<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
  android:orientation="vertical"
  android:layout_width="fill_parent"
  android:layout_height="fill_parent"
  >
<TextView android:id="@+id/tv"
  android:layout_width="fill_parent" 
  android:layout_height="wrap_content" 
  android:textColor="@android:color/white" 
  android:ellipsize="marquee" 
  android:focusable="true" 
  android:marqueeRepeatLimit="marquee_forever" 
  android:focusableInTouchMode="true" 
  android:scrollHorizontally="true"  
  android:text="Please input the text:"
  />
<EditText android:id="@+id/ET" 
  android:layout_width="match_parent" 
  android:layout_height="wrap_content"
  android:inputType="number"/>
</LinearLayout>

package com.android.text;

import android.app.Activity;
import android.os.Bundle;
import android.text.Editable;
import android.text.TextWatcher;
import android.util.Log;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;

public class TextWatcherDemo extends Activity {
  private TextView mTextView;
  private EditText mEditText;
  /** Called when the activity is first created. */
  @Override
  public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);
    mTextView = (TextView)findViewById(R.id.tv);
    mEditText = (EditText)findViewById(R.id.ET);
    mEditText.addTextChangedListener(mTextWatcher);
  }
  TextWatcher mTextWatcher = new TextWatcher() {
    private CharSequence temp;
    private int editStart ;
    private int editEnd ;
    @Override
    public void beforeTextChanged(CharSequence s, int arg1, int arg2,
        int arg3) {
      temp = s;
    }

    @Override
    public void onTextChanged(CharSequence s, int arg1, int arg2,
        int arg3) {
      mTextView.setText(s);
    }

    @Override
    public void afterTextChanged(Editable s) {
      editStart = mEditText.getSelectionStart();
      editEnd = mEditText.getSelectionEnd();
      if (temp.length() > 10) {
        Toast.makeText(TextWatcherDemo.this,
            "你輸入的字數已經超過了限制！", Toast.LENGTH_SHORT)
            .show();
        s.delete(editStart-1, editEnd);
        int tempSelection = editStart;
        mEditText.setText(s);
        mEditText.setSelection(tempSelection);
      }
    }
  };
}