Why does String.replace not work?

String is immutable in java so you have to do

test =test.replace("KP", "");

Strings are immutable so you need to assign your test reference to the result of String.replace:

test = test.replace("KP", "");

You did not assign it to test. Strings are immutable.

test = test.replace("KP", "");

You need to assign it back to test.