Check if an ArrayList contains a given object

Question

Assuming I have class like this:

class A {
  int elementA;
  int elementB
}

I also have an ArrayList like this: ArrayList<A> listObj.

How can I check if that list contains an object using only some of the properties of A? E.g. considering only elementA for finding if object newA{elementA} is already in the list?

For object A I have defined an equals method, where I consider only the elementA, however this is not enough.

If you have overridden your equals() correctly, then listObj.contains(new A(elemAvalue)) should work. Note that you should also override hashCode only considering elementA value, though it's not functionally relevant for this example. — misberner, Apr 29 '13 at 07:42
for equals i use [here is example](http://javarevisited.blogspot.dk/2011/02/how-to-write-equals-method-in-java.html) as source (equals and hashCode) — Saulius S, Apr 29 '13 at 08:12

score 33 · Answer 1 · answered Apr 29 '13 at 07:33

33

List#contains() method uses the equals() method to evaluate if two objects are the same. So, you need to override equals() in your Class A and also override the hashCode().

@Override
public boolean equals(Object object)
{
    boolean isEqual= false;

    if (object != null && object instanceof A)
    {
        isEqual = (this.elementA == ((A) object).elementA);
    }

    return isEqual;
}

@Override
public int hashCode() {
    return this.elementA;
}

answered Apr 29 '13 at 07:33

AllTooSir

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This works fine as long as you have to compare the A elements, but as soon as the requirement changes you can throw this out, not to mention that you might want to use the equals or hashcode methods other places, I would go with a Comparator thats for sure. – Peter Jaloveczki Apr 29 '13 at 08:07
2

Fair enough! But List `contains()` uses the `equals` method, not `Comparable.compareTo()` ! – AllTooSir Apr 29 '13 at 08:17
Thanks for the solution. For newbies like me: If `elementA` is `String` type, then use `isEqual = (this.elementA.equals(((A) object).elementA));` – Muhammad Saqib Aug 24 '16 at 15:39
1

In case "elementA" is a String, we need not override hashCode() method – Anubhav Apr 01 '20 at 13:20

NilsH · Answer 2 · 2013-04-29T08:23:55.567

You could adapt your equals to accomodate for your needs, but you could also use one of the powerful collection libraries that already exists, like commons-collections, Guava or lambdaj. They all have iteration and predicate constructs that allow you to "explore" your collections based on some predicate.

Example with Commons Collections:

boolean contains = CollectionUtils.exists(myArrayList, new Predicate<A>() {
    public boolean evaluate(A theA) {
        // Do the comparison
    }
});

score 1 · Answer 3 · answered Apr 29 '13 at 07:38

I wouldn't override equals for this. Even though {1, 1} and {1, 2} cannot both occur in your list, the two objects are not equal.

I would use a Map instead:

Map<Integer, A> map = new HashMap<>();
A a1 = new A(1, 1);
A a2 = new A(1, 2);

map.put(a1.elementA, a1);

// test if key is contained
boolean contains = map.containsKey(a2.elementA);

// overwrite a1 with a2
map.put(a2.elementA, a2);

score 0 · Answer 4 · edited May 23 '17 at 11:47

The hashCode() and equals() of arrays are a bit broken when it comes to this (it is a long different discussion why).

A possible work around is to use ArrayList<ArrayList<String>> instead of ArrayList<String[]>, the equals() method for ArrayList will be as you expect it to.

For example:

   ArrayList<String> l1 = new ArrayList<>();
    ArrayList<String> l2 = new ArrayList<>();
    l1.add("asdf");
    l2.add("asdf");
    ArrayList<ArrayList<String>> coll = new ArrayList<>();
    coll.add(l1);
    System.out.println(coll.contains(l2));

Will yield true, as expected

Also refer below link

Most efficient way to see if an ArrayList contains an object in Java

score 0 · Answer 5 · answered May 03 '13 at 10:17

All the above solution will suits your question. What i like to suggest you here is !!! When ever you use collection, always have your variable datatype to its super class. which will help you to make more manipulation and convert to any of its sub-class.

Ex:
List<String> myList = new ArrayList<String>();

The advantage that the implementation of the List can change (to a LinkedList for example), without affecting the rest of the code. This is will be difficult to do with an ArrayList, not only because you will need to change ArrayList to LinkedList everywhere, but also because you may have used ArrayList specific methods.

score 0 · Answer 6 · edited Jan 17 '18 at 13:28

0

For comparing the object value in arrayList. Follow this Link. How to compare Objects attributes in an ArrayList?

I hope this solution will help you. Thanks

edited Jan 17 '18 at 13:28

alessandrio

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answered Jan 17 '18 at 12:32

Aman Goyal

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This should not be set as an answer I guess, but as a comment. – NatNgs Jan 17 '18 at 12:52
You cannot just link to answer. You have to give the answer here. – patapouf_ai Jan 17 '18 at 13:26

score 0 · Answer 7 · answered Sep 11 '19 at 00:15

0

List<Upload> mUploadPost = new ArrayList<Upload>();
Upload upload = new Upload();
upload.addName("Mpendulo");
upload.addLastName("Mtshali");
if(!mUploadPost.contains(upload)){
    mUploadPost.add(upload);
}

answered Sep 11 '19 at 00:15

Mpendulo

1

Check if an ArrayList contains a given object

7 Answers7

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