CODEWITHSUNDEEP
javalistarraylist
Saulius S
Saulius S

Reputation: 401

Check if an ArrayList contains a given object

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.

Upvotes: 26

Views: 104528

Answers (7)

Mpendulo
Mpendulo

Reputation: 1

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

Upvotes: 0

Aman Goyal
Aman Goyal

Reputation: 449

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

Upvotes: 0

user2346260
user2346260

Reputation:

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.

Upvotes: 0

NilsH
NilsH

Reputation: 13821

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
    }
});

Upvotes: 4

Vincent van der Weele
Vincent van der Weele

Reputation: 13177

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);

Upvotes: 1

AllTooSir
AllTooSir

Reputation: 49372

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;
}

Upvotes: 33

Hariharan
Hariharan

Reputation: 3263

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

Upvotes: 0

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