为什么我在比较优先级队列堆中两个索引的行上得到 null?
Why am I getting null on a line that compares two indices in my priority queue heap?
我正在创建一个类型为 T 的优先级队列堆。当我向堆中添加多个整数时,我在 第 55 行 上得到一个空指针异常,这是reheapUp 方法使用比较器来决定哪个整数获得优先权。
我已经坚持了几个小时。起初我以为我必须实现一个通用的比较方法,但这没有意义,因为没有足够具体的东西可以比较。我使用的比较方法来自一个旧项目,我在该项目中制作了一个比较字符串的二叉搜索树图。
/*
* PQHeap.java
* 11/12/18
*/
import java.util.Comparator;
import java.util.*;
public class PQHeap<T>{
private Object[] heap; //hash table
private int heapSize;
private int capacity;
private Comparator<T> comparator;
public PQHeap(Comparator<T> comparator){
heapSize = 0;
capacity = 100;
heap = new Object[capacity];
}
public int size(){
return this.heapSize;
}
public void add(T obj){
ensureCapacity();
//add to lower right most leaf
heap[heapSize++] = obj;
reheapUp();
}
public void ensureCapacity(){
if(heapSize < heap.length/2)
return;
Object newHeap[] = new Object[2*heap.length];
for(int i=0; i<heap.length; i++)
newHeap[i] = heap[i];
heap = newHeap;
}
@SuppressWarnings("unchecked")
private void reheapUp(){
int outOfPlaceInd = heapSize - 1;
while(outOfPlaceInd > 0){
int parentInd = (outOfPlaceInd - 1)/2;
**if (comparator.compare((T)heap[outOfPlaceInd], (T)heap[parentInd]) < 0)**
{
swap(outOfPlaceInd, parentInd);
outOfPlaceInd = (outOfPlaceInd-1)/2;
}
else{
return;
}
}
}
private void swap(int i, int j){
Object copy = heap[i];
heap[i] = heap[j];
heap[j] = copy;
}
@SuppressWarnings("unchecked")
public T remove(){
if(heapSize == 0)
throw new IllegalStateException("Trying to remove from an empty PQ!");
Object p = heap[0];
heap[0] = heap[--heapSize];
reheapDown();
return (T)p;
}
@SuppressWarnings("unchecked")
private void reheapDown(){
int outOfPlaceInd = 0;
int leftInd = 2*outOfPlaceInd+1; //left child
int rightInd = 2*outOfPlaceInd+2; //right child
while(leftInd <= heapSize-1){
int smallerChildInd = leftInd;
if ((rightInd < heapSize) && (comparator.compare((T)heap[rightInd], (T)heap[leftInd]) < 0))
smallerChildInd = rightInd;
// is the parent smaller or equal to the smaller child
int compare = comparator.compare((T)heap[outOfPlaceInd], (T)heap[smallerChildInd]);
// if the parent is larger than the child...swap with smaller child
if (compare > 0)
{
swap(outOfPlaceInd, smallerChildInd);
// update indices
outOfPlaceInd = smallerChildInd;
leftInd = 2*outOfPlaceInd + 1;
rightInd = 2*outOfPlaceInd + 2;
}
else
{
return;
}
}
}
public static void main( String[] args ) {
PQHeap<Integer> pq = new PQHeap<Integer>(new TestIntComparator());
pq.add( 10 );
pq.add( 20 );
System.out.println(pq.size());
pq.add( 20 );
pq.add( 30 );
class TestIntComparator implements Comparator<Integer> {
public TestIntComparator() {;}
public int compare(Integer o1, Integer o2) {
return o1-o2;
}
}
}
}
// class NaturalComparator<T extends Comparable<T>> implements Comparator<T> {
// public int compar(T a, T b) {
// return a.compareTo(b);
// }
// }
在 PQHeap 构造函数中,您没有将输入比较器对象分配给 class 字段。添加这样的行:
this.comparator = comparator;
在你的构造函数中
我正在创建一个类型为 T 的优先级队列堆。当我向堆中添加多个整数时,我在 第 55 行 上得到一个空指针异常,这是reheapUp 方法使用比较器来决定哪个整数获得优先权。 我已经坚持了几个小时。起初我以为我必须实现一个通用的比较方法,但这没有意义,因为没有足够具体的东西可以比较。我使用的比较方法来自一个旧项目,我在该项目中制作了一个比较字符串的二叉搜索树图。
/*
* PQHeap.java
* 11/12/18
*/
import java.util.Comparator;
import java.util.*;
public class PQHeap<T>{
private Object[] heap; //hash table
private int heapSize;
private int capacity;
private Comparator<T> comparator;
public PQHeap(Comparator<T> comparator){
heapSize = 0;
capacity = 100;
heap = new Object[capacity];
}
public int size(){
return this.heapSize;
}
public void add(T obj){
ensureCapacity();
//add to lower right most leaf
heap[heapSize++] = obj;
reheapUp();
}
public void ensureCapacity(){
if(heapSize < heap.length/2)
return;
Object newHeap[] = new Object[2*heap.length];
for(int i=0; i<heap.length; i++)
newHeap[i] = heap[i];
heap = newHeap;
}
@SuppressWarnings("unchecked")
private void reheapUp(){
int outOfPlaceInd = heapSize - 1;
while(outOfPlaceInd > 0){
int parentInd = (outOfPlaceInd - 1)/2;
**if (comparator.compare((T)heap[outOfPlaceInd], (T)heap[parentInd]) < 0)**
{
swap(outOfPlaceInd, parentInd);
outOfPlaceInd = (outOfPlaceInd-1)/2;
}
else{
return;
}
}
}
private void swap(int i, int j){
Object copy = heap[i];
heap[i] = heap[j];
heap[j] = copy;
}
@SuppressWarnings("unchecked")
public T remove(){
if(heapSize == 0)
throw new IllegalStateException("Trying to remove from an empty PQ!");
Object p = heap[0];
heap[0] = heap[--heapSize];
reheapDown();
return (T)p;
}
@SuppressWarnings("unchecked")
private void reheapDown(){
int outOfPlaceInd = 0;
int leftInd = 2*outOfPlaceInd+1; //left child
int rightInd = 2*outOfPlaceInd+2; //right child
while(leftInd <= heapSize-1){
int smallerChildInd = leftInd;
if ((rightInd < heapSize) && (comparator.compare((T)heap[rightInd], (T)heap[leftInd]) < 0))
smallerChildInd = rightInd;
// is the parent smaller or equal to the smaller child
int compare = comparator.compare((T)heap[outOfPlaceInd], (T)heap[smallerChildInd]);
// if the parent is larger than the child...swap with smaller child
if (compare > 0)
{
swap(outOfPlaceInd, smallerChildInd);
// update indices
outOfPlaceInd = smallerChildInd;
leftInd = 2*outOfPlaceInd + 1;
rightInd = 2*outOfPlaceInd + 2;
}
else
{
return;
}
}
}
public static void main( String[] args ) {
PQHeap<Integer> pq = new PQHeap<Integer>(new TestIntComparator());
pq.add( 10 );
pq.add( 20 );
System.out.println(pq.size());
pq.add( 20 );
pq.add( 30 );
class TestIntComparator implements Comparator<Integer> {
public TestIntComparator() {;}
public int compare(Integer o1, Integer o2) {
return o1-o2;
}
}
}
}
// class NaturalComparator<T extends Comparable<T>> implements Comparator<T> {
// public int compar(T a, T b) {
// return a.compareTo(b);
// }
// }
在 PQHeap 构造函数中,您没有将输入比较器对象分配给 class 字段。添加这样的行:
this.comparator = comparator;
在你的构造函数中