Use the addTextChangedListener(TextWatcher watcher) method of EditText to monitor EditText. TextWatcher is an interface class, so the abstract method in TextWatcher must be implemented:

When the content in EditText changes, the TextChangedListener event will be triggered, and the abstract method in TextWatcher will be called.
Layout:

1. <?xml version="1.0" encoding="utf-8"?>  
2. <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"  
3.     android:orientation="vertical"  
4.     android:layout_width="fill_parent"  
5.     android:layout_height="fill_parent" >  
6. <EditText  
7.     android:id="@+id/main_et"   
8.     android:layout_width="fill_parent"     
9.     android:layout_height="wrap_content" />   
10. <TextView  
11.     android:id="@+id/main_tv"   
12.     android:layout_width="fill_parent"     
13.     android:layout_height="wrap_content" />   
14. </LinearLayout>  

1, Monitor the number of text boxes and prompt how many characters can be input
MainActivity:

1. package chay.mian;  
2.   
3. import android.app.Activity;  
4. import android.os.Bundle;  
5. import android.text.Editable;  
6. import android.text.TextWatcher;  
7. import android.view.View;  
8. import android.widget.EditText;  
9. import android.widget.TextView;  
10.   
11. public class MainActivity extends Activity {  
12.   
13.     private EditText editText;  
14.     private TextView tip;  
15.     private final int charMaxNum = 10; //Number of words allowed  
16.   
17.     @Override  
18.     protected void onCreate(Bundle savedInstanceState) {  
19.         super.onCreate(savedInstanceState);  
20.         setContentView(R.layout.activity_main);  
21.         init();  
22.     }  
23.   
24.     private void init() {  
25.         editText = (EditText) findViewById(R.id.main_et);  
26.         editText.addTextChangedListener(new EditChangedListener());  
27.           
28.         tip = (TextView) findViewById(R.id.main_tv);  
29.         tip.setText("0/"+charMaxNum);  
30.     }  
31.   
32.     class EditChangedListener implements TextWatcher {  
33.         private CharSequence temp; //Text before listening  
34.         private int editStart; //Cursor start position  
35.         private int editEnd; //Cursor end position  
36.           
37.         //Status before entering text  
38.         @Override  
39.         public void beforeTextChanged(CharSequence s, int start, int count,int after) {  
40.             temp = s;  
41.         }  
42.   
43.         //Enter the status in the text, count is the number of characters entered at one time  
44.         @Override  
45.         public void onTextChanged(CharSequence s, int start, int before, int count) {  
46. //          if (charMaxNum - s.length() <= 5) {  
47. //tip.setText("can also input" + (charMaxNum - s.length()) + "character");  
48. //          }  
49.             tip.setText((s.length()) + "/" + charMaxNum);  
50.         }  
51.   
52.         //Status after entering text  
53.         @Override  
54.         public void afterTextChanged(Editable s) {  
55.             /** Get the start and end position of the cursor, record the number index just exceeded after the maximum number for control*/  
56.             editStart = editText.getSelectionStart();  
57.             editEnd = editText.getSelectionEnd();  
58.             if (temp.length() > charMaxNum) {  
59. //Toast.makeText(getApplicationContext(), "input up to 10 characters", Toast.LENGTH_SHORT).show();  
60.                 s.delete(editStart - 1, editEnd);  
61.                 editText.setText(s);  
62.                 editText.setSelection(s.length());  
63.             }  
64.         }  
65.     };  
66. }  

2, For data type inspection, when the input is not an integer number, the input box will pop up immediately and prompt for correction
MainActivity:

1. package chay.mian;  
2.   
3. import android.app.Activity;  
4. import android.app.AlertDialog;  
5. import android.content.DialogInterface;  
6. import android.os.Bundle;  
7. import android.text.Editable;  
8. import android.text.TextWatcher;  
9. import android.util.Log;  
10. import android.widget.EditText;  
11. import android.widget.TextView;  
12.   
13. public class MainActivity extends Activity {  
14.   
15.     private EditText editText;  
16.     private TextView tip;  
17.     String str;  
18.   
19.     @Override  
20.     protected void onCreate(Bundle savedInstanceState) {  
21.         super.onCreate(savedInstanceState);  
22.         setContentView(R.layout.activity_mian);  
23.         init();  
24.     }  
25.   
26.     private void init() {  
27.         editText = (EditText) findViewById(R.id.mian_et);  
28.         editText.addTextChangedListener(new EditChangedListener());  
29.         tip = (TextView) findViewById(R.id.mian_tv);  
30.         tip.setText("Please enter an integer number");  
31.     }  
32.   
33.     class EditChangedListener implements TextWatcher {  
34.   
35.         //Status before entering text  
36.         @Override  
37.         public void beforeTextChanged(CharSequence s, int start, int count, int after) {  
38.             Log.d("TAG", "beforeTextChanged--------------->");  
39.         }  
40.   
41.         //Enter the status in the text, count is the number of characters entered at one time  
42.         @Override  
43.         public void onTextChanged(CharSequence s, int start, int before, int count) {  
44.             Log.d("TAG", "onTextChanged--------------->");  
45.         }  
46.   
47.         //Status after entering text  
48.         @Override  
49.         public void afterTextChanged(Editable s) {  
50.             Log.d("TAG", "afterTextChanged--------------->");  
51.             str = editText.getText().toString();  
52.             try {  
53. //              if ((editText.getText().toString()) != null)  
54.                     Integer.parseInt(str);  
55.             } catch (Exception e) {  
56.                 showDialog();  
57.             }  
58.         }  
59.     };  
60.   
61.     private void showDialog() {  
62.         AlertDialog dialog;  
63.         AlertDialog.Builder builder = new AlertDialog.Builder(this);  
64.         builder.setTitle("news").setIcon(android.R.drawable.stat_notify_error);  
65.         builder.setMessage("The integer number you output is wrong, please correct it");  
66.         builder.setPositiveButton("Determine", new DialogInterface.OnClickListener() {  
67.             @Override  
68.             public void onClick(DialogInterface dialog, int which) {  
69.                 // TODO Auto-generated method stub  
70.             }  
71.         });  
72.         dialog = builder.create();  
73.         dialog.show();  
74.     }  
75. }  

View the order in which these methods are called in LogCat:
beforeTextChanged-->onTextChanged-->onTextChanged

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