The source attachment does not contain the source for the file TextWatcher.class.
You can change the source attachment by clicking Change Attached Source below:

You have to add JDK src.zip path to dialog as on image below.

class file editor source not found Eclipse warning

Or go to Project > Properties > Java Build Path > Libraries
Expand JRE System Library. Expand rt.jar.
Select Source attachment and double click or Edit.
Type path the source code file (External File…) and press OK.

Java project build path

Or type path in Java JRE definition

Java JRE deifiniton path in Eclipse

How open String.class or others keywords definition with Eclipse.

Now if mouse move about keywords String or F3 on keywords and press button in yellow field will opened String.class