Add an element to listB without adding it to listA in java

Question

I have two list (listA and listB) and I want them to be independent. So I can add an element to the listB without the elements of the listA being modified, to remain the same.

Tryed Collection listB = Collections.unmodifiableCollection(listA);

    ArrayList<String> listA = new ArrayList<>();
    listA.add("alex");
    listA.add("brian");
    listA.add("charles");
    ArrayList<String> listB = new ArrayList<>();
    listB = listA;
    listB.add("williams");
    System.out.println(listA);

run: [alex, brian, charles, williams]

When I run it I wanted to show only theese
run: [alex, brian, charles]
(Without "williams")

You should clone `listA` as `listB`, before adding to `listB` — Scary Wombat, Aug 27 '19 at 05:26

score 5 · Answer 1 · answered Aug 27 '19 at 05:28

5

When you are doing listB = listA you are just assigning the reference of listA to listB, you are not creating a new Object.
To create a new Object, do a copy of the list as below

List<String> listB = new ArrayList<>(listA);

answered Aug 27 '19 at 05:28

raviraja

676
10
27

score 1 · Answer 2 · answered Aug 27 '19 at 06:23

1

In order to add all elements of a List to another, you may simply use the method addAll.

In your case:

listB.addAll(listA);

answered Aug 27 '19 at 06:23

Jakob Schwarz

42
4

score 0 · Answer 3 · answered Aug 27 '19 at 05:32

    ArrayList<String> listA = new ArrayList<>();
    listA.add("alex");
    listA.add("brian");
    listA.add("charles");
    ArrayList<String> listB = (ArrayList<String>) listA.clone();
    listB.add("williams");
    System.out.println(listA);
    System.out.println(listB);

score 0 · Answer 4 · answered Aug 27 '19 at 05:48

0

You can use the SerializationUtils.clone() method.

private static void deepCopy(){
    ArrayList<String> listA = new ArrayList<>();
    listA.add("alex");
    listA.add("brian");
    listA.add("charles");
    ArrayList<String> listB = SerializationUtils.clone(listA);
    listB.add("williams");
    System.out.println(listA);
    System.out.println(listB);

}

Result:

[alex, brian, charles]
[alex, brian, charles, williams]

answered Aug 27 '19 at 05:48

Randika

31
4

and where is this class `SerializationUtils` class defined? a reference link to docs might be encouraged... – Michał Krzywański Aug 27 '19 at 05:55
It's include in "commons-lang.jar" .First add that jar as the dependency and after that, import the 'import org.apache.commons.lang3.SerializationUtils' – Randika Aug 27 '19 at 08:12

score 0 · Answer 5 · answered Aug 27 '19 at 09:07

Your problem is how clone List in Java,there are many method to do this:

Using a Copy Constructor: Using the ArrayList constructor in Java, a new list can be initialized with the elements from another collection.

List<String> listB = new ArrayList<>(listA);

Using the addAll() method: The List class has a method called addAll(), which appends all the elements of one collection to the list.

List<String> listB = new ArrayList<>();
listB.addAll(listA);

Using streams in Java 8 Using the Streams API introduced in JAVA 8, cloning of a list is possible. The collect() method (along with toList() method) is used to clone a list.

List<String> listB = listA.stream().collect(Collectors.toList());

Using Gson library to convert list to JSON: Using Google’s Gson library, a list can be converted to JSON. This JSON string can be converted to an object of type List and can be used to initialize the new List.

String jsonListA = gson.toJson(listA);
List<String> listB = gson.fromJson(jsonListA,List.class);

score 0 · Answer 6 · answered Aug 27 '19 at 14:07

You need to copy the original list to a new object. You can stream the first list and collect the elements in a new one:

List<String> listB = listA.stream().collect(Collectors.toList());

Please notice that using a copy constructor (like new ArrayList<>(listA)) or using the addAll method, the contents of both lists will reference the same objects, every amends made in one element will affect both lists.

Add an element to listB without adding it to listA in java

6 Answers6