Java how to copy list

How to copy a list to another list in Java?

A List of elements can be copied to another List using multiple ways.

Way #1

Create a List by passing another list as a constructor argument.

List copyOflist = new ArrayList<>(list);

Create a List and use addAll method to add all the elements of the source list.

Way #2

List copyOfList = new ArrayList<>(); copyOfList.addAll(list);

Way #3

Use Collections.copy method to copy the contents of source list to destination. Existing elements will be overridden by indexes if any.

Collections.copy(copyOfList, list);

Way #4

Use Streams to create a copy of a list.

List copyOfList = list.stream().collect(Collectors.toList());

Example

Following is the example to explain the creation of copy of List object using multiple ways −

package com.tutorialspoint; import java.util.ArrayList; import java.util.Arrays; import java.util.Collections; import java.util.List; import java.util.stream.Collectors; public class CollectionsDemo < public static void main(String[] args) < Listlist = Arrays.asList("Zara","Mahnaz","Ayan" ); System.out.println("Source: " + list); List copyOfList1 = new ArrayList<>(list); System.out.println("Copy 1: " + copyOfList1); List copyOfList2 = new ArrayList<>(); copyOfList2.addAll(list); System.out.println("Copy 2: " + copyOfList2); List copyOfList3 = Arrays.asList("Rob","Julie","John" ); Collections.copy(copyOfList3, list); System.out.println("Copy 3: " + copyOfList3); List copyOfList4 = list.stream().collect(Collectors.toList()); System.out.println("Copy 4: " + copyOfList4); > >

Output

This will produce the following result −

Source: [Zara, Mahnaz, Ayan] Copy 1: [Zara, Mahnaz, Ayan] Copy 2: [Zara, Mahnaz, Ayan] Copy 3: [Zara, Mahnaz, Ayan] Copy 4: [Zara, Mahnaz, Ayan]

Источник

How to copy java collections list?

When working with Java collections, it’s often necessary to create a copy of a list to ensure that the original list remains unmodified. However, simply assigning a reference to the original list to a new variable doesn’t create a new copy of the list, it only creates a reference to the same list. To create a new copy of a Java collection, you need to use one of several methods available in Java.

Method 1: Clone

To copy a Java Collections list using the clone() method, follow these steps:

ListString> originalList = new ArrayList>(); originalList.add("apple"); originalList.add("banana"); originalList.add("cherry");

ListString> copiedList = (ListString>) originalList.clone();

1. Verify that the copied list is a separate object from the original list by modifying the original list:

System.out.println(copiedList); // Output: [apple, banana, cherry]

Note that this method only creates a shallow copy of the list, meaning that the elements themselves are not cloned. If the elements of the original list are mutable objects, changes to those objects will be reflected in both the original and copied lists.

Method 2: Copy Constructor

To copy a Java Collections list using a Copy Constructor, you can create a new instance of the list and pass the original list as a parameter to the constructor. Here is an example code for copying an ArrayList using a Copy Constructor:

import java.util.ArrayList; public class Main  public static void main(String[] args)  ArrayListString> originalList = new ArrayList>(); originalList.add("Java"); originalList.add("Python"); originalList.add("C++"); // Copy Constructor ArrayListString> copiedList = new ArrayList>(originalList); System.out.println("Original List: " + originalList); System.out.println("Copied List: " + copiedList); > >

In this example, the Copy Constructor is used to copy the original ArrayList originalList to a new ArrayList copiedList . The ArrayList constructor takes the original list as a parameter and creates a new instance of the list with the same elements.

You can also use a Copy Constructor to copy other types of Java Collections, such as LinkedList, HashSet, and TreeSet. Here’s an example code for copying a HashSet using a Copy Constructor:

import java.util.HashSet; public class Main  public static void main(String[] args)  HashSetString> originalSet = new HashSet>(); originalSet.add("Java"); originalSet.add("Python"); originalSet.add("C++"); // Copy Constructor HashSetString> copiedSet = new HashSet>(originalSet); System.out.println("Original Set: " + originalSet); System.out.println("Copied Set: " + copiedSet); > >

In this example, the Copy Constructor is used to copy the original HashSet originalSet to a new HashSet copiedSet . The HashSet constructor takes the original set as a parameter and creates a new instance of the set with the same elements.

Overall, using a Copy Constructor is a simple and efficient way to copy Java Collections lists. Simply create a new instance of the list and pass the original list as a parameter to the constructor.

Method 3: Serialization

To copy a Java Collections list using Serialization, you can follow these steps:

1. Create a new instance of the list you want to copy.
2. Serialize the original list into a byte array.
3. Deserialize the byte array into the new list.

Here’s an example code snippet that demonstrates how to do this:

import java.io.*; import java.util.*; public class ListCopyExample  public static void main(String[] args) throws Exception  // Create the original list ListString> originalList = new ArrayList>(); originalList.add("apple"); originalList.add("banana"); originalList.add("cherry"); // Serialize the original list into a byte array ByteArrayOutputStream baos = new ByteArrayOutputStream(); ObjectOutputStream oos = new ObjectOutputStream(baos); oos.writeObject(originalList); byte[] bytes = baos.toByteArray(); // Deserialize the byte array into a new list ByteArrayInputStream bais = new ByteArrayInputStream(bytes); ObjectInputStream ois = new ObjectInputStream(bais); ListString> newList = (ListString>) ois.readObject(); // Print both lists to verify the copy was successful System.out.println("Original list: " + originalList); System.out.println("New list: " + newList); > >

In this example, we first create the original list and add some elements to it. Then, we serialize the list using an ObjectOutputStream and write the resulting byte array to a ByteArrayOutputStream .

Next, we deserialize the byte array using an ObjectInputStream and cast the resulting object to a List to create a new list.

Finally, we print both the original and new lists to verify that the copy was successful.

Note that this approach only works if all the elements in the list are also serializable. If any of the elements are not serializable, you will need to use a different approach to copy the list.

Method 4: Java 8 Streams API

To copy a Java Collections list using Java 8 Streams API, you can use the stream() method to convert the list to a stream, then use the collect() method to create a new list from the stream. Here’s an example code:

ListString> originalList = new ArrayList>(Arrays.asList("apple", "banana", "cherry")); ListString> copiedList = originalList.stream().collect(Collectors.toList());

In this code, we first create an ArrayList called originalList with three elements. We then use the stream() method to convert this list to a stream, and then use the collect() method with the Collectors.toList() method to create a new list called copiedList .

You can also use the toArray() method to create an array from the stream instead of a list. Here’s an example code:

String[] copiedArray = originalList.stream().toArray(String[]::new);

In this code, we use the stream() method to convert the originalList to a stream, and then use the toArray() method with a method reference to String[]::new to create a new array called copiedArray .

That’s it! With just a few lines of code, you can easily copy a Java Collections list using Java 8 Streams API.

Method 5: Apache Commons Collections

To copy a Java Collections list using Apache Commons Collections, you can use the ListUtils class. Here are the steps to do it:

import org.apache.commons.collections4.ListUtils;

ListString> originalList = new ArrayList>(); // add elements to originalList ListString> copiedList = new ArrayList>();

ListUtils.copy(copiedList, originalList);

Here is an example code that demonstrates how to copy a list using Apache Commons Collections:

import org.apache.commons.collections4.ListUtils; import java.util.ArrayList; import java.util.List; public class CopyListExample  public static void main(String[] args)  ListString> originalList = new ArrayList>(); originalList.add("apple"); originalList.add("banana"); originalList.add("orange"); ListString> copiedList = new ArrayList>(); ListUtils.copy(copiedList, originalList); System.out.println("Original list: " + originalList); System.out.println("Copied list: " + copiedList); > >

Original list: [apple, banana, orange] Copied list: [apple, banana, orange]

In this example, the ListUtils.copy() method is used to copy the elements from the originalList to the copiedList . The output shows that both lists contain the same elements.

Источник

How do I copy the contents of one ArrayList into another?

I have some data structures, and I would like to use one as a temporary, and another as not temporary.

ArrayList myObject = new ArrayList(); ArrayList myTempObject = new ArrayList(); //fill myTempObject here . //make myObject contain the same values as myTempObject myObject = myTempObject; //free up memory by clearing myTempObject myTempObject.clear(); 

now the problem with this of course is that myObject is really just pointing to myTempObject , and so once myTempObject is cleared, so is myObject . How do I retain the values from myTempObject in myObject using java?

You can use List.addAll . But if you need to retain all the objects then clearing the temp list is not really going to clear a whole lot of memory. Because your are only trying to clear the references, as far as objects you are trying to keep them.

13 Answers 13

myObject = new ArrayList(myTempObject); 

myObject = (ArrayList)myTempObject.clone(); 

You can get some information about clone() method here

But you should remember, that all these ways will give you a copy of your List, not all of its elements. So if you change one of the elements in your copied List, it will also be changed in your original List.

assign by value in Java is something that is reserved for primitive types (int, byte, char, etc.) and literals. Unless you explicitly tell Java to copy something that is derived from Object it’ll always assign by reference. By the way clone() is a shallow copy and will not copy the contained Objects but rather references to them. If you want to make a deep-copy take a look at: stackoverflow.com/questions/64036/…

@PeterT — Java is only call by and assign by value. Non-primitive type variables contain reference values which are assigned or passed.

@X09 oldList = newList will copy reference of the newList to oldList, any change in newList will be reflected in the oldList also. That is not the intended behaviour.

originalArrayList.addAll(copyArrayList);

Please Note: When using the addAll() method to copy, the contents of both the array lists (originalArrayList and copyArrayList) refer to the same objects or contents. So if you modify any one of them the other will also reflect the same change.

If you don’t wan’t this then you need to copy each element from the originalArrayList to the copyArrayList, like using a for or while loop.

There are no implicit copies made in java via the assignment operator. Variables contain a reference value (pointer) and when you use = you’re only coping that value.

In order to preserve the contents of myTempObject you would need to make a copy of it.

This can be done by creating a new ArrayList using the constructor that takes another ArrayList :

ArrayList myObject = new ArrayList(myTempObject); 

Edit: As Bohemian points out in the comments below, is this what you’re asking? By doing the above, both ArrayList s ( myTempObject and myObject ) would contain references to the same objects. If you actually want a new list that contains new copies of the objects contained in myTempObject then you would need to make a copy of each individual object in the original ArrayList

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