Mutability.java

import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;
import java.util.Set;

public class Mutability {

    public static void main(String[] args) {

        ArrayList<String> l1 = new ArrayList<>();
        l1.add("foo");
        l1.add("bar");

        ArrayList<String> l2 = new ArrayList<>();
        l2.add("foo");
        l2.add("bar");

        Set<List<String>> s = new HashSet<>();
        s.add(l1);

        System.out.printf("s.size() = %s; s.contains(l1) = %s; s.contains(l2) = %s;\n", s.size(), s.contains(l1), s.contains(l2));

        l1.add("quux");
        System.out.printf("s.size() = %s; s.contains(l1) = %s; s.contains(l2) = %s;\n", s.size(), s.contains(l1), s.contains(l2));
    }
}

javac Mutability.java
java Mutability
s.size() = 1; s.contains(l1) = true; s.contains(l2) = true;
s.size() = 1; s.contains(l1) = false; s.contains(l2) = false;