Java library's Arrays class

  • The Java library provides many useful operations on arrays

  • These operations provided in the java.util.Arrays class include:

      • Searching for some value stored in an array

      • Sorting an array

      • Comparing the content of 2 arrays

      • Make a copy of an array

      • Converting the data in an array to String representation (for printing)

  • To gain access to the operations in java.util.Arrays, you must import it as follows: 

         import java.util.Arrays

  • You can google the documentation of the java.util.Arrays class:

Searching an array with binary search

  • You can use the binary search method in the Arrays class as follows: 

     Arrays.binarySearch( arrayName, key ): returns the index of array 
                                        element containing key
					neg. index = not found

    Example: 

    import java.util.Arrays;

public class BinSearch
{
   public static void main(String[] args)
   {
     int[] list = {2, 4, 7, 10, 11, 45, 50, 59, 60, 70, 79};

     int index1 = Arrays.binarySearch(list, 11);  // returns 4
     int index2 = Arrays.binarySearch(list, 999); // returns a neg value
   }
}

DEMO: demo/08-array/14-arrays-class/BinSearch.java

Sorting an entrie array

  • You can sort an entire array using: 

        Arrays.sort( arrayName )

    Example: 

    import java.util.Arrays;

public class myProg
{
   public static void main(String[] args)
   {
       double[] myList = {8, 4, 9, 7, 3, 5, 6};
 
       Arrays.sort(myList); // Sort the whole array

       // Result:  myList = { 3, 4, 5, 6, 7, 8, 9 }
   }
}

DEMO: demo/08-array/14-arrays-class/SortArray.java

Sorting a partial array

  • You can sort a part/portion of an array using: 

     Arrays.sort(arrayName,  s ,  e ): sorts elems s to (e−1) in "arrayName"

    Example: 

    import java.util.Arrays;

public class myProg
{
   public static void main(String[] args)
   {
       char[] myList = { 'X', 'B', 'Z', 'A', 'Y', 'C' };
 
       Arrays.sort(myList, 2 , 5 ); // Sort elems 2, 3, 4

       // Result:  myList = { 'X', 'B', 'A', 'Y', 'Z', 'C' }
   }
}

DEMO: demo/08-array/14-arrays-class/SortArrayPart.java

Check if 2 arrays are equal

  • You can check if 2 arrays contains the same list of values with equals( ): 

     Arrays.equals( arrayName1, arrayName2 )

    Example: 

       public static void main(String[] args)
   {
      int[] list1 = {2, 4, 7, 10};
      int[] list2 = {2, 4, 7, 10};
      int[] list3 = {4, 2, 7, 10};

      boolean result;

      result = Arrays.equals(list1, list2); // true
      result = Arrays.equals(list2, list3); // false
   }

DEMO: demo/08-array/14-arrays-class/Equals.java

Make a copy of an part or whole array

  • You can make a copy of an array with copyOf( ): 

     Arrays.copyOf(arrayName, N): returns a copy of the 
                              first N elements in array

    Example: 

    import java.util.Arrays;

public class myProg
{
   public static void main(String[] args)
   {
        int[] list = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
        int[] copy

	copy = Arrays.copyOf(list, 4);    // copy = {1, 2, 3, 4}
	copy = Arrays.copyOf(list, list.length); // Copy entire array
   }
}

DEMO: demo/08-array/14-arrays-class/CopyOf.java

Fill whole or part of array with a specific value

  • You can use the fill( ) method to fill in all or a part of an array: 

     Arrays.fill(arrayName, val):         fill whole array with val
 Arrays.fill(arrayName, s , e , val): fill elems [s]-[e−1] with val

    Example: 

    import java.util.Arrays;

public class myProg
{
   public static void main(String[] args)
   {
      int[] list = new int[10];

      Arrays.fill(list, 4); // Fill whole array with the value 4

      Arrays.fill(list, 2 , 5 , 7); // Fill list[2-4] with the value 7
   }
}

DEMO: demo/08-array/14-arrays-class/Fill.java

Convert all values in an array into a single string (for printing without using a loop)

  • The Arrays.toString(arrayName) method to returns a string representation of all elements in the array: 

     Arrays.toString(arrayName): return a string of 
                             all elements in array

    Example: 2 ways to print an array: 

       public static void main(String[] args)
   {
       int[] list = {1, 2, 3, 4};

       for (int i = 0; i < list.length; i++) // Method 1 to print array
           System.out.println( list[i] );

       // Method 2 to print array
       String s;

       s = Arrays.toString(list); // Convert all elements to a string
       System.out.println(s);
   }

DEMO: demo/08-array/14-arrays-class/ToString.java