Question

I have to add only unique elements to an ArrayList. How can I override the equals method for that purpose? I want something like:

public boolean equals (Object in) {
    if(in == null) {
        return false;
    }
    else if(in instanceof UniqueFruits) {
        // ?? check whether the newly added element exists or not           
        return true;
    }
    else {
        return false;
    }       
} 

How to check whether the newly added element exists or not? I have to check on the Fruit Names.

Answer 1

It sounds like you want to use a Set implementation. If order doesn’t matter, just use HashSet. If you want to keep insertion-order, use LinkedHashSet. Or, to maintain natural ordering, use TreeSet.

In any case, make sure to override your element object’s equals method. What you have is a good start. After checking in instanceof UniqueFruits, cast in to UniqueFruits:

UniqueFruits uf = (UniqueFruits)in;

You can then check each relevant field using equals in turn (make sure to check for null first if a field is nullable). Any modern IDE will help you generate equals automatically. It may be educational to try it yourself first, then compare with the generated version.

Make sure to override hashCode also (IDEs will similarly help you with this, and there is plenty of reading online about the matter – just search).

If you use TreeSet (or some other SortedSet implementation), your element object should either implement Comparable or else you should provide the SortedSet with a Comparator.