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Computational Problem Solving with Java and Interactive Object Graphics

Mark F. Russo, PhD

Section 7.5 Arrays Library

Working with Java arrays can be tedious. Typically, we have to size new arrays and handle each element individually. Fortunately, the Java Core Library includes a class named Arrays providing a long list of static methods that we can use to operate on arrays. The Arrays class static methods include a wide range of utilities that manipulate and manage arrays of many types, including the following. See examples of several of these methods in Listing 7.5.2. Note that the Arrays class is in the java.util.Arrays module and must be imported before its methods may be used.
Table 7.5.1. Common static utility methods of the Arrays class
Arrays method Description
Arrays.copyOf(…) copying an entire array
Arrays.copyOfRange(…) copying a slice of an array
Arrays.equals(…) element-by-element equality test
Arrays.fill(…) filling with values
Arrays.toString(…) converting to a String
Arrays.sort(…) sort array elements
Arrays.binarySearch(…) search array elements 
// ArraysMethods.java
import java.util.Arrays;
import java.util.Random;

public class ArraysMethods {
  public static void main(String[] args) {
    Random rnd = new Random();  // Random object

    // Create an array of 1000 random integers in [100, 500)
    // and print ends
    int[] nums = new int[1000];
    for (int i = 0; i < nums.length; i++) {
      nums[i] = 100 + rnd.nextInt(400);
    }
    printEnds( "nums", nums );

    // Create a copy of the array and print copy
    int[] copy = Arrays.copyOf(nums, nums.length);
    printEnds( "copy", copy );

    // Check if they are the identical array
    System.out.println("nums and copy" + 
        (nums == copy ? " ARE" : " ARE NOT") + " identical");

    // Check equivalence element-by-element
    System.out.println("nums and copy" +
        (Arrays.equals(nums, copy) ? " ARE" : " ARE NOT") + " equivalent");

    // Sort array and print
    Arrays.sort(nums);
    printEnds( "nums (sorted)", nums );

    // Copy array ranges into two arrays and print
    int[] firstHalf  = Arrays.copyOfRange(nums, 0, 500);
    int[] secondHalf = Arrays.copyOfRange(nums, 500, 1000);
    printEnds( "firstHalf ", firstHalf );
    printEnds( "secondHalf", secondHalf );

  }

  // Print array name and elements from both ends of array
  public static void printEnds( String name, int[] arr ) {
    System.out.print(name + ": ");
    for (int i=0; i<5;  i++) {          // Print first 5
      System.out.print( arr[i] + " ");
    }
    System.out.print( "... ");         // Print last 5
    for (int i=arr.length-5; i<arr.length;  i++) {
      System.out.print( arr[i] + " ");
    }
    System.out.println();
  }
}
Listing 7.5.2. ArraysMethods.java
javac ArraysMethods.java
java ArraysMethods
nums: 277 217 458 299 117 ... 482 447 433 136 199 
copy: 277 217 458 299 117 ... 482 447 433 136 199
nums and copy ARE NOT identical
nums and copy ARE equivalent
nums (sorted): 100 100 100 101 102 ... 498 498 498 499 499
firstHalf : 100 100 100 101 102 ... 301 302 302 303 303
secondHalf: 303 304 305 305 305 ... 498 498 498 499 499
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