Concatenate two arrays in Java

This post will discuss concatenating two arrays in Java into a new array. The new array should maintain the original order of elements in individual arrays, and all elements in the first array should precede all elements of the second array.

1. Using Java 8 Stream

We can use Stream in Java 8 and above to concatenate two arrays. There are various ways to do that:

⮚ Stream.of() method

// Method to concatenate two arrays in Java 8 and above

public static String[] concatenate(String[] first, String[] second)

{

return Stream.of(first, second)

.flatMap(Stream::of) // or, use `Arrays::stream`

.toArray(String[]::new);

}

⮚ Stream.concat() method

// Method to concatenate two arrays in Java 8 and above

public static String[] concatenate(String[] first, String[] second)

{

return Stream.concat(Arrays.stream(first), Arrays.stream(second))

.toArray(String[]::new);

}

2. Using `System.arraycopy()` method

We start by allocating enough memory to the new array to accommodate all the elements present in both arrays. Then we use System.arraycopy() to copy given arrays into the new array, as shown below:

// Method to concatenate two arrays in Java

public static String[] concatenate(String[] first, String[] second)

{

String[] arr = new String[first.length + second.length];

System.arraycopy(first, 0, arr, 0, first.length);

System.arraycopy(second, 0, arr, first.length, second.length);

return arr;

}

Here’s a version that works with generics:

// Generic method to concatenate arrays of the same type in Java

public static<T> T[] concatenate(T[] first, T[] second)

{

T[] ob = (T[]) Array.newInstance(first.getClass().getComponentType(),

first.length + second.length);

System.arraycopy(first, 0, ob, 0, first.length);

System.arraycopy(second, 0, ob, first.length, second.length);

return ob;

}

Here’s another generic version that uses Arrays.copyOf() along with System.arraycopy():

// Generic method to concatenate arrays of the same type in Java

public static<T> T[] concatenate(T[] first, T[] second)

{

T[] result = Arrays.copyOf(first, first.length + second.length);

System.arraycopy(second, 0, result, first.length, second.length);

return result;

}

3. Using List

We can also use a list to concatenate two arrays, as shown below. This approach is not suggested over Java 8, and the System.arraycopy() method discussed earlier since it involves creating an intermediary list object.

// Method to concatenate two arrays in Java 8 and above

public static String[] concatenate(String[] first, String[] second)

{

List<String> result = new ArrayList<>();

Stream.of(first, second)

.flatMap(Stream::of)

.map(x -> (String)x)

.forEach(result::add);

return result.toArray(new String[0]);

}

For Java 7 and before, we can use Collections.addAll() method:

// Method to concatenate two arrays in Java

public static String[] concatenate(String[] first, String[] second)

{

List<String> list = new ArrayList<>();

Collections.addAll(list, first);

Collections.addAll(list, second);

return list.toArray(new String[0]);

}

// Method to concatenate two arrays in Java

public static String[] concatenate(String[] first, String[] second)

{

List<String> list = new ArrayList<>(Arrays.asList(first));

Collections.addAll(list, second);

return list.toArray(new String[0]);

}

4. Using Guava Library

Guava library provides ObjectArrays class that has the concat() method, which returns a new array of the specified type containing concatenated contents of two arrays.

// Method to concatenate two arrays in Java

public static String[] concatenate(String[] first, String[] second) {

return ObjectArrays.concat(first, second, String.class);

}

That’s all about concatenating two arrays in Java.

Concatenate two arrays in Java

1. Using Java 8 Stream

2. Using System.arraycopy() method

3. Using List

4. Using Guava Library

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2. Using `System.arraycopy()` method