There is a number of ways do to Java List copy. Which one is best? Which one apply to your situation at hand? In this post you will find this out!

The following Java program demonstrates how to do Java List copy using copy constructor and Collections.copy(List dest, List src) method:

package com.farenda.java;

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

import static java.util.Arrays.asList;

public class CollectionsCopy {

    public static void main(String[] args) {
        usingCopyConstructor();

        copySourceToSmallerDest();

        copySameSizeLists();

        copySourceToBiggerDest();

        copyToUnmodifiableDest();
    }

    private static void usingCopyConstructor() {
        List<Integer> source = asList(1, 2, 3);
        List<Integer> dest = new ArrayList<>(source);
        System.out.printf("%nCopy %s to %s using copy constructor.%n",
                          source, dest);
    }

    private static void copySourceToSmallerDest() {
        List<Integer> source = asList(1, 2, 3);
        // The same for: new ArrayList<>(source.size());
        List<Integer> dest = asList(4, 5);

        System.out.printf("%nCopy %s to (smaller) %s%n", source, dest);
        try {
            Collections.copy(dest, source);
        } catch (IndexOutOfBoundsException e) {
            e.printStackTrace();
        }
    }

    private static void copySameSizeLists() {
        List<Integer> source = asList(1, 2);
        List<Integer> dest = asList(3, 4);

        System.out.printf("%nCopy %s to (same size) %s%n", source, dest);
        Collections.copy(dest, source);
        System.out.println("source: " + source);
        System.out.println("destination: " + dest);
    }

    private static void copySourceToBiggerDest() {
        List<Integer> source = asList(1, 2);
        List<Integer> dest = asList(3, 4, 5);

        System.out.printf("%nCopy %s to (bigger) %s%n", source, dest);
        Collections.copy(dest, source);
        System.out.println("source: " + source);
        System.out.println("destination: " + dest);
    }

    private static void copyToUnmodifiableDest() {
        List<Integer> source = asList(1, 2);
        List<Integer> dest = Collections.unmodifiableList(asList(4, 5));

        System.out.printf("%nCopy %s to (unmodifiable) %s%n", source, dest);
        try {
            Collections.copy(dest, source);
        } catch (UnsupportedOperationException e) {
            e.printStackTrace();
        }
    }
}

As you can see the easiest way of copying Lists is to use copy constructor that’s available in List implementations – here it’s ArrayList. It copies references to all elements (so it is a shallow copy) from source list into the new list.

Sometimes you may want to update an existing List with a new data. In such case Collections.copy(List dest, List src) performs really well. The method sets elements on the same positions in dest List and doesn’t re-size target list when needed. So, remember that dest List have to be of at least size of source List and allow for modifications. These corner cases are illustrated in the 4 copy… methods in the example above.

Running the above Java code gives the following result:

Copy [1, 2, 3] to [1, 2, 3] using copy constructor.

Copy [1, 2, 3] to (smaller) [4, 5]
java.lang.IndexOutOfBoundsException: Source does not fit in dest
        at java.util.Collections.copy(Collections.java:556)
        at com.farenda.java.CollectionsCopy.copySourceToSmallerDest(CollectionsCopy.java:36)
        at com.farenda.java.CollectionsCopy.main(CollectionsCopy.java:14)

Copy [1, 2] to (same size) [3, 4]
source: [1, 2]
destination: [1, 2]

Copy [1, 2] to (bigger) [3, 4, 5]
source: [1, 2]
destination: [1, 2, 5]

Copy [1, 2] to (unmodifiable) [4, 5]
java.lang.UnsupportedOperationException
        at java.util.Collections$UnmodifiableList.set(Collections.java:1311)
        at java.util.Collections.copy(Collections.java:561)
        at com.farenda.java.CollectionsCopy.copyToUnmodifiableDest(CollectionsCopy.java:68)
        at com.farenda.java.CollectionsCopy.main(CollectionsCopy.java:20)

If you know a good use case for Collections.copy(List dest, List src) then let me know. Thanks! :-)