Arrays Utility Class - Sip of Java

When needing to compare, copy, or sort arrays, a developer’s first instinct might be to write out the loops, if statements, and other logic necessary to handle this operation. This, in most cases, would be a mistake as it would be duplicating the behavior available in the java.util.Arrays utility class. Let’s take a look at how to use the Arrays class for handling everyday operations on arrays.

History of Arrays

The java.util.Arrays was added to the JDK in 2, so it has been a while. It has seen several updates in subsequent releases. The most significant update was in JDK 8; the most recent update was with JDK 9. So if you are still on JDK 8, there might be a few changes you’re unfamiliar with.

Copying Arrays

Arrays provides several options for copying an array with copy(T[], int length).

int[] numbers = new int[] {1,2,3,4,5};
int[] copyOfNumbers = Arrays.copyOf(numbers, numbers.length);
int[] copyOfSomeNumbers = Arrays.copyOf(numbers, numbers.length - 3);
int[] copyOfNumbersWithPadding = Arrays.copyOf(numbers, numbers.length + 1);
  • copyOfNumbers - is an example of a simple copy of an array.

  • copyOfSomeNumbers - is an example of copying only a portion of an array.

  • copyOfNumbersWithPadding - is an example of copying an array and adding padding. The padded values are the default value for the type; in the case of int, that is 0.

Copying Subranges

Arrays also provides an option for copying a specific subrange from an array with copyOfRange(T[]. int startIndex, int endIndex) like in the example below:

int[] numbers = new int[] {1,2,3,4,5};
int[] copyOfNumbers = Arrays.copyOfRange(numbers, 0, 3);

Converting Arrays to Other Types

Arrays provides several methods for converting an array to a different type, including a List, Stream, Spliterator, and String:

int[] numbers = new int[] { 1, 2, 3, 4, 5 };


Comparing Arrays

For comparing arrays, Arrays provides several options that often provide better results than default options. Let’s take a look:

int[] numbers = new int[] {1,2,3,4,5};
int[] copyOfNumbers = Arrays.copyOf(numbers, 5);
System.out.println(Arrays.equals(numbers, copyOfNumbers));//True

In the above example, numbers and copyOfNumbers hold identical values, but int[].equals(int[]) returns the likely unexpected false. This is because it is comparing by reference, not value. Whereas Arrays.equals(T[]) compares two arrays by the values stored in the arrays.

Comparing Subranges

Like with copying, Arrays also has an option for comparing the subrange of arrays, like in this example:

int[] numbers = new int[] { 1, 2, 3, 4, 5 };
int[] moreNumbers = new int[] { 1, 2, 3, 6, 7 };

System.out.println(numbers.equals(moreNumbers));// False
System.out.println(Arrays.equals(numbers, moreNumbers));// False
System.out.println(Arrays.equals(numbers, 0, 2, moreNumbers, 0, 2));// True

Deep Comparison

When working with nested/multi-dimensional arrays, there is deepEquals(T[]) which step through the comparison of an array and any nested arrays:

int[][] coordinates = new int[][] { {10,20}, {100,200} };
int[][] altCoordinates = new int[][] { {10,20}, {100,200} };
System.out.println(Arrays.equals(coordinates, altCoordinates));//False
System.out.println(Arrays.deepEquals(coordinates, altCoordinates));//True

Comparing Comparable Arrays

Arrays also allows comparing arrays containing primitive values and types implementing Comparable<T> with compare(T[], T[]):

int[] littleNumbers = new int[] {10, 20};
int[] copyOfLittleNumbers = Arrays.copyOf(littleNumbers, 2);
int[] bigNumbers = new int[] {100, 200};
System.out.println(, bigNumbers));//-1
System.out.println(, littleNumbers));//1
System.out.println(, copyOfLittleNumbers));//0

Sorting Arrays

For arrays containing primitives and types implementing Comparable<T>, Arrays can be used to sort the array with sort(T[]). Arrays can also only sort a subrange of an array with sort(T[], startIndex, endIndex). Note that Arrays sorts the passed array; it doesn’t return a copy of a sorted array:

int[] numbers = new int[] { 3, 5, 2, 1, 4 };
int[] moreNumbers = new int[] { 3, 5, 2, 1, 4 };

System.out.println(Arrays.toString(numbers));// 3,5,2,1,4
System.out.println(Arrays.toString(numbers));// 1,2,3,4,5

System.out.println(Arrays.toString(moreNumbers));// 3,5,2,1,4
Arrays.sort(moreNumbers, 0, 3);
System.out.println(Arrays.toString(moreNumbers));// 2,3,5,1,4

Searching Arrays

For sorted arrays, Arrays also provides an operation for searching them with binarySearch(T[], T key) returning the index if the key is matched, -1 if the key is below the range of the array, and the negative of the length of the array + 1 (i.e -6 if the array length is 5) is return if the key is above range. Unsorted arrays will not return expected results. Below is an example of using binarySearch(T[], T key):

int[] numbers = new int[] { 1, 2, 3, 4, 5};

System.out.println(Arrays.binarySearch(numbers, 4)); // 3 - insertion point in array
System.out.println(Arrays.binarySearch(numbers, -0));// -1 - low value not in array
													 // (one less than 0)
System.out.println(Arrays.binarySearch(numbers, 8)); // -6 - high value not in array
													 // (negative value of length + 1)

Other Arrays Operations

There are several other operations Arrays provides that aren’t covered in this article that are also worth checking out:

  • deepHashCode(T[]) - Javadoc
  • deepToString(T[]) - Javadoc
  • fill(T[], T val) - Javadoc
  • hashCode(T[]) - Javadoc
  • mismatch(T[], T[]) - Javadoc
  • parallelSort(T[]) - Javadoc
  • parallelPrefix(T[] array, int fromIndex, int toIndex, BinaryOperator o) - JavaDoc

Additional Reading

Arrays JavaDoc

Happy coding!