How can I return the difference between two lists?

Question

I have two array lists e.g.

List a;
contains : 10/10/2014, 10/11/2016

List b;
contains : 10/10/2016

How can i do

Answer 1

I was looking for a different problem and came across this, so I will add my solution to a related problem: comparing two Maps.

    // make a copy of the data
    Map a = new HashMap(actual);
    Map e = new HashMap(expected);
    // check *every* expected value
    for(Map.Entry val : e.entrySet()){
        // check for presence
        if(!a.containsKey(val.getKey())){
            System.out.println(String.format("Did not find expected value: %s", val.getKey()));
        }
        // check for equality
        else{
            if(0 != a.get(val.getKey()).compareTo(val.getValue())){
                System.out.println(String.format("Value does not match expected: %s", val.getValue()));
            }
            // we have found the item, so remove it 
            // from future consideration. While it 
            // doesn't affect Java Maps, other types of sets
            // may contain duplicates, this will flag those
            // duplicates. 
            a.remove(val.getKey());
        }
    }
    // check to see that we did not receive extra values
    for(Map.Entry val : a.entrySet()){
        System.out.println(String.format("Found unexpected value: %s", val.getKey()));
    }

It works on the same principle as the other solutions but also compares not only that values are present, but that they contain the same value. Mostly I've used this in accounting software when comparing data from two sources (Employee and Manager entered values match; Customer and Corporate transactions match; ... etc)