android源代码提示文本框还能输入多少个字符

public class TestAndroidActivity extends Activity {
    /** Called when the activity is first created. */

     Button clearbtn;
     EditText et;
     TextView tv;
     final int MAX_LENGTH = 50;
     int Rest_Length = MAX_LENGTH;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        //tv =(TextView)findViewById(R.);
    tv=(TextView) this.findViewById(R.id.btn);
        //tv=this.findViewById(R.)
        et = (EditText) this.findViewById(R.id.bg);

        et.addTextChangedListener(new TextWatcher() {

            public void onTextChanged(CharSequence s, int start, int before, int count) {
                if(Rest_Length>0){
                    Rest_Length = MAX_LENGTH - et.getText().length();
                }
            }

            public void beforeTextChanged(CharSequence s, int start, int count,
                    int after) {
                tv.setText("还可以输入"+Rest_Length+"个字");

            }

            public void afterTextChanged(Editable s) {
                tv.setText("还可以输入"+Rest_Length+"个字");
            }
        });

        clearbtn = (Button)findViewById(R.id.btn);
        clearbtn.setOnClickListener(new Button.OnClickListener() {

            public void onClick(View v) {
                et.setText("");
                Rest_Length = MAX_LENGTH;
            }
        });
    }
}