Java- How can I add a list of objects to another list of Objects

Question

Scenario: got two lists as following:

List<Object[]> listOne = null;
List<Object[]> listTwo = null;

aMethod (see at the bottom) is invoked on each lists which returns a compatible type list

listOne = aMethod(arg1, arg2, arg3);
listTwo = aMethod(argx, argy, argz);

When i try

listOne.add(listTwo);

I get error about the add function . Recommends to use addAll(), which i cant use for my reasons. So, any one have idea how to add a list of objects [] to another list of objects []? Thanks.

public List<Object[]> aMethod(a1, a2, a3) {
        List<Object[]> aList = service.getSomeinfo();
        return aList;
    }

Your question is unclear - you want to add all items of the second list to the first list? — assylias
– assylias, Commented Jun 16, 2020 at 3:58
Currently, listOne = aMethod(arg1, arg2, arg3); is part of the code. It is processed as for(Object[] obj : listOne) down the road. Wonder if I use addAll(), will for(Object[] obj : listOne) will work as before? — Shubbi
– Shubbi, Commented Jun 16, 2020 at 4:13

Jimmy · Accepted Answer · 2020-06-16 04:52:06Z

4

If you do not want to use addAll() method then @Shubbi you can add the second list of object array through the iteration like this :

for(Object[] o:listTwo){
    listOne.add(o);
}

There is another way and is very efficient also by using Stream Api if you are using Java 8 or upper version

listOne = Stream.concat(listOne.stream(), listTwo.stream())
                .collect(Collectors.toList());

answered Jun 16, 2020 at 4:52

Jimmy

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Comments

Brian H. · Accepted Answer · 2020-06-16 03:58:31Z

1

Use Stream

listOne = Stream.concat(listOne.stream(), listTwo.stream())
                         .collect(Collectors.toList());

https://docs.oracle.com/javase/8/docs/api/java/util/stream/package-summary.html

answered Jun 16, 2020 at 3:58

Brian H.

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ProGamer · Accepted Answer · 2020-06-16 04:03:19Z

1

You can use addAll function of list which adds all the elements of one array to other.

listOne.addAll(listTwo);

listOne will contain elements from both the lists.
You could add both list to third list as:

List<Object[]> listThree = new ArrayList<Object[]>(listOne);
listThree.addAll(listTwo);

Refer: https://docs.oracle.com/javase/8/docs/api/java/util/List.html#addAll-java.util.Collection-

answered Jun 16, 2020 at 4:03

ProGamer

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4 Comments

ProGamer Over a year ago

as mentioned above, you can add both listOne and listTwo to listThree. It wont have any effect on other two lists.

Shubbi Over a year ago

if listOne.addAll(listTwo); then listThree.addAll(listTwo); will not be ncecessory. right?

ProGamer Over a year ago

@Shubbi The above two are totally 2 different ways of adding OR merging two list. Please dont combine them into a single example.

ProGamer Over a year ago

First one adds two list in listOne. Second one adds both the list in listThree. Select either first or second

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