Compare two arraylist and remove common elements

Question

I know this question has been asked many times before. But in my problem there are two arraylist of same object type. Let us say arraylist A contains 5 elements and arraylist B contains 2 elements.

Arraylist A elements = obj1,obj2,obj3,obj4,obj5

Arraylist B elements = obj1,obj2

What I want is I want to remove all the elements of Arraylist A that exist in arraylist B i.e obj1,obj2. Then the final Arraylist A should look like:

Arraylist A = obj3,obj4,obj5

The code I implemented:

for (int i=0;i<arraylistB.size();i++){
    if(arraylistA.contains(arraylistB.get(i))){
        arraylistA.remove(arraylistB.get(i));
        arraylistB.remove(i);
   }
}

But this code deleted nothing from Arraylist A, I don't know what I'm doing wrong. Please help me!

What is the type of the elements? Is it a custom type? If so, did you override equals() and hashCode()? — cpp beginner
– cpp beginner, Commented Mar 30, 2018 at 16:09
Not easily as I’m using my phone. Someone will find a link for a duplicate in a few minutes. — cpp beginner
– cpp beginner, Commented Mar 30, 2018 at 16:11
Possible duplicate of ArrayList removeAll() not removing objects — Ousmane D.
– Ousmane D., Commented Mar 30, 2018 at 16:16

Pritam Banerjee · Accepted Answer · 2018-03-30 16:12:35Z

3

You will need to override the equals() and hashcode() of class of Object1, Object2 etc which should be instances of the same class.

Then you can do a removeAll().

edited Mar 30, 2018 at 16:12

answered Mar 30, 2018 at 16:10

Pritam Banerjee

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Comments

hoan · Accepted Answer · 2018-03-30 16:09:20Z

1

Guessing the elements in arraylistA and arraylistB are not the same, meaning not reference to the same address, so this check is not correct

if(arraylistA.contains(arraylistB.get(i)))

Should check by e.g. id of the element instead.

answered Mar 30, 2018 at 16:09

hoan

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3 Comments

Rektirino Over a year ago

Both arraylist have same object type

Rektirino Over a year ago

No, it does not

hoan Over a year ago

meaning that obj1 in arraylistA and obj1 in arraylistB are different. You can see it by checking both objects addresses via debugging.

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