Java的方法参考

Question

According to Oracle Documentation , the String::compareToIgnoreCase is also a valid method reference, my question is that compareToIgnoreCase is not a static method, in other words, compareToIgnoreCase must be attached to a specific String instance. So how does JDK know which instance of String I refer when I use String::compareToIgnoreCase ?

Answer 1

Consider the following example using toUpperCase which is also an instance method.

It works in this case because the Stream item that is being handled is of the same type as the class of the method being invoked. So the item actually invokes the method directly.

So for

Stream.of("abcde").map(String::toUpperCase).forEach(System.out::println);

the String::toUpperCase call will be the same as "abcde".toUpperCase()

If you did something like this:

Stream.of("abcde").map(OtherClass::toUpperCase).forEach(System.out::println);

"abcde" is not a type of OtherClass so the OtherClass would need to look like the following for the stream to work.

class OtherClass {
    public static String toUpperCase(String s) {
       return s.toUpperCase();
    }
} 

Answer 2

String::compareToIgnoreCase is not used such as str1.compareToIgnoreCase(str2) would be. It actually is used as a comparator. Eg you could compare it to

Arrays.sort(someIntegerArray, Collections.reverseOrder()) 

but in this case it would be

Arrays.sort(someStringArray, String::compareToIgnoreCase) 

Answer 3

It is like there is an additional parameter, the actual instance, involved.

Example for String::compareToIgnoreCase :

ToIntBiFunction<String, String> func = String::compareToIgnoreCase;
int result = func.applyAsInt("ab", "cd");    // some negative number expected

We get a ToIntBiFunction - a two parameter function returning int - since the result is an int , the first parameter correspond to this of compareToIgnoreCase and the second function parameter is the parameter passed to compareToIgnoreCase .

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maybe a bit easier:

ToIntFunction<String> f = String::length;  // function accepting String, returning int
int length = f.applyAsInt("abc");          // 3

length does not accept any parameter, but the first argument of the function is used as the instance length is called on

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The examples above are very abstract, just to show the types involved. The functions are mostly used directly in some method call, no need to use an intermediate variable