On the definition of "sameness"
As Joachim noted, for most application, List.equals(Object o)
definition works:
Compares the specified object with this list for equality. Returns true
if and only if the specified object is also a list, both lists have the same size, and all corresponding pairs of elements in the two lists are equal. (Two elements e1
and e2
are equal if (e1==null ? e2==null : e1.equals(e2))
.) In other words, two lists are defined to be equal if they contain the same elements in the same order. This definition ensures that the equals
method works properly across different implementations of the List
interface.
Depending on how you're using it, though, this may not work as expected. If you have a List<int[]>
, for example, it doesn't quite work because arrays inherit equals
from Object
which defines equality as reference identity.
List<int[]> list1 = Arrays.asList(new int[] { 1, 2, 3 });
List<int[]> list2 = Arrays.asList(new int[] { 1, 2, 3 });
System.out.println(list1.equals(list2)); // prints "false"
Also, two lists with different type parameter can be equals
:
List<Number> list1 = new ArrayList<Number>();
List<String> list2 = new ArrayList<String>();
System.out.println(list1.equals(list2)); // prints "true"
You also mentioned that the list must contain elements with the same type. Here's yet another example where the elements don't have the same type, and yet they're equals
:
List<Object> list1 = new ArrayList<Object>();
List<Object> list2 = new ArrayList<Object>();
list1.add(new ArrayList<Integer>());
list2.add(new LinkedList<String>());
System.out.println(list1.equals(list2)); // prints "true"
So unless you clearly define what equality means to you, the question can have very different answers. For most practical purposes, though, List.equals
should suffice.
On implementing equals
Information after update suggests that List.equals
will do the job just fine, provided that the elements implement equals
properly (because List<E>.equals
invokes E.equals
on the non-null
-elements, per the API documentation above).
So in this case, if we have, say, a List<Player>
, then Player
must @Override equals(Object o)
to return true
if o instanceof Player
and on the relevant fields, they're all equals
(for reference types) or ==
(for primitives).
Of course, when you @Override equals
, you should also @Override int hashCode()
. The barely acceptable minimum is to return 42;
; slightly better is to return name.hashCode();
; best is to use a formula that involves all the fields on which you define equals
. A good IDE can automatically generate equals/hashCode
methods for you.
See also
- Effective Java 2nd Edition
- Item 8: Obey the general contract when overriding equals
- Item 9: Always override hashcode when you override equals
API links
Related questions
On equals/hashCode
combo:
On equals
vs ==
: