public class PracticalTest01MainActivity extends Activity {
  private ButtonClickListener buttonClickListener = new ButtonClickListener();

  private class ButtonClickListener implements Button.OnClickListener {
         
    @Override
    public void onClick(View view) {
      EditText leftEditText = (EditText)PracticalTest01MainActivity.this.findViewById(R.id.left_edit_text);
      EditText rightEditText = (EditText)PracticalTest01MainActivity.this.findViewById(R.id.right_edit_text);
      int leftButtonClickedNumber = Integer.parseInt(leftEditText.getText().toString());
      int rightButtonClickedNumber = Integer.parseInt(rightEditText.getText().toString());
    
      switch(view.getId()) {
        case R.id.left_button:
          leftButtonClickedNumber++;
          leftEditText.setText(String.valueOf(leftButtonClickedNumber));
          break;
        case R.id.right_button:
          rightButtonClickedNumber++;
          rightEditText.setText(String.valueOf(rightButtonClickedNumber));
          break;
      }
    }
  } 
    
  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    
    // ...

    Button leftButton = (Button)findViewById(R.id.left_button);
    leftButton.setOnClickListener(buttonClickListener);
    Button rightButton = (Button)findViewById(R.id.right_button);
    rightButton.setOnClickListener(buttonClickListener);       
  }
  
  // ...
  
}