LinkedHashSet .equals() vs LinkedList .equals() wi

问题:

Consider the following SSCCE:

public static void main(String[] args) {
    LinkedHashSet<String> set1 = new LinkedHashSet<>();
    set1.add("Bob");
    set1.add("Tom");
    set1.add("Sam");
    LinkedHashSet<String> set2 = new LinkedHashSet<>();
    set2.add("Sam");
    set2.add("Bob");
    set2.add("Tom");

    System.out.println(set1);
    System.out.println(set2);
    System.out.println(set1.equals(set2));
}

This prints:

[Bob, Tom, Sam]
[Sam, Bob, Tom]
true

Yet if you change LinkedHashSet to LinkedList:

public static void main(String[] args) {
    LinkedList<String> set1 = new LinkedList<>();
    set1.add("Bob");
    set1.add("Tom");
    set1.add("Sam");
    LinkedList<String> set2 = new LinkedList<>();
    set2.add("Sam");
    set2.add("Bob");
    set2.add("Tom");

    System.out.println(set1);
    System.out.println(set2);
    System.out.println(set1.equals(set2));
}

it produces:

[Bob, Tom, Sam]
[Sam, Bob, Tom]
false

My question is one of clarification. Can someone help make sense of this? Why would a LinkedHashSet be considered equals whereas the same LinkedList would not? I'm assuming the definition of List and Set plays a role, but I'm not sure.

Basically, I'm saying if you consider the Sets to be the same, wouldn't you consider the Lists to be the same too? And vice-versa (assuming no duplicate elements)?

回答1:

The guarantee that LinkedHashSet makes is about iteration order. However, it's still a Set and a set doesn't care about order in itself. A List on the other hand, does. A List with an element in 3rd position is not the same as another List with the same element in the 1st position.

Set javadoc for the equals(Object) method

Returns true if the specified object is also a set, the two sets have the same size, and every member of the specified set is contained in this set (or equivalently, every member of this set is contained in the specified set). This definition ensures that the equals method works properly across different implementations of the set interface.

The LinkedHashSet javadoc states

Hash table and linked list implementation of the Set interface, with predictable iteration order.

A LinkedHashSet is a Set. It has the same rules, ie. those that apply to the set ADT.

回答2:

As mentioned above: LinkedHashSet extends HashSet which extends AbstractSet which implements equals method: https://docs.oracle.com/javase/8/docs/api/java/util/AbstractSet.html#equals-java.lang.Object-

Compares the specified object with this set for equality. Returns true if the given object is also a set, the two sets have the same size, and every member of the given set is contained in this set. This ensures that the equals method works properly across different implementations of the Set interface.

The easiest way to compare LinkedHashSet if order if important to you is to serialize it and compare them then:

    LinkedHashSet<Integer> reverseOrder = new LinkedHashSet<>();
    reverseOrder.add(2);
    reverseOrder.add(1);
    LinkedHashSet<Integer> ordered = new LinkedHashSet<>();
    ordered.add(1);
    ordered.add(2);
    System.out.println("Equals via set: " + ordered.equals(reverseOrder));
    System.out.println("Equals With Arrays: " + ordered.ordered.toString().equals(reverseOrder.ordered.toString()));

Result:

Equals via Set: true
Equals With Arrays: false