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; 
            } 
        }); 
    } 
}