null pointer exception problems
can any one tell me why i keep getting this error?
publicvoid mergesort(int[] a,int l,int r){
if(r <= l)return;
int m = (r+l)/2;
mergesort(a,l,m);
mergesort(a, m+1,r);
merge(a,l,m,r);
}
int[] aux;
publicvoid merge(int[] a,int l,int m,int r){
int i, j;
for(i = m+1; i > l; i--) aux[i-1] = a[i-1];
for(j = m; j < r; j++) aux[r+m-j] = a[j+1];
for(int k = l; k <= r; k++)
if(less(aux[j], aux[i]))
a[k] = aux[j--];else a[k] = aux[i++];
}
heres the class that calls it
publicclass QSort{
int x = 25000;
int[] a =newint[x];
publicvoid createArray(){
Random myRandom =new Random();
for(int i = 0; i < x; i++){
a[i] = myRandom.nextInt(100);
}
}
publicvoid getMergeTime(){
Quicksort mySort =new Quicksort();
createArray();
long beginningTime = System.currentTimeMillis();
mySort.mergesort(a,7, x -1);
long endingTime = System.currentTimeMillis();
long difference = endingTime - beginningTime;
System.out.println("Time = " + difference);
}
