Keywords are words that have already been defined for Java compiler. They have special meaning for the compiler. Java Keywords must be in your information because you can not use them as a variable, class or a method name.
Java Reserved Keywords List
You can't use keyword as identifier in your Java programs, its reserved words in Java library and used to perform an internal operation.
true, false and null are not reserved words but cannot be used as identifiers, because it is literals of built-in types.