Android TextWather object. How java anonymous class is working ?

Question

private TextWatcher billEditTextWatcher = new TextWatcher() 
   {
      // called when the user enters a number
      @Override
      public void onTextChanged(CharSequence s, int start, 
         int before, int count) 
      {         
         // convert billEditText's text to a double
         try
         {
            currentBillTotal = Double.parseDouble(s.toString());
         } // end try
         catch (NumberFormatException e)
         {
            currentBillTotal = 0.0; // default if an exception occurs
         } // end catch 

         // update the standard and custom tip EditTexts
         updateStandard(); // update the 10, 15 and 20% EditTexts
         updateCustom(); // update the custom tip EditTexts
      } // end method onTextChanged

      @Override
      public void afterTextChanged(Editable s) 
      {
      } // end method afterTextChanged

      @Override
      public void beforeTextChanged(CharSequence s, int start, int count,
         int after) 
      {
      } // end method beforeTextChanged
   }; // end billEditTextWatcher

This is a segment of a code from a tip calculator app written by a professional. Can someone explain how this works?

Usually I just write the following to create a new object.

TextWatcher billEditTextWatcher = new TextWatcher();

I understand what the private does. But how come there are methods in the creation of the new object? Is it basically doing what it says? overriding the original methods in the TextWatcher class?

I hope this question makes sense because I'm quiet confused.

Thanks in advance!

Answer 1

This is an example of an anonymous class in Java. You don't have any TextWatcher java file, and you declare the content of the class while initializing it.

http://docs.oracle.com/javase/tutorial/java/javaOO/anonymousclasses.html

Answer 2

As name is indicating TextWatcher will sense all event of EditText

Like when you are writing something in Editable area.

Its have following callback(Watching state.)

1. Key Pressed - beforeTextChanged();
2. Key Down - afterTextChanged();
3. Text Change - onTextChanged();

All the callback method contain their relative data which passed by event generator. like which is key is pressed , Unicode(ASCII) of Key etc.

Basically, a TextWatcher is used to keep watch on EditText or MultiLine EditText while entering data into it. We can perform operation and keep watch on which characters are being entered or how many characters are being entered in the EditText.

Technical Description:

1. If you want to use TextWatcher then you need to resister your EditText with TextWather object.

e.g.

EditText editTextPassword;  // Some EditText object.

TextWatcher billEditTextWatcher = new TextWatcher();  // TextWather object creation

editTextPassword.addTextChangedListener(billEditTextWatcher );  // EditText registation with Textwather object.

1. By default all call back of TextWather are empty that's that why you need yo give your definition of all callback according to your requirement.

   private TextWatcher billEditTextWatcher = new TextWatcher() 
    {
   
     @Override
     public void onTextChanged(CharSequence s, int start, int before, int count) 
     {         
   
         // you code is here while onTextChanged.
   
     } 
   
     @Override
     public void afterTextChanged(Editable s) 
     {
   
         you code is here while afterTextChanged.
   
     } 
   
     @Override
     public void beforeTextChanged(CharSequence s, int start, int count,
        int after) 
     {
   
        // you code is here while beforeTextChanged.
   
     } 
   };