我下了一个记账程序要学习一下，发现看不懂代码啊，哪位高手给我瞧瞧，这个keyPad类是什么作用
代码如下：
public class KeyPad extends Activity implements OnClickListener {
private Button btnDisplay = null;
private Button btnOne = null;
private Button btnTwo = null;
private Button btnThree = null;
private Button btnFour = null;
private Button btnFive = null;
private Button btnSix = null;
private Button btnSeven = null;
private Button btnEight = null;
private Button btnNine = null;
private Button btnZero = null;
private Button btnDot = null;
private Button btnDelete = null;
private Button btnCancel = null;
private Button btnClean = null;
private Button btnDone = null;
private String value = "0";
private boolean isValueEmpty = false;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.widget_digit_keypad);
btnDisplay = (Button) findViewById(R.id.display);
btnOne = (Button) findViewById(R.id.one);
btnTwo = (Button) findViewById(R.id.two);
btnThree = (Button) findViewById(R.id.three);
btnFour = (Button) findViewById(R.id.four);
btnFive = (Button) findViewById(R.id.five);
btnSix = (Button) findViewById(R.id.six);
btnSeven = (Button) findViewById(R.id.seven);
btnEight = (Button) findViewById(R.id.eight);
btnNine = (Button) findViewById(R.id.nine);
btnZero = (Button) findViewById(R.id.zero);
btnDot = (Button) findViewById(R.id.dot);
btnDelete = (Button) findViewById(R.id.delete);
btnCancel = (Button) findViewById(R.id.cancel);
btnClean = (Button) findViewById(R.id.clean);
btnDone = (Button) findViewById(R.id.done);
btnDisplay.setOnClickListener(this);
btnOne.setOnClickListener(this);
btnTwo.setOnClickListener(this);
btnThree.setOnClickListener(this);
btnFour.setOnClickListener(this);
btnFive.setOnClickListener(this);
btnSix.setOnClickListener(this);
btnSeven.setOnClickListener(this);
btnEight.setOnClickListener(this);
btnNine.setOnClickListener(this);
btnZero.setOnClickListener(this);
btnDot.setOnClickListener(this);
btnDelete.setOnClickListener(this);
btnCancel.setOnClickListener(this);
btnClean.setOnClickListener(this);
btnDone.setOnClickListener(this);
if(this.getIntent().hasExtra("value"))
{
value=this.getIntent().getStringExtra("value");
}
if(value==null || value.equals(""))
{
value="0";
isValueEmpty = true;
}
btnDisplay.setText(value);
}
/*
* (non-Javadoc)
*
* @see android.view.View.OnClickListener#onClick(android.view.View)
*/
@Override
public void onClick(View v) {
// TODO Auto-generated method stub
switch (v.getId()) {
case R.id.display:
break;
case R.id.delete:
if (value.length() > 1) {
value = value.substring(0, value.length() - 1);