LinkedHashSet .equals() vs LinkedList .equals() with same elements but different order
collections, equals, java, linked-list, linkedhashset
Solution
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.
Problem
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 `Set`s to be the same, wouldn't you consider the `List`s to be the same too? And vice-versa (assuming no duplicate elements)?