1. New OnClickListener in  setOnClickListener’s parameter

public class MainActivity extends Activity {

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

        Button b = (Button) findViewById(R.id.button);
        b.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Log.d("BG","1st way handling button click");
            }
        });

    }
}

2. Inner class OnClickListener

public class MainActivity extends Activity {

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

        Button b = (Button) findViewById(R.id.button);
        b.setOnClickListener(buttonListener);

    }

    private View.OnClickListener buttonListener = new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Log.d("BG","2nd way handling button click");
        }
    };
}

3. Implements OnClickListener interface & override onClick()

public class MainActivity extends Activity implements View.OnClickListener {

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

        Button b = (Button) findViewById(R.id.button);
        b.setOnClickListener(this);

    }
    @Override
    public void onClick(View view) {
        switch(view.getId()){
            case R.id.button:
                Log.d("BG","3rd way handling button click");
                break;
        }
    }
}

4. Assign method at onClick attribute in layout file.

Activity class

public class MainActivity extends Activity implements View.OnClickListener {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }
    public void buttonClick(View view) {
        Log.d("BG","4th way handling button click");
    }
}

layout file

<Button
    android:text="Button"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:id="@+id/button"
    android:onClick="buttonClick" />