The Collections.nCopies() method in Java is used to create an immutable list which contains multiple copies of the same object. This method is helpful if we want to initialize a list with n copies of given object. The newly allocated data object is tiny, i.e, it contains a single reference to the data object. This method is defined in the java.util.Collections class.
Syntax of Collections.nCopies() Method
public static <T> List<T> nCopies(int n, T object)
Parameters:
- n: The number of copies we want in the list.
- object: The object that should be repeated.
- T: The type of object.
Returns: It returns an immutable list with n references to the same object.
Important Points:
- We cannot modify the returned list, because it is immutable.
- If n is less than 0, it throws an IllegalArgumentException.
- This method is useful for creating pre-filled data structures.
Examples of Collections.nCopies() Method
Example 1: Creating a list with 4 identical strings
// Java program to demonstrate Collections.nCopies()
// with 4 copies of a string
import java.util.*;
class Geeks {
public static void main(String[] args) {
// Creating a list with 4 copies of "GeeksforGeeks"
List<String> list = Collections.nCopies(4, "GeeksforGeeks");
System.out.println("The list returned is:");
for (String item : list) {
System.out.print(item + " ");
}
System.out.println();
}
}
Output
The list returned is: GeeksforGeeks GeeksforGeeks GeeksforGeeks GeeksforGeeks
Explanation: This example creates a list with 4 references to the string "GeeksforGeeks" and prints each one.
Example 2: Another list with 3 identical elements
// Java program to demonstrate Collections.nCopies()
// with 3 copies of another string
import java.util.*;
class Geeks {
public static void main(String[] args) {
// Creating a list with 3 copies of "GeeksQuiz"
List<String> list = Collections.nCopies(3, "GeeksQuiz");
System.out.println("The list returned is:");
for (String item : list) {
System.out.print(item + " ");
}
}
}
Output
The list returned is: GeeksQuiz GeeksQuiz GeeksQuiz
Explanation: In this example, the method returns a new list with 3 copis of "GeeksQuiz" also demonstrating another example of how to initialize lists with duplicate values.
Example 3: Using Collections.nCopies() with custom objects
// Java program to demonstrate Collections.nCopies()
// with a custom class object
import java.util.*;
class Book {
String title;
// Constructor
Book(String title) {
this.title = title;
}
// toString method for printing
public String toString() {
return title;
}
}
public class Geeks {
public static void main(String[] args) {
// Create a custom Book object
Book book = new Book("Java Basics");
// Create 5 copies of the book object in a list
List<Book> bookList = Collections.nCopies(5, book);
System.out.println("Book List:");
for (Book b : bookList) {
System.out.println(b);
}
}
}
Output
Book List: Java Basics Java Basics Java Basics Java Basics Java Basics
This method is helpful where we need to work with a fixed number of repeated elements in a list without manually adding them each time.