Ques: What is TextWatcher ?

TextWatcher

Basically, a TextWatcher is used to keep watch on 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.

For Ex. We can use TextWatcher to have checks on the password field that minimum number of characters are entered or not, or We can check the password Strength depending on number of characters entered in Password field .

TextWatcher class has 3 methods and we need override these methods to implement our own functionality. 

1: public abstract void afterTextChanged (Editable s)

This method is called to notify you that, somewhere within s, the text has been changed. It is legitimate to make further changes to s from this callback, but be careful not to get yourself into an infinite loop, because any changes you make will cause this method to be called again recursively. (You are not told where the change took place because other afterTextChanged() methods may already have made other changes and invalidated the offsets. But if you need to know here, you can use setSpan(Object, int, int, int) in onTextChanged(CharSequence, int, int, int) to mark your place and then look up from here where the span ended up.

2: public abstract void beforeTextChanged (CharSequence s, int start, int count, int after)

This method is called to notify you that, within s, the count characters beginning at start are about to be replaced by new text with length after. It is an error to attempt to make changes to s from this callback.

3: public abstract void onTextChanged (CharSequence s, int start, int before, int count)

This method is called to notify you that, within s, the count characters beginning at start have just replaced old text that had length before. It is an error to attempt to make changes to s from this callback.