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In java, how to concatenate two arrays

Anonymous User · 2015-07-01T02:04:46.480Z

Need to concatenate two String arrays in Java. What is the easiest way to do this?

Anonymous User 3674 01 Jul 2015

Need to concatenate two String arrays in Java.

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

What is the easiest way to do this?

Ask by Anonymous User

Updated 01 Jul 2015

1 Answers

Anonymous User 01 Jul 2015

Solution 1:

Use Apache Commons Lang library

String[] both = ArrayUtils.addAll(first, second);

Solution 2:

public Foo[] concat(Foo[] a, Foo[] b) {
   int aLen = a.length;
   int bLen = b.length;
   Foo[] c= new Foo[aLen+bLen];
   System.arraycopy(a, 0, c, 0, aLen);
   System.arraycopy(b, 0, c, aLen, bLen);
   return c;
}

Here is a version that works with generics:

public <T> T[] concatenate (T[] a, T[] b) {
    int aLen = a.length;
    int bLen = b.length;
    @SuppressWarnings("unchecked")
    T[] c = (T[]) Array.newInstance(a.getClass().getComponentType(), aLen+bLen);
    System.arraycopy(a, 0, c, 0, aLen);
    System.arraycopy(b, 0, c, aLen, bLen);
    return c;
}

Note like all generics it will not work with primitives but with Objects.

Solution 3:

use System.arraycopy() to ensure that copying large arrays is as fast as possible.

For two arrays it looks like this:

public static <T> T[] concat(T[] first, T[] second) {
  T[] result = Arrays.copyOf(first, first.length + second.length);
  System.arraycopy(second, 0, result, first.length, second.length);
  return result;
}

And for a arbitrary number of arrays (>= 1) it looks like this:

public static <T> T[] concatAll(T[] first, T[]... rest) {
  int totalLength = first.length;
  for (T[] array : rest) {
    totalLength += array.length;
  }
  T[] result = Arrays.copyOf(first, totalLength);
  int offset = first.length;
  for (T[] array : rest) {
    System.arraycopy(array, 0, result, offset, array.length);
    offset += array.length;
  }
  return result;
}

Solution 4:

in Java 8:

String[] both = Stream.concat(Arrays.stream(a), Arrays.stream(b))
                      .toArray(String[]::new);
Or:
String[] both = Stream.of(a, b).flatMap(Stream::of)
                      .toArray(String[]::new);

Solution 5:

Or with the beloved Guava:

String[] both = ObjectArrays.concat(first, second, String.class);

Using the Java API:

Solution 6:

String[] f(String[] first, String[] second) {
    List<String> both = new ArrayList<String>(first.length + second.length);
    Collections.addAll(both, first);
    Collections.addAll(both, second);
    return both.toArray(new String[both.size()]);
}

Solution 7:

without System.arraycopy (not available in GWT client for example):

static String[] concat(String[]... arrays) {
    int lengh = 0;
    for (String[] array : arrays) {
        lengh += array.length;
    }
    String[] result = new String[lengh];
    int pos = 0;
    for (String[] array : arrays) {
        for (String element : array) {
            result[pos] = element;
            pos++;
        }
    }
    return result;
}

Solution 8:

The Functional Java library has an array wrapper class that equips arrays with handy methods like concatenation.

import static fj.data.Array.array;

...and then

Array<String> both = array(first).append(array(second));

To get the unwrapped array back out, call

String[] s = both.array();

Solution 9:

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

both.addAll(Arrays.asList(second));

both.toArray();

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