How do I add an object to the beginning of Stream?

To add an object to the beginning of a list using Java Streams, we typically cannot directly prepend an object in a stream-friendly way because Streams themselves are immutable and don’t directly modify the original collection. However, we can achieve this by creating a new list with the desired order.

Here’s how we can approach it:

package org.kodejava.stream;

import java.util.*;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class StreamBeginningAdd {
    public static void main(String[] args) {
        List<String> originalList = Arrays.asList("B", "C", "D");
        String newElement = "A";

        // Add the new element at the beginning using Stream
        List<String> updatedList = Stream.concat(Stream.of(newElement), originalList.stream())
                .collect(Collectors.toList());

        // Output: [A, B, C, D]
        System.out.println(updatedList);
    }
}

Explanation:

Stream.of(newElement): Wraps the new element as a single-element stream.
originalList.stream(): Converts the existing list into a stream.
Stream.concat(): Combines the two streams — placing the newElement stream first and the original list’s stream second.
collect(Collectors.toList()): Materializes (collects) the combined stream into a new list.

This ensures immutability of the original list and creates a new list with the desired order.

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Wayan

Founder at Kode Java

A Java programmer who lives on the island of Bali, Indonesia. Programming in Spring Framework, Hibernate / JPA, MySQL. To support me working on this project, buy me a cup of coffee, every little bit helps. Thank you.

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