MainActivity.java

public class MainActivity extends Activity implements OnClickListener {
    ...
        
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        ...
        button.setOnClickListener(this);
    }
    
    public void onClick(View v) {
        Toast.makeText(v.getContext(), "Click", Toast.LENGTH_SHORT).show();    
    }
}