java - ArrayExceptionOutOfBounds when returning the method -

i attempting create fillarray method fills , array 20 random values , sums every third value. getting arrayexceptionoutofbounds @ line 21 when method called. through debug have watched array fill proper values , sum calculated. wondering error is.

public static void fillarray(){     //adding a[0], a[3], a[6}, ...      double[] = new double[20];     for(int = 0; < a.length; i++)         a[i] = math.random();      double sum = 0;     int k = 0;     do{         k += 3;         sum += a[k];     }while(k < 20);      system.out.println("sum = " + sum); } 

again looking determine reason error not way fix it.

here problem:

do{     k += 3;     sum += a[k]; }while(k < 20); 

k equal 0, 3, 6, etc, until reaches 21 , try , access a[21] out of bounds.

this because when k = 18 on 6th iteration of while loop, (k < 20) true , therefore while loop continues , adds 3 k making 21. after that, while loop stops k not less 20 anymore leaving k value of 21.