Arrays.fill() – Fill Java Arrays

In Java to fill the arrays, the Arrays.fill() method is available. The Arrays.fill() method Assigns the specified value to each element of the specified range of the specified array. There are many overloaded forms of fill() method are given, they are listed here:-

• public static void fill(byte[] a, byte val)
• public static void fill(short[] a, short val)
• public static void fill(int[] a, int val)
• public static void fill(long[] a, long val)
• public static void fill(char[] a, char val)
• public static void fill(float[] a, float val)
• public static void fill(double[] a, double val)
• public static void fill(boolean[] a, boolean val)
• public static void fill(Object[] a, Object val)

• public static void fill(byte[] a, int fromIndex, int toIndex, byte val)
• public static void fill(short[] a, int fromIndex, int toIndex, short val)
• public static void fill(int[] a, int fromIndex, int toIndex, int val)
• public static void fill(long[] a, int fromIndex, int toIndex, long val)
• public static void fill(char[] a, int fromIndex, int toIndex, char val)
• public static void fill(float[] a, int fromIndex, int toIndex, float val)
• public static void fill(double[] a, int fromIndex, int toIndex,double val)
• public static void fill(boolean[] a, int fromIndex, int toIndex, boolean val)
• public static void fill(Object[] a, int fromIndex, int toIndex, Object val)

In these methods the arguments represents,

• a:- the array to be filled.
• val:- the value to be stored in all elements of the array.
• fromIndex:- the index of the first element (inclusive) to be filled with the specified value.
• toIndex:- the index of the last element (exclusive) to be filled with the specified value.

Fill Java Arrays Using Arrays.fill(array, value)

Let us first discuss and demonstrate the fill() method with two arguments.

Arrays.fill(array, value):- It assigns the specified double value to each element of the specified array. Here the given value will be stored in all elements of the given array.

import java.util.Arrays;

public class ArrayTest {
   public static void main(String[] args) {

      // int array example
      int arr[] = {10, 20, 30, 40, 50};
      System.out.println("Original = " + Arrays.toString(arr));
      Arrays.fill(arr, 99);
      System.out.println("After filling = " + Arrays.toString(arr));
   }
}

Output:-

Original = [10, 20, 30, 40, 50]
After filling = [99, 99, 99, 99, 99]

Let us demonstrate some other examples with double, char, and boolean array.

// double array example
double doubelArr[] = {15.9, 20.5, 30.5, 40.5, 55.9};
System.out.println("Original = " + Arrays.toString(doubelArr));
Arrays.fill(doubelArr, 18.9);
System.out.println("After filling = " + Arrays.toString(doubelArr));

Original = [15.9, 20.5, 30.5, 40.5, 55.9]
After filling = [18.9, 18.9, 18.9, 18.9, 18.9]

// char array example
char charArr[]={'K', 'n', 'o', 'w', 'P', 'r', 'o', 'g', 'r', 'a', 'm'};
System.out.println("Original = " + Arrays.toString(charArr));
Arrays.fill(charArr, 'A');
System.out.println("After filling = " + Arrays.toString(charArr));

Original = [K, n, o, w, P, r, o, g, r, a, m]
After filling = [A, A, A, A, A, A, A, A, A, A, A]

// boolean array example
boolean boolArr[] = {true, false, true, false, false};
System.out.println("Original = " + Arrays.toString(boolArr));
Arrays.fill(boolArr, true);
System.out.println("After filling = " + Arrays.toString(boolArr));

Original = [true, false, true, false, false]
After filling = [true, true, true, true, true]

How Arrays.fill() method is implemented? The Arrays.fill() is using normal for loop to iterate through the array and update the array element by given value. The Arrays.fill(int[] a, int val) is implemented as given below. The remaining fill(array, value) is also implemented in a very similar way.

public static void fill(int[] a, int val) {
    for (int i = 0, len = a.length; i < len; i++)
        a[i] = val;
}

Arrays.fill(array, fromIndex, toIndex, value)

The another form of fill() method takes four arguments, where:-

• a:- the array to be filled.
• val:- the value to be stored in all elements of the array.
• fromIndex:- the index of the first element (inclusive) to be filled with the specified value.
• toIndex:- the index of the last element (exclusive) to be filled with the specified value.

Let us demonstrate it through some examples.

// int array example
int arr1[] = {10, 20, 30, 40, 50};
// Example1:- filling first 3 elements
Arrays.fill(arr1, 0, 3, 99);
System.out.println("After filling first 3 elements = " 
                  + Arrays.toString(arr1));

After filling first 3 elements = [99, 99, 99, 40, 50]

int arr2[] = {10, 20, 30, 40, 50};
// Example2:- filling last 3 elements in given array
Arrays.fill(arr2, 2, arr2.length, 99);
System.out.println("After filling last 3 elements = " 
                  + Arrays.toString(arr2));

After filling last 3 elements = [10, 20, 99, 99, 99]

int arr3[] = {10, 20, 30, 40, 50};
// Example3:- filling all elements in given array
Arrays.fill(arr3, 0, arr3.length, 99);
System.out.println("After filling all elements = " 
                  + Arrays.toString(arr3));

After filling all elements = [99, 99, 99, 99, 99]

Exceptions thrown by Arrays.fill(-,-,-,-) method:-

• IllegalArgumentException:- if fromIndex > toIndex
• ArrayIndexOutOfBoundsException:- if fromIndex < 0 or toIndex > a.length

How Arrays.fill() method with four arguments is implemented? The Arrays.fill(-,-,-,-) is implemented similar to Arrays.fill(-,-) but first it checking the range. If fromIndex, toIndex are valid to perform the operation then only it is going to the next step. It is also using normal for loop to iterate through the array from fromIndex to toIndex and update the array element by given value. The Arrays.fill(int[] a, int val) is implemented as given below. The remaining fill(array, value) is also implemented in a very similar way.

public static void fill(int[] a, int fromIndex, int toIndex, int val) {
    rangeCheck(a.length, fromIndex, toIndex);
    for (int i = fromIndex; i < toIndex; i++)
        a[i] = val;
}
static void rangeCheck(int arrayLength, int fromIndex, int toIndex) {
    if (fromIndex > toIndex) {
        throw new IllegalArgumentException(
           "fromIndex(" + fromIndex + ") > toIndex(" + toIndex + ")");
    }
    if (fromIndex < 0) {
        throw new ArrayIndexOutOfBoundsException(fromIndex);
    }
    if (toIndex > arrayLength) {
        throw new ArrayIndexOutOfBoundsException(toIndex);
    }
}

Filling multidimensional array in Java

The Arrays.fill() method directly can be used only for the single-dimensional array. It doesn’t perform deep operations. To fill multi-dimensional array, we need to take the help of loops and Arrays.fill() method. 

Java Program to fill 2D Java array,

import java.util.Arrays;

public class ArrayTest {
   public static void main(String[] args) {
      // 2D array
      int matrix[][] = { { 1, 2 }, { 4, 5 }, { 7, 8 } };
      // filling all elements with 9 in given 2D array
      // using for loop
      for(int i=0; i<matrix.length; i++) {
         Arrays.fill(matrix[i], 9);
      }
      System.out.println("After filling all elements = " 
               + Arrays.deepToString(matrix));
   }
}

Output:-

After filling all elements = [[9, 9], [9, 9], [9, 9]]

Using for each loop,

// using for each loop
for (int[] row : matrix) {
   Arrays.fill(row, 9);
}

Java Program to fill 3D Java array,

// 3D array 2x3x2
int[][][] arr = {{{1,2},{3,4},{5,6}},{{7,8},{9,1},{2,3}}};
// filling all elements with 5 in given 3D array
// using for loop
for (int i = 0; i < arr.length; i++) {
   for (int j = 0; j < arr[i].length; j++) {
      Arrays.fill(arr[i][j], 5);
   }
}
System.out.println("After filling all elements = " 
                  + Arrays.deepToString(arr));

After filling all elements = [[[5, 5], [5, 5], [5, 5]], [[5, 5], [5, 5], [5, 5]]]

Using for each loop,

// using for-each loop
for (int[][] row : arr) {
   for (int[] column : row) {
      Arrays.fill(column, 5);
   }
}

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