Android setOnClickListener method - How does it work?

Question

I have trouble understanding this code. I get that findViewById will get the button widget and then it'll cast it. Then, it's going to use the button to call the setOnClickListener method. However, I don't know what is that argument being passed into the setOnClickListener and I have never seen code like that before. How is it that it creates a new object but is able to create a method of its own within another method's argument? Would be great if someone could explain that. Also, what type of object is the setOnClickListener method taking in?

btn = (Button)findViewById(R.id.firstButton);
btn.setOnClickListener(new View.OnClickListener()
{
    @Override
    public void onClick(View v)
    {
        tv.setText(months[rand.nextInt(12)]);
        tv.setTextColor(Color.rgb(rand.nextInt(255)+1, rand.nextInt(255)+1, rand.nextInt(255)+1));
    }
});

Solution 1

It works like this. View.OnClickListenere is defined -

public interface OnClickListener {
    void onClick(View v);
}

As far as we know you cannot instantiate an object OnClickListener, as it doesn't have a method implemented. So there are two ways you can go by - you can implement this interface which will override onClick method like this:

public class MyListener implements View.OnClickListener {
    @Override
    public void onClick (View v) {
         // your code here;
    }
}

But it's tedious to do it each time as you want to set a click listener. So in order to avoid this you can provide the implementation for the method on spot, just like in an example you gave.

setOnClickListener takes View.OnClickListener as its parameter.

Solution 2

This is the best way to implement Onclicklistener for many buttons in a row implement View.onclicklistener.

public class MainActivity extends AppCompatActivity implements View.OnClickListener {

This is a button in the MainActivity

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    bt_submit = (Button) findViewById(R.id.submit);

    bt_submit.setOnClickListener(this);
}

This is an override method

    @Override
    public void onClick(View view) {
        switch (view.getId()){
            case R.id.submit:
                //action

                break;

            case R.id.secondbutton:
                //action
                break;
        }
    }

Comments

[rayleigh]

Awesome explanation!! Makes perfect sense now and learned something new!! I never knew you can instantiate an object of an interface by implementing its method on the spot. Very cool.

[YTerle]

If you want to know more read about Observer Design Pattern :)

[Maccesch]

This is not an answer to the question asked.