Java 检查数组是否为空而不获取NullPointerException
当我检查数组是否为空时,得到一个NullPointerException错误。当数组为空时,它应该返回null,但我无法找到它不工作的原因。有没有关于我做错了什么的线索?这是我的全部代码:Java 检查数组是否为空而不获取NullPointerException,java,nullpointerexception,priority-queue,binary-heap,Java,Nullpointerexception,Priority Queue,Binary Heap,当我检查数组是否为空时,得到一个NullPointerException错误。当数组为空时,它应该返回null,但我无法找到它不工作的原因。有没有关于我做错了什么的线索?这是我的全部代码: import java.lang.reflect.Array; public class EventPriorityQueue { class Node { Event event; int priority; Node(Event event,
import java.lang.reflect.Array;
public class EventPriorityQueue {
class Node {
Event event;
int priority;
Node(Event event, int priority) {
this.event = event;
this.priority = priority;
}
}
private int size;
private Node[] array;
public EventPriorityQueue() {
this.size = 0;
array = new Node[20];
}
public int size() {
return size;
}
public void insert(Event event, int priority) {
if (size == 0) {
array[0] = new Node(event, priority);
size++;
return;
}
if (size == array.length) {
Node[] array2 = new Node[size*2];
System.arraycopy(array, 0, array2, 0, array.length);
array = array2;
}
if (size > 0) {
array[size] = new Node(event, priority);
size++;
percolateUp(size - 1);
}
}
private void percolateUp(int index){
int parentIndex = (index - 1) / 2;
while (array[index].priority < array[parentIndex].priority) {
Node temp = array[parentIndex];
array[parentIndex] = array[index];
array[index] = temp;
index = parentIndex;
parentIndex = (index - 1) / 2;
}
}
public Event peek() {
return array[0].event;
}
public Event poll() {
if (array[0] == null) {
return null;
}
Node deletedElement = array[0];
array[0] = array[size - 1];
array[size - 1] = null;
size--;
percolateDown(0);
return deletedElement.event;
}
private void percolateDown(int index) {
int smallest = index;
int rightChildIndex = (index * 2) + 2;
int leftChildIndex = (index * 2) + 1;
if (array[leftChildIndex] != null) {
if (array[leftChildIndex].priority < array[smallest].priority) {
smallest = leftChildIndex;
}
}
if (array[rightChildIndex] != null) {
if (array[rightChildIndex].priority < array[smallest].priority) {
smallest = rightChildIndex;
}
}
if (smallest != index) {
Node temp = array[index];
array[index] = array[smallest];
array[smallest] = temp;
index = smallest;
percolateDown(smallest);
}
}
public int[] toPrioritiesArray() {
// DO NOT CHANGE THIS FUNCTION
int[] result = new int[this.size];
for (int i = 0; i < this.size; i++) {
result[i] = this.array[i].priority;
}
return result;
}
public static boolean checkHeapProperty(int[] priorities) {
for (int i = 0; i < priorities.length; i++) {
if (2 * i + 1 < priorities.length && priorities[i] > priorities[2 * i + 1]) {
return false;
}
if (2 * i + 2 < priorities.length && priorities[i] > priorities[2 * i + 2]) {
return false;
}
}
return true;
}
public static void main(String[] args) {
// initialize the queue
EventPriorityQueue queue = new EventPriorityQueue();
// add some numbers to the queue
int NUM_EVENTS = 1;
for (int i = NUM_EVENTS; i > 0; i--) {
queue.insert(new Event(i), i);
System.out.println("inserted " + i);
}
// poll everything
for (int i = 1; i <= NUM_EVENTS + 1; i++) {
int next = queue.poll().getTime();
System.out.println("polled " + next);
}
}
}
任何提示/帮助都将不胜感激 我不太理解这个问题,但是我想我看到了这个问题,试试这个-
public Event poll(){
如果(array.length==0){
返回null;
}
希望这有帮助!
for(int i=1;i)说明程序逻辑很奇怪。当数组中的项为null时,您希望在poll()中返回null
,可以,但调用poll()的方法不正确
麻烦检查返回值是否为null
。当代码无法正确处理null时,使用null是没有意义的。您的poll()可能重复
不是问题所在。代码中的其他内容不必检查返回值是否为null
,这就是为什么会出现NullPointerException。可能重复int next=queue.poll().getTime();
。真的吗?queue.poll()
可以返回null
。您没有检查它。因此它会引发NullPointerException。正如预期的那样发生。如果您发布了实际的堆栈跟踪,您本可以节省很多人的时间。@Tom哦,天哪,我没有正确阅读问题;(很抱歉,我现在就编辑它。好的,因为这是NPE OP的实际问题。您的代码如何修复它?我特意添加了+1
,以检查在空数组上调用poll()
时,代码是否会正确返回null。在这种情况下,您应该检查(queue.poll()!=null),然后执行queue.poll().getTime();数组[0]==null为true,因为该数组包含null对象。
for (int i = 1; i <= NUM_EVENTS + 1; i++) {
int next = queue.poll().getTime();
System.out.println("polled " + next);
}
for (int i = 1; i <= NUM_EVENTS + 1; i++) {
int next = queue.poll().getTime();
System.out.println("polled " + next);
}