Android : setting event listener(onClickListener) for a button in android app development

Why do we create an event listener such as onClickListener for a button in the following way:-


        ...
        @Override
        public void onCreate(Bundle savedInstanceState)
       {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_quiz);
            mTrueButton = (Button)findViewById(R.id.true_button);

            mTrueButton.setOnClickListener(new View.OnClickListener() 
             {
                @Override     
                public void onClick(View v) 
                {  
                  // Does nothing yet, but soon!            
                }        });}

Why is mTrueButton using setOnClickListener method and not OnClickListener. Why this approach?