Java 8 and method references - specifically compareToIgnoreCase

Question

I read the Java 8 tutorial on Lambda Expressions and do not quite understand the Method Reference example for "Reference to an instance method of an arbitrary object of a particular type"

In the same tutorial there is an example "Reference to an Instance Method of a Particular Object" which look likes.

public int compareByName(Person a, Person b) {
        return a.getName().compareTo(b.getName());
}
Arrays.sort(rosterAsArray, myComparisonProvider::compareByName);

I can see this working because the method compareByName has the same signature as Comparator.compare, lambda (a, b) -> myComparisonProvider.compareByName(a, b) takes two arguments and calls a method with the same two arguments.

Now the "Reference to an instance method of an arbitrary object of a particular type" example uses String::compareToIgnoreCase

String[] stringArray = { "Barbara", "James", "Mary", "John",
    "Patricia", "Robert", "Michael", "Linda" };
Arrays.sort(stringArray, String::compareToIgnoreCase);

The signature for that method is int compareTo(String anotherString) and is different than Comparator.compare. The tutorial is not very clear but seem to imply you end up with a lambda such as (a, b) -> a.compareToIgnoreCase(b) I dont understand how the compiler decides what is acceptable for the second param of Arrays.sort I thought maybe it is smart enough to understand how to call that method, so I created an example.

public class LambdaTest {

    public static void main(String... args) {
        String[] stringArray = { "Barbara", "James", "Mary", "John", "Patricia", "Robert", "Michael", "Linda" };

        Arrays.sort(stringArray, String::compareToIgnoreCase);      // This works

        // using Static methods
        Arrays.sort(stringArray, FakeString::compare);              // This compiles
        Arrays.sort(stringArray, FakeString::compareToIgnoreCase);  // This does not

        // using Instance methods
        LambdaTest lt = new LambdaTest();
        FakeString2 fs2 = lt.new FakeString2();
        Arrays.sort(stringArray, fs2::compare);                 // This compiles
        Arrays.sort(stringArray, fs2::compareToIgnoreCase);     // This does not

        for(String name : stringArray){
            System.out.println(name);
        }
    }

    static class FakeString {
         public static int compareToIgnoreCase(String a) {
             return 0;
         }


        public static int compare(String a, String b) {
            return String.CASE_INSENSITIVE_ORDER.compare(a, b);
        }
    }

    class FakeString2 implements Comparator<String> {
         public int compareToIgnoreCase(String a) {
             return 0;
         }

        @Override
        public int compare(String a, String b) {
            return String.CASE_INSENSITIVE_ORDER.compare(a, b);
        }
   }
}

Can some one explain why the above two Arrays.sort don't compile even though they are using methods that are the same as String.compareToIgnoreCase method

Answer 1

This is the difference between a method reference on some object and a method reference on the object being processed.

First the Oracle examples

Lets look at this first case:

public int compareByName(Person a, Person b) {
        return a.getName().compareTo(b.getName());
}
Arrays.sort(rosterAsArray, myComparisonProvider::compareByName);

Here, the method compareByName is called on the passed in instance of myComparisonProvider with each pair of arguments in the sort algorithm.

So here, when comparing a and b we actually call:

final int c = myComparisonProvider.compareByName(a,b);

Now, in the second case:

String[] stringArray = { "Barbara", "James", "Mary", "John",
    "Patricia", "Robert", "Michael", "Linda" };
Arrays.sort(stringArray, String::compareToIgnoreCase);

You are sorting a String[] so the method compareToIgnoreCase is called on the String instance currently being sorted with the other String as an argument.

So here, when comparing a and b we actually call:

final int c = a.compareToIgnoreCase(b);

So these are two different cases:

• one where you pass in a method on an arbitrary object instance; and
• one where you pass in a method to be called on the instance being processed.

Onto your examples

Now in your first example, you also have a String[] and you try and sort it. So:

Arrays.sort(stringArray, FakeString::compare);

So here, when comparing a and b we actually call:

final int c = FakeString.compare(a, b);

The only difference is compare is static.

Arrays.sort(stringArray, FakeString::compareToIgnoreCase);

Now, the String[] is not a FakeString[] so we cannot call this method on String. Therefore we must be calling a static method on FakeString. But we cannot do this either, because we require a method (String, String) -> int but we only have (String) -> int - compilation error.

In the second example the problem is exactly the same, as you still have a String[]. And compareToIgnoreCase has the wrong signature.

TL;DR:

The point you are missing is that in the String::compareToIgnoreCase example; the method is called on the String currently being processed.