Collections copy() method in Java with Examples

The copy() method of the java.util.Collections class copies all elements from a source list to a destination list. Each element is copied to the same index in the destination list. The destination list must already contain at least as many elements as the source list; otherwise, an IndexOutOfBoundsException is thrown.

Destination list must be at least the same size as the source list.
Existing elements in the destination list are overwritten.
Runs in O(n) time, where n is the number of elements copied.

Java

import java.util.*;

public class GFG {
    public static void main(String[] args) {

        List<Integer> source = Arrays.asList(10, 20, 30);

        List<Integer> destination =
                new ArrayList<>(Arrays.asList(0, 0, 0));

        Collections.copy(destination, source);

        System.out.println(destination);
    }
}

Output

[10, 20, 30]

Syntax

public static <T> void copy(List<? super T> dest, List<? extends T> src)

Parameters

dest: The destination list where elements are copied.
src: The source list from which elements are copied

Return Value: This method does not return any value.

Exception: IndexOutOfBoundsException: Thrown if the destination list contains fewer elements than the source list.

Example: Copying Elements Between Two Lists

Java

import java.util.*;

public class GFG {
    public static void main(String[] args) {

        List<String> source = new ArrayList<>();
        source.add("Ram");
        source.add("Gopal");
        source.add("Verma");

        List<String> destination =
                new ArrayList<>(Arrays.asList("1", "2", "3"));

        System.out.println("Source List: " + source);
        System.out.println("Destination List: " + destination);

        Collections.copy(destination, source);

        System.out.println("\nAfter Copying:");
        System.out.println("Source List: " + source);
        System.out.println("Destination List: " + destination);
    }
}

Output

Source List: [Ram, Gopal, Verma]
Destination List: [1, 2, 3]

After Copying:
Source List: [Ram, Gopal, Verma]
Destination List: [Ram, Gopal, Verma]

Explanation: This example copies all elements from the source list to the destination list. Since both lists have the same size, the copy operation succeeds and replaces the destination elements.

Example: Destination List Too Small

Java

import java.util.*;

public class GFG {
    public static void main(String[] args) {

        List<String> source = Arrays.asList(
                "Ram", "Gopal", "Verma");

        List<String> destination =
                new ArrayList<>(Arrays.asList("1", "2"));

        System.out.println("Source List: " + source);
        System.out.println("Destination List: " + destination);

        try {
            Collections.copy(destination, source);
        }
        catch (IndexOutOfBoundsException e) {
            System.out.println("\nException: " + e);
        }
    }
}

Output

Source List: [Ram, Gopal, Verma]
Destination List: [1, 2]

Exception: java.lang.IndexOutOfBoundsException: Source does not fit in dest

Explanation: Since the destination list contains fewer elements than the source list, Collections.copy() throws an IndexOutOfBoundsException.

Difference Between Collections.copy() and List.addAll()

Feature	Collections.copy()	List.addAll()
Purpose	Replaces existing elements	Appends new elements
Destination Size	Must already contain enough elements	Can be empty
Existing Elements	Overwritten	Preserved and new elements added
Return Type	`void`	boolean

Advantages

Copies elements while preserving their positions.
Easy way to replace elements in an existing list.
Works with any implementation of the List interface.
Efficient with linear time complexity.

Collections copy() method in Java with Examples

Syntax

Difference Between Collections.copy() and List.addAll()

Advantages

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