Android- Logic behind setOnClickListener

Question

I am new to Java/Android programming and unfortunately I don't fully under stand the logic behind this piece of code:

         button.setOnClickListener(new View.OnClickListener() {
         public void onClick(View v) {
             // Perform action on click
         }
     }); 

I have alredy read a whole lot of tutorials about it but unfortunately in no one so far the code gets explained in detail. Maybe because it's so basic that every decent object oriented programmer would understand the reason for the structure right away.

I am wondering why do I need to use new View.OnClickListener()as a parameter? for the setOnClickListener method. In other words why doesn't something like

         button.setOnClickListener(
         public void onClick(View v) {
             // Perform action on click
     });  

this work?

Besides that I am not quite sure why the onClick method requires the paramterer of View v.

I would be quite thankful for some help since I am currently rather puzzled.

Answer 1

View.OnClickListener is an interface which you need to implement when you want to handle click events. In your code, you are doing that by doing new View.OnClickListener(). Here you are actually creating an anonymous class that implements View.OnClickListener. And any class that implements View.OnClickListener must also implement all the methods declared in it (e.g. the onClick method). Also, in public void onClick(View v) {..}, the View v denotes the view or the button that was clicked so that you can perform whatever you want with it. For example, get it's id, change it's color etc.

Answer 2

Imagine the case where it would be like what you defined - you do not pass in a parameter and simply describe the actions to be taken when that button is pressed. It will work perfectly but for that button only. You will have to redo this method for every other button. What in the case where there are quite a few buttons that do the same thing? You will end up redefining it every time. Wouldn't it be easier to simply just do it once and then pass that in? Also wouldn't it be easier to simply just have one onClick() method which identifies the id of the element being clicked and then performs the logic just for the widget with that id rather than having laborious separate lines of code?

Also don't forget that the parameter being passed belongs to the View class which is the superclass of every other widget. So you can pass the onClickListener() method defined for other widgets into this method if you want (though this case is practically rare).

To answer your question, the Android team could have designed it to be the way you described but chose to do it this way due to best practices and programming paradignms.

Answer 3

Ok so first of all onClick method wants a parameter as a function. The thing is in Java you can't send a function as a parameter as you can do in other languages (like C++). The Java solution is to use a Functional Interface.

In this case OnClickListener is an interface that have one method you must implement : public void onClick(View v). View v it's the view you just clicked so the method it's called.

button.setOnClickListener(OnClickListener) means the set... function needs an argument that implements the OnClickListener.

So that what this code does :

new View.OnClickListener() {
         public void onClick(View v) {
             // Perform action on click
         }
     }); 

It is creating an anonymous class that implements the OnClickListener interface required.