Hey guys so i'm working on a lab assignment for my programming class and we have to create a heap where the user enters integers into an array then displays it, then we are suppose to use those same values and use HeapSort well the first part was fairly easy, i'm having trouble trying to call the HeapSort method in order to sort the array everytime i do i come up with this error
Exception in thread "main" java.lang.NullPointerException
at HeapApp.heapSort(HeapApp.java:11)
at HeapApp.main(HeapApp.java:88)
The error specifically points out
int count = hp.length;
and
HeapApp.heapSort(HP);
please help! Its one of my last assignments for this class!
Heap Class
import java.util.ArrayList;
import java.util.NoSuchElementException;
public class Heap<T extends Comparable<T>> {
private ArrayList<T> items;
public Heap() {
items = new ArrayList<T>();
}
private void siftUp() {
int k = items.size() - 1;
while (k > 0) {
int p = (k-1)/2;
T item = items.get(k);
T parent = items.get(p);
if (item.compareTo(parent) > 0) {
// swap
items.set(k, parent);
items.set(p, item);
// move up one level
k = p;
} else {
break;
}
}
}
public void insert(T item) {
items.add(item);
siftUp();
}
private void siftDown() {
int k = 0;
int l = 2*k+1;
while (l < items.size()) {
int max=l, r=l+1;
if (r < items.size()) { // there is a right child
if (items.get(r).compareTo(items.get(l)) > 0) {
max++;
}
}
if (items.get(k).compareTo(items.get(max)) < 0) {
// switch
T temp = items.get(k);
items.set(k, items.get(max));
items.set(max, temp);
k = max;
l = 2*k+1;
} else {
break;
}
}
}
public T delete()
throws NoSuchElementException {
if (items.size() == 0) {
throw new NoSuchElementException();
}
if (items.size() == 1) {
return items.remove(0);
}
T hold = items.get(0);
items.set(0, items.remove(items.size()-1));
siftDown();
return hold;
}
public int size() {
return items.size();
}
public boolean isEmpty() {
return items.isEmpty();
}
public String toString() {
return items.toString();
}
}
import java.util.Scanner;
public class HeapApp {
/**
* @param args
*/
public static void main(String[] args) {
Heap<Integer> hp = new Heap<Integer>();
Scanner sc = new Scanner(System.in);
HeapApp HP = new HeapApp();
System.out.print("Enter next int, 'done' to stop: ");
String line = sc.next();
while (!line.equals("done")) {
hp.insert(Integer.parseInt(line));
System.out.println(hp);
System.out.print("Enter next int, 'done' to stop: ");
line = sc.next();
}
while (hp.isEmpty()) {
//int max = hp.delete();
System.out.println(hp);
}
System.out.println(hp);
HP.heapSort(HP);
System.out.println("After sorting " + hp);
}
private static int [] hp;
public static void heapSort(HeapApp HP){
int count = hp.length;
//first place a in max-heap order
heapify(hp, count);
int end = count - 1;
while(end > 0){
//swap the root(maximum value) of the heap with the
//last element of the heap
int tmp = hp[end];
hp[end] = hp[0];
hp[0] = tmp;
//put the heap back in max-heap order
siftDown(hp, 0, end - 1);
//decrement the size of the heap so that the previous
//max value will stay in its proper place
end--;
}
}
public static void heapify(int[] hp, int count){
//start is assigned the index in a of the last parent node
int start = (count - 2) / 2; //binary heap
while(start >= 0){
//sift down the node at index start to the proper place
//such that all nodes below the start index are in heap
//order
siftDown(hp, start, count - 1);
start--;
}
//after sifting down the root all nodes/elements are in heap order
}
public static void siftDown(int[] hp, int start, int end){
//end represents the limit of how far down the heap to sift
int root = start;
while((root * 2 + 1) <= end){ //While the root has at least one child
int child = root * 2 + 1; //root*2+1 points to the left child
//if the child has a sibling and the child's value is less than its sibling's...
if(child + 1 <= end && hp[child] < hp[child + 1])
child = child + 1; //... then point to the right child instead
if(hp[root] < hp[child]){ //out of max-heap order
int tmp = hp[root];
hp[root] = hp[child];
hp[child] = tmp;
root = child; //repeat to continue sifting down the child now
}else
return;
}
}
}
