you will find src.zip in your JAVA_HOME directory.
I am using eclipse.
In eclipse press ctrl and click on Integer word
in your code, it will open a new Integer.class and you can see attach source button, just click on that button and give it the location of your src.zip file.
source code will be available in your eclipse, if you click on Integer word with ctrl button pressed you can see source code of Integer.class. You can use ctrl+o (o for onion) to find equals() method.