The Arrays Class

The java.util.Arrays class contains useful methods for common array operations such as sorting and searching.

The java.util.Arrays class contains various static methods for sorting and searching arrays, comparing arrays, filling array elements, and returning a string representation of the array. These methods are overloaded for all primitive types.

You can use the sort or parallelSort method to sort a whole array or a partial array. For example, the following code sorts an array of numbers and an array of characters.

double[] numbers = {6.0, 4.4, 1.9, 2.9, 3.4, 3.5};
java.util.Arrays.sort(numbers); // Sort the whole array
java.util.Arrays.parallelSort(numbers); // Sort the whole array

char[] chars = {'a', 'A', '4', 'F', 'D', 'P'};
java.util.Arrays.sort(chars, 1, 3); // Sort part of the array
java.util.Arrays.parallelSort(chars, 1, 3); // Sort part of the array

Invoking sort(numbers) sorts the whole array numbers. Invoking sort(chars, 1, 3) sorts a partial array from chars[1] to chars[3-1]. parallelSort is more efficient if your computer has multiple processors.

You can use the binarySearch method to search for a key in an array. The array must be presorted in increasing order. If the key is not in the array, the method returns –(insertionIndex + 1). For example, the following code searches the keys in an array of integers and an array
of characters.

int[] list = {2, 4, 7, 10, 11, 45, 50, 59, 60, 66, 69, 70, 79};
System.out.println("1. Index is " +
java.util.Arrays.binarySearch(list, 11));
System.out.println("2. Index is " +
java.util.Arrays.binarySearch(list, 12));

char[] chars = {'a', 'c', 'g', 'x', 'y', 'z'};
System.out.println("3. Index is " +
java.util.Arrays.binarySearch(chars, 'a'));
System.out.println("4. Index is " +
java.util.Arrays.binarySearch(chars, 't'));

The output of the preceding code is

1. Index is 4
2. Index is -6
3. Index is 0
4. Index is -4

You can use the equals method to check whether two arrays are strictly equal. Two arrays are strictly equal if their corresponding elements are the same. In the following code, list1 and list2 are equal, but list2 and list3 are not.

int[] list1 = {2, 4, 7, 10};
int[] list2 = {2, 4, 7, 10};
int[] list3 = {4, 2, 7, 10};
System.out.println(java.util.Arrays.equals(list1, list2)); // true
System.out.println(java.util.Arrays.equals(list2, list3)); // false

You can use the fill method to fill in all or part of the array. For example, the following code fills list1 with 5 and fills 8 into elements list2[1] through list2[5-1].

int[] list1 = {2, 4, 7, 10};
int[] list2 = {2, 4, 7, 7, 7, 10};
java.util.Arrays.fill(list1, 5); // Fill 5 to the whole array
java.util.Arrays.fill(list2, 1, 5, 8); // Fill 8 to a partial array

You can also use the toString method to return a string that represents all elements in the array. This is a quick and simple way to display all elements in the array. For example, the following code

int[] list = {2, 4, 7, 10};
System.out.println(Arrays.toString(list));

displays [2, 4, 7, 10].