Duplicate objects are added to the list

Question

While adding objects to list I was able to see the object is replacing all values in the list.

Please check the below image and notice the code in for loop for duplicates of object in the list.

public static void main(String args[]) {

    ArrayList<Modelclass> al = new ArrayList<Modelclass>();

    Modelclass obj = new Modelclass();
    for (int i = 0; i < 10; i++) {

        obj.setName(2 + i);
        obj.setRoolno(4 + i);
        System.out.println(obj);

        //if (!al.equals(obj)) {

            al.add(obj);
            System.out.println(obj.getName() + "" + obj.getRoolno());

        //}

    }
}

Post code inline as text. – chrylis -cautiouslyoptimistic- Jul 24 '15 at 02:01 — chrylis -cautiouslyoptimistic-, Jul 24 '15 at 02:01

Cacho Santa · Accepted Answer · 2015-07-24T02:08:05.243

7

You are always adding the same

Modelclass obj = new Modelclass();

That you created outside of the for loop. Then, inside the loop, you are modifying the values.

Since it is always a reference to the same object, you are modifying all of the items in the ArrayList.

Try this instead:

for (int i = 0; i < 10; i++) {
    Modelclass obj = new Modelclass(); //This is the key to solve it.
    obj.setName(2 + i);
    obj.setRoolno(4 + i);
    System.out.println(obj);

    al.add(obj);
    System.out.println(obj.getName() + "" + obj.getRoolno());
}

edited Jul 24 '15 at 02:08

answered Jul 24 '15 at 02:05

Cacho Santa

6,846
6
41
73

If I use same duplicate values are adding to the list – Kartheek Sarabu Jul 24 '15 at 02:06
1

@Kartheeks did you try this? It's not possible to get duplicate values with that code..... Maybe you are printing them wrong, check with the debugger again. – Cacho Santa Jul 24 '15 at 02:09
THankyou so much I got it – Kartheek Sarabu Jul 24 '15 at 02:10
1

@kartheeks glad I could help :) – Cacho Santa Jul 24 '15 at 02:12

score 2 · Answer 2 · answered Jul 24 '15 at 02:05

2

Your obj variable is only being instantiated once, but being added to the list multiple times. Whenever you update the members of obj, you are updating the same piece of memory and thus every list reference shows the same (last added) data.

answered Jul 24 '15 at 02:05

VirtualMichael

719
9
18

score 0 · Answer 3 · answered Jul 24 '15 at 02:05

I guess you are new in Java? Just create new instances in the loop will work.

ArrayList<ModelClass> al = new ArrayList<ModelClass>();
for(int i=0; i<10; i++){
    ModelClass obj = new ModelClass();
    obj.setName(2+i);
    obj.setRoolno(4+i);
    al.add(obj);
}

score 0 · Answer 4 · answered Jul 24 '15 at 02:06

Your problem is that you're referencing the same object cause you created the object before the loop. Should be

public static void main(String args[]) {

    ArrayList<Modelclass> al = new ArrayList<Modelclass>();


    for (int i = 0; i < 10; i++) {
        Modelclass obj = new Modelclass();
        obj.setName(2 + i);
        obj.setRoolno(4 + i);
        System.out.println(obj);

        //if (!al.equals(obj)) {

            al.add(obj);
            System.out.println(obj.getName() + "" + obj.getRoolno());

        //}

    }
}

Duplicate objects are added to the list

4 Answers4

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