Question

18.12 What does the following program do? (Please explain briefly)

1 // Exercise 18.12: MysteryClass.java

2 public class MysteryClass {

3 public static int mystery(int[] array2, int size)

4 if (size == 1) {

5 return array2[0];

6 }

7 else {

8 return array2[size - 1] + mystery(array2, size - 1);

9 }

10 }

11

12 public static void main(String[] args) {

13 int[] array = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};

14

15 int result = mystery(array, array.length);

16 System.out.printf(“Result is: %d%n”, result);

17 }

18 }

Answer 1

There are some errors in your code.

1) First the code didn't have curly brackets "{ }" over the method mystery().

2) There is an error in the print statement in main method.

After debugging the above errors the output of the code is " The sum of first 10 natural numbers."

Code tracing(Explanation):

• First the program starts execution with main method, in main method there is an array declared with 10 elements.
• From main method there is a method call to mystery method with values array and it's size i.e, mystery(array, array.length)
• mystery() method returns first element of the array if size of the array is 1, else it adds all the elements recursively and return its sum.
• The sum of the first 10 elements is 1+2+3+4+5+6+7+8+9+10 = 55.
• And this sum is returned by mystery method to main method, and main method prints this value and the program is terminated.

​​​​​​​Modified code:

public class MysteryClass { //Mystery class
   public static int mystery(int[] array2, int size){ //mystery method
       if (size == 1) { //returns first elements if size is 1
           return array2[0];
       }
       else { //returns sum of the array
           return array2[size - 1] + mystery(array2, size - 1);
       }
   }
   public static void main(String[] args) { //starts execution from here
       int[] array = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10}; //array declaration
       int result = mystery(array, array.length); //method calling
       System.out.println("Result is: "+result); //printing result
   }
}

Execution: