Java 8 function interface assignment context

The question is about the allocation background of the functional interface –

Predicate<String> p = String::isEmpty;

It works normally when the isempty method declaration in the string class is – public Boolean isempty() {}

If I try to declare the same in a custom class –

class Test{
   public boolean isEmpty(){
       ...
   }
}

Do the same task –

Predicate<String> p = Test::isEmpty;

This will be a compilation error –

And predicate < T > represents a predicate of a parameter (Boolean function), and the function method is Boolean test (T, t) {}

Any explanation? Did I miss anything?

Solution

You should have:

Predicate<Test> p = Test::isEmpty;

Not at all

Predicate<String> p = Test::isEmpty;

No string

class Test {
    public boolean isEmpty(){
           ...
    }
}

So why predict < string >?

See tutorial of method references What you get here is the third case "instance method referencing any object of a specific type"

have

Predicate<String> p = String::isEmpty();
String s = "";

Call p.test (s); Same as call s.isempty(); So that's why you can't call a method from test with a string as a parameter

If both string and test use the method Boolean isempty() to implement the interface empty, and then use predicate < empty >, there may be a common predicate Then both p.test (string) and p.test (test) can work; Otherwise it won't. Java has powerful input and doesn't support duck typing