Collections.nCopies() Method in Java

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

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

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

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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.