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I repeatedly get a StackOverflow error at line marked with ** The stack overflows if I try to sort more than 3 numbers but works for arrays of 3 or less, therefore I do not think there is an infinite recursion.
Can someone explain to me why line 246 seems to be the source of the stack overflow?
Thanks
public static void heapSort(double [] a,int node, int index, boolean upcheck){
if(node < 0){
}
else if(node > a.length-index){
}
else if(upcheck){
if(!testHeap(a,node,(2*node)+1) || !testHeap(a,node,(2*node)+2) ){
int min = 0;
if(a[(2*node)+1] > a[(2*node)+2]){
min = (2*node)+2;
}
else{
min = (2*node)+1;
}
switchHeap(a,node,min);
****************************heapSort(a,(node-1)/2,index,true);*********************
}
}
else if(node == (a.length-index-1)/2){
if((2*node)+1 <= a.length-index){
if(testHeap(a,node,(2*node)+1)){
}
else{
heapSort(a,(node-1)/2,index,true);
}
if((2*node)+2 <= a.length-index){
if(testHeap(a,node,(2*node)+2)){
}
else{
heapSort(a,(node-1)/2,index,true);
}
}
}
switchHeap(a,0,a.length-index);
index++;
heapSort(a,0,index,false);
}
else{
if((2*node)+1 <= a.length-index){
if(testHeap(a,node,(2*node)+1)){
}
else{
heapSort(a,(node-1)/2,index,true);
}
heapSort(a,(2*node)+1,index,false);
if((2*node)+2 <= a.length-index){
if(testHeap(a,node,(2*node)+2)){
}
else{
heapSort(a,(node-1)/2,index,true);
}
heapSort(a,(2*node)+2,index,false);
}
}
}
}//heapSort - method

You are getting StackOverFlowError due to the recursive call you make in your code. i.e.
static void heapSort(double [] a,int node, int index, boolean upcheck)
calls itself recursively. You need to make use of break; in this kind of recursive method calls when the work is done
From the documentation,
public class StackOverflowError
extends VirtualMachineError
Thrown when a stack overflow occurs because an application recurses too deeply.
Please read this link to know more about the error. The accepted answer explains it all

Everytime you call
heapSort(a,(node-1)/2,index,true);
the stack will end up having its own set of variables (double[] , int node, int index...) .. this might be causing the problem if your input size is >3 (too much recursion..).
One solution could be to declare only one array which is to be sorted at instance level, not method level and pass just the indices in your functions instead of passing the array.
this might help - http://www.vogella.com/articles/JavaAlgorithmsMergesort/article.html