Java Combine Arrays

Use for-loops and Collections.addAll to combine arrays into a single array.

Combine arrays. Two arrays exist. We can combine them into a third array. We can do this in different ways. Some are more efficient, and others have simpler syntax.

By creating another array, we can copy all elements into a single region of storage. For-loops can be used to quickly copy these elements.

For-loop example. Let us begin with this example. We have two arrays—they can have any number of elements, but in this example they both have 6 elements.For

New: We create a new array called "merged." This must have the total number of elements required.

For: In the first for-loop we copy the first elements into the merged array. The second for-loop must use an offset.

Tip: The offset for the second for-loop ensures we do not overwrite the elements from the first array.

Java program that uses for-loops to combine arrays import java.util.Arrays; public class Program { public static void main(String[] args) { int[] array1 = { 1, 2, 3, 4, 5 }; int[] array2 = { 5, 4, 3, 2, 1 }; // Create empty array of required length. int[] merged = new int[array1.length + array2.length]; // Copy first array into new array. for (int i = 0; i < array1.length; i++) { merged[i] = array1[i]; } // Copy second array into new array. // ... Use offset to assign elements. for (int i = 0; i < array2.length; i++) { merged[array1.length + i] = array2[i]; } // Print results. System.out.println(Arrays.toString(array1)); System.out.println(Arrays.toString(array2)); System.out.println(Arrays.toString(merged)); } } Output [1, 2, 3, 4, 5] [5, 4, 3, 2, 1] [1, 2, 3, 4, 5, 5, 4, 3, 2, 1]

Collections.addAll. This approach is a little simpler. We create an ArrayList of the required element type. Then we use Collections.addAll to add both arrays to it.ArrayList

Finally: We can convert our ArrayList back into a single array with the toArray method.

Note: We must pass an array reference to the toArray method. This is where our new array will be placed.

Java program that uses Collections.addAll to combine arrays import java.util.ArrayList; import java.util.Arrays; import java.util.Collections; public class Program { public static void main(String[] args) { // Two string arrays we want to combine. String[] array1 = { "cat", "bird", "fish" }; String[] array2 = { "ant", "bee" }; System.out.println(Arrays.toString(array1)); System.out.println(Arrays.toString(array2)); // Create an ArrayList. // ... Add all string arrays with addAll. ArrayList<String> list = new ArrayList<>(); Collections.addAll(list, array1); Collections.addAll(list, array2); // Display ArrayList contents. System.out.println(list.toString()); // Convert ArrayList to String array. // ... This is the final merged array. String[] merged = new String[list.size()]; list.toArray(merged); System.out.println(Arrays.toString(merged)); } } Output [cat, bird, fish] [ant, bee] [cat, bird, fish, ant, bee] [cat, bird, fish, ant, bee]

Notes, Collections.addAll. With Collections.addAll, we cannot add int arrays to an ArrayList of Integers. For int arrays, using for-loops is a better solution.Collections.addAll ArrayList int, Integer

A summary. With for-loops or an ArrayList and Collections.addAll we can combine arrays. For more than two arrays, Collections.addAll may be simpler—fewer offsets will be needed.

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Java Combine Arrays

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