How to create JsonArray using loop?

Question

I am trying to create a JsonArray using a loop and adding the loop index to value, but getting last index in the JsonArray.

JSONArray array = new JSONArray();
JSONObject item = new JSONObject();
for(int i=0;i<2;i++) {
    item.put("information", "test"+i);
    item.put("id", 0+i);
    item.put("subName", "course"+i);
    //System.out.println(item.toString());
    array.add(item);
    System.out.println(array.toString());
}

Expected result:

[{"subName":"course0","information":"test0","id":0},
{"subName":"course1","information":"test1","id":1}]

Actual result:

[{"subName":"course1","information":"test1","id":1},{"subName":"course1","information":"test1","id":1}]

Michel K · Accepted Answer · 2022-05-08 05:51:23Z

1

Your code only creates one JSONObject and then keeps modifying it. The following example illustrates this:

JSONArray array = new JSONArray();
JSONObject item = new JSONObject();
item.put("id", 0); 
array.add(item); 
// The next line overwrites the "id" in item. This also affects 
// the one already in the array because it's the same instance
item.put("id", 1); 
array.add(item); 
System.out.println(array.toString());

The array now contains the same instance twice. Result: [{"id":0}, {"id":0}]

JSONArray array = new JSONArray();
JSONObject item1 = new JSONObject();
item1.put("id", 0); 
array.add(item1); 
JSONObject item2 = new JSONObject();
item2.put("id", 1); 
array.add(item2); 
System.out.println(array.toString());

The array now contains two distinct instances. Result: [{"id":0}, {"id":1}]

In your code, you can solve the problem by moving the creation of the JSONObject instances into the loop:

for(int i=0;i<2;i++) {
    JSONObject item = new JSONObject(); // moved inside loop
    item.put("information", "test"+i);
    ...
    array.add(item); //

edited May 8, 2022 at 5:51

answered May 8, 2022 at 5:36

Michel K

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2 Comments

suraj Over a year ago

Thanks for the solution it's working but I have one doubt as item.put uses HashMap so it's suppose to override the Id value to 1 right Output:- Result: [{"id":1}]

Michel K Over a year ago

You mean in the loop? The thing is that a new instance is created on every iteration (in the code in my answer). The put operation only affects that instance and not the one created in the previous iteration of the loop. In your original code, there is only one instance of JSONObject and there, put is called on that same instance repeatedly, overwriting the previous values every time which is why you get the output you describe.

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