A_testMethodReference0_in.java

package p;

import java.util.*;

class B {
	public void bar() {
		List<A> people = new ArrayList<>();
		Collections.sort(people, Comparator.comparing(A::m)); // Case1 w/o space
		Collections.sort(people, Comparator.comparing(A :: m));// Case2 with space
	}
}

public class A {
	String m() {
		return null;
	}
}