Question

How can I use contains method on a list that contains HashMaps so that it returns true when I ask whether or not the list contains a specified HashMap

For Example:

    List ssibenefit = new ArrayList();
    HashMap map = new HashMap();
    map.put("one", "1");
    ssibenefit.add(map);
    HashMap map4 = new HashMap();
    map4.put("one", "1");

    System.out.println("size: " + ssibenefit.size());
    System.out.println("does it contain orig: " + ssibenefit.contains(map));
    System.out.println("does it contain new: " + ssibenefit.contains(new HashMap().put("one", "1")));

should return:

size: 1
does it contain orig: true
does it contain new: true

but in this case the actual output is

size: 1
does it contain orig: true
does it contain new: false

Update

Sorry, I’ve updated the question. As I was posting the question, I realized what I wanted to do and what I was asking were not the same thing.

Answer 1

I’m afraid the answers you’ve been given so far (prior to Vlad’s answer) are all complete rubbish (which is surprising, because this is pretty basic stuff!).

The reason your test prints false where you would like to see true is that put("one", "1") returns the previous value of the mapping, not the map you’ve just manipulated. So, your call to ssibenefit.contains is asking if the list contains null. Which it doesn’t.

On the other hand, if you rewrite your test:

HashMap newMap = new HashMap();
newMap.put("one", "1");
System.out.println("does it contain new: " + ssibenefit.contains(newMap));

You will see that it prints true.

If you read the javadoc for List.contains, you’ll see that it

returns true if and only if this list contains at least one element e such that (o==null ? e==null : o.equals(e))

So, it works by equality, not identity, as others have asserted. And if you read the javadoc for Map.equals, you’ll see that it

Returns true if the given object is also a map and the two maps represent the same mappings

So, equality is defined as equality of content, not identity. Putting those two together, you can see that what you’re trying to do must work on any compliant implementation of Java.