Why does String::isEmpty works when non-static method cannot be referenced from a static context?

Mariano L :

I understand the error message. I know that I cannot access non-static methods in a static context. But why I can do the following:

Predicate<String> t = String::isEmpty; // this works

When isEmpty() is a non-static method for the class String? Look at the following example class. I understand the logic to not allow TestLamba::isEmptyTest; but what I don't understand is why String:isEmpty can break this rule:

import java.util.function.Predicate;

public class TestLamba {

    public static void main(String... args) {

        Predicate<String> t = String::isEmpty; // this works
        Predicate<String> t2 = TestLamba::isEmptyTest; // this doesn't
    }

    public boolean isEmptyTest() {
        return true;
    }

}

This is the source for String.isEmpty. It's a pretty common method and you can see that it is not static:

public boolean isEmpty() {
    return this.value.length == 0;
}

Khalid Shah :

isEmpty is the function of String Class and isEmptyTest is the function of TestLamba class.

import java.util.function.Predicate;

public class TestLamba {

    public static void main(String... args) {

        Predicate<String> t = String::isEmpty; // this works
        Predicate<TestLamba > t2 = TestLamba::isEmptyTest; //Now this will work
    }

    public boolean isEmptyTest() {
        return true;
    }

}