How can I combine two arrays in Java efficiently?

General Discussions
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1

What is the easiest way to combine two arrays in Java?

I have two String arrays in Java, and I need to merge them into a single array.

For example:

void f(String[] first, String[] second) {
    String[] both = ???;
}

How can I achieve this efficiently? Is there a built-in method in Java that simplifies this process, or do I need to use a loop? I’m looking for the best way to combine two arrays in Java while keeping the code clean and efficient.

2

Ah, I’ve worked with array operations quite a bit. If you’re using Java 8 or later, the Stream.concat() method makes combining two arrays super clean and concise:

import java.util.stream.Stream;

void f(String[] first, String[] second) {
    String[] both = Stream.concat(Stream.of(first), Stream.of(second))
                          .toArray(String[]::new);
}

:small_blue_diamond: Why this? It’s neat, readable, and avoids manual looping. Plus, it works with any array type—making it a solid option for most use cases. This is a great way to Java combine two arrays efficiently.

3

Building on @jacqueline-bosco’s point—if performance is your main concern, System.arraycopy() is the way to go. It’s faster and more efficient for larger arrays because it directly manipulates memory:

void f(String[] first, String[] second) {
    String[] both = new String[first.length + second.length];
    
    System.arraycopy(first, 0, both, 0, first.length);
    System.arraycopy(second, 0, both, first.length, second.length);
}

:small_blue_diamond: Why this?

When handling large datasets, this method is optimized for speed. It’s my go-to when I need to Java combine two arrays without sacrificing performance.

4

Using ArrayList (If You Need Flexibility) If you might modify the array later, an ArrayList can be a better option:

import java.util.ArrayList; import java.util.Arrays; import java.util.List;

void f(String first, String second) { List list = new ArrayList<>(Arrays.asList(first)); list.addAll(Arrays.asList(second)); String both = list.toArray(new String[0]); }

:small_blue_diamond: Why this? It allows you to easily add or remove elements before converting it back to an array.