEvent e) {
    System.out.println("Mouse entry event: " + e
                       + " on button: " + buttonLabel);
  }
});

3.In Java 8, we have lambda expression, we can replace some anonymous class into a more concise lambada expression, but this is limited on function interface,which only have one method.

//Old way:
button.addActionListener(new ActionListener() {
    @Override
    public void actionPerformed(ActionEvent e) {
        System.out.println("The button was clicked using old fashion code!");
    }
});

//New way:
button.addActionListener( (e) -> {
        System.out.println("The button was clicked. From lambda expressions !");
});