Inline comparator vs custom comparator in Java

Question

When sorting a list, is there any performance difference between using a java Comparator in-line (with an anonymous inner class) vs implementing a separate custom Comparator class?

1.

public class SortByErrorComparator implements Comparator<WorkflowError> {
    public int compare(WorkflowError obj1, WorkflowError obj2) {
        return obj1.getErrorCode().compareTo(obj2.getErrorCode());
    }
}
Collections.sort(list, new SortByErrorComparator()) ;

2.

Collections.sort(list, new Comparator<WorkflowError>() {
    public int compare(WorkflowError obj1, WorkflowError obj2) {
        return obj1.getErrorCode().compareTo(obj2.getErrorCode());
    }
});

Also, when will the compare() method be invoked?

Answer 1

There's also option 3 - a lambda Function:

Collections.sort(list, (a, b) -> a.getErrorCode().compareTo(b.getErrorCode()));

which should be about 2 x faster, according to this benchmark data.

... or (thanks to @JB Nizet) option 4:

list.sort(Comparator.comparing(WorkflowError::getErrorCode))

Answer 2

I made a little test and found no difference (just in some small run the inline comparator shows a slightly better performace). This is the code used to make the test:

public class ComparatorTest {

    private static final int MAX = 1000000;
    private static final int RUN = 10000;

    public static void main(String[] args) {

        List<A> list = new ArrayList<A>();

        long externalComparatorClassTotalTime = 0;
        long inlineCompartorTotalTime = 0;

        for (int i = RUN; i > 0; i--) {
            init(list);
            externalComparatorClassTotalTime += externalComparatorClassTest(list);
            init(list);
            inlineCompartorTotalTime += inlineCompartorTest(list);
        }

        System.out.format("List with %d elements and %d runs%n", MAX, RUN);
        System.out.println("external Comparator class average millis: " + externalComparatorClassTotalTime / RUN);
        System.out.println("inline Comparator class average millis: " + inlineCompartorTotalTime / RUN);
    }

    private static void init(List<A> list) {
        list.clear();
        for (int i = MAX; i > 0; i--) {
            list.add(new A(i));
        }
    }

    private static long inlineCompartorTest(List<A> secondList) {
        long start = System.currentTimeMillis();

        Collections.sort(secondList, new Comparator<A>() {
                public int compare(A obj1, A obj2) {
                    return obj1.getVal().compareTo(obj2.getVal());
                }
        });

        return System.currentTimeMillis() - start;
    }

    private static long externalComparatorClassTest(List<A> firstList) {
        long start = System.currentTimeMillis();

        Collections.sort(firstList, new MyComparatorOne());

        return System.currentTimeMillis() - start;
    }
}

Comparator class:

public class MyComparatorOne implements Comparator<A> {
    public int compare(A obj1, A obj2) {
        return obj1.getVal().compareTo(obj2.getVal());
    }
}

and the output is:

List with 1000000 elements and 10000 runs
external Comparator class average millis: 3
inline Comparator class average millis: 3

If you have several invocation to the comparator keeping an instance of it would be helpful