Why this lambda can be used here where expects a comparator in a Arrays.sort()

Question

I am new about Lambda, here is what the textbook (java in nutshell) says:

Only certain types are eligible to be the target of a lambda . Target types are also called functional interfaces and they must:

Be interfaces

Have only one nondefault method(but may have other methods that are default)

and the book gives me an example :

File dir =new File("/src");
String[] fileList =dir.list((d,fName)->fName.endWith(".java"))

I understand this lambda will convert into a FilenameFilter interface which is an @FunctionalInterface It does match the description in the book

However I can use lambda here to when the sort() is expecting to receive a Comparator

        int n =12;
        Function<String,String[]> f = (s) -> {
            String[] ans = new String[n];();
            for (int i = 0; i < n; i++) {
                ans[i] = s;
            }
            return ans;};
        Arrays.sort(f.apply("colin"),(String s1 ,String s2)->{
            return s1.length() - s2.length();});

I look over its source code to find Comparator is a @FunctionInterface

but it have two nondefault methods compare and equals.It is different from what the book says.

I've taken the liberty of removing the second request at the bottom of your (otherwise well-put) question, because SO is not well suited for handling multiple questions in one. Also, I'm afraid the kind of request you posted there isn't particularly well-suited for SO as a whole (general "how do I approach learning X" type guidance doesn't fit neatly into the Questions-and-Answers model it SO is built on). — Joachim Sauer, Apr 13 '21 at 15:31

Joachim Sauer · Accepted Answer · 2021-04-13T15:31:58.077

equals is a public method of java.lang.Object. Those methods are explicitly not counted when identifying functional interfaces.

The real, autoritative definition of what a functional interface is (and what can therefore be "implemented" via a lambda) is found in the Java Langauge Specification.

Almost by necessity the definition of what a functional interface is in a guide book will be less strict and not as precise as the one in the Java Language Specification (because the later is written in a language that's not well-suited for initial learning of a concept and much better for creating multiple compatible implementations of the same thing).

See JLS §9.8 Functional Interfaces (emphasis mine):

For an interface I, let M be the set of abstract methods that are members of I that do not have the same signature as any public instance method of the class Object. [...]

It's worth noting that the reason that this method shows up in the interface's JavaDoc at all (as opposed to, say, [Iterator](https://docs.oracle.com/javase/8/docs/api/java/util/Iterator.html) where it does not) is because Comparator declares it explicitly, for the purpose of adding a more specific JavaDoc to it than the one provided by Object. — Michael, Apr 13 '21 at 15:35

Why this lambda can be used here where expects a comparator in a Arrays.sort()

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