Unable to find a method belonging to an object

Question

I believe I am making a really simple mistake/ overlooking something trivial.

import java.util.Comparator;

public class NaturalComparator {

            


        

Answer 1

You accidentally created a generic type parameter Integer which has nothing to do with the Integer class, and you didn't implement Comparator. Try

public class NaturalComparator implements Comparator<Integer>

Placing an identifier in <> on the class declaration declares a generic type parameter, but placing an identifier in <> on the implements/extends clause is a passing a type argument.

Answer 2

You are shadowing the type java.lang.Integer with a type parameter variable you decided to name Integer.

Your code is equivalent to

public class NaturalComparator<T> {

    public int compare(T o1, T o2) {
        return o1.intValue() - o2.intValue();
    }
}

Which obviously doesn't compile since Object (the bound of T) doesn't declare a intValue() method.

What you want is

public class NaturalComparator implements Comparator<Integer> {

    @Override
    public int compare(Integer o1, Integer o2) {
        return o1.intValue() - o2.intValue();
    }
}

in which case java.lang.Integer is used as a type argument.