Ordering for key value pairs

Question

I have the following two classes:

class KeyClass {
    private prop1;
    private prop2;

    hashcode() {
    //implemented properly
    }

    equals() {
    //implemented properly
    }
}

class ValueClass {
    private prop1;
    private prop2;

    hashcode() {
    //implemented properly
    }

    equals() {
    //implemented properly
    }
}

I am trying to find out the max pair from a map where objects of these classes are key and value pairs respectively. I also have an com.google.common.collect.Ordering<ValueClass> which uses multiple comparators. I can easily find out the max of values using this ordering, but what I am interested into is the key of the max value.

I can write a certain implementation wherein I can keep track of my key w.r.t the value in a loop and use the ordering to compare values (similar to the conventional way of finding a max value) but I want to know if we already have such case handled by Guava or any other library?

Answer 1

You say guava or any other library and that's straightforward with Java 8 streams. If your Ordering<ValueClass> instance is called ordering:

Entry<KeyClass, ValueClass> maxEntry = map.entrySet().stream()
        .max(Comparator.comparing(Entry::getValue, ordering))
        .orElse(null);

Add .map(Entry::getKey) before orElse to get just the key.

The above is possible since guava's Ordering implements java.util.Comparator so you can pass it as argument to comparing.