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.

Answer 1

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; } 

Answer 2

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     } });