我得到一个
Exception in thread "main" java.lang.NullPointerException
at datastructuresPart2.LinkedStack.pop(LinkedStack.java:20)
at tests.StackTest.main(StackTest.java:66)
尝试实现链接堆栈时。几乎相同的代码在不同的项目中工作，所以我不确定发生了什么。我将发布所有相关代码。（我已经成功地将它作为一个数组运行。但我需要使用一个链表......）
package datastructuresPart2;
import datastructuresPart2.Node;
public class LinkedStack<E> extends AbstractCollection<E> implements Stack<E> {
private Node<E> top = null;
@Override
public void push(E element) {
top = new Node<E>(element, top);
size++;
}
@Override
public E pop() {
if (isEmpty())
throw new EmptyCollectionException("empty stack");
E element = top.data;
top = top.next;
size--;
return element;
}
@Override
public E top() {
if (isEmpty())
throw new EmptyCollectionException("empty stack");
return top.data;
}
@Override
public void clear() {
super.clear();
top = null;
}
public boolean contains(E element) {
for (Node<E> current = top; current != null; current = current.next)
if (element.equals(current.data))
return true;
return false;
}
// Returns a string representation for this collection.
@Override
public String toString() {
String buffer = "[";
if (! isEmpty()) {
buffer += top.data;
for (Node<E> current = top.next; current != null; current = current.next)
buffer += ", " + current.data;
}
return buffer + "]";
}
@Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (getClass() != obj.getClass())
return false;
@SuppressWarnings("unchecked")
LinkedStack<E> other = (LinkedStack<E>) obj;
int count = 0;
for (Node<E> trav = other.top; trav != null; trav = trav.next){
if (top.getData().equals(trav.getData())) {
count++;
}
top = top.getNext();
}
if (count == size)
return true;
else
return false;
}
}
我也使用：
/*
* StackTest.java
* This source file contains a class that tests the Stack interface and
* implementation.
*/
package tests;
import datastructuresPart2.*;
public class StackTest {
// Serves as the entry point for this application.
public static void main(String[] args) {
Stack<String> stk = new LinkedStack<>();
Stack<String> stk2 = new LinkedStack<>();
System.out.println("After creating a new stack...");
CollectionTest.print(stk);
System.out.println();
stk.push("cat");
stk.push("dog");
stk.push("tree");
stk.push("house");
stk.push("boat");
stk.push("woman");
stk.push("man");
stk.push("car");
stk.push("pool");
stk.push("motorcycle");
stk.push("mailbox");
System.out.println("After adding some elements...");
CollectionTest.print(stk);
System.out.println();
System.out.println("After creating a new stack2...");
CollectionTest.print(stk2);
System.out.println();
stk2.push("cat");
stk2.push("dog");
stk2.push("tree");
stk2.push("house");
stk2.push("boat");
stk2.push("woman");
stk2.push("man");
stk2.push("car");
stk2.push("pool");
stk2.push("motorcycle");
stk2.push("mailbox");
System.out.println("The top element is " + stk.top());
System.out.println("Does it contains a man? " + stk.contains("man"));
System.out.println();
System.out.println("Are the stacks equal? " + stk.equals(stk2));
// System.out.println("Traversing the stack...");
// for (String element : stk)
// System.out.println(element);
// System.out.println();
System.out.println("Removing: " + stk.pop());
System.out.println("Removing: " + stk.pop());
System.out.println("After removing the top two elements...");
CollectionTest.print(stk);
System.out.println();
System.out.println("Are the stacks equal? " + stk.equals(stk2));
System.out.println();
System.out.println("The top element is " + stk.top());
System.out.println("Does it contains a man? " + stk.contains("man"));
System.out.println();
stk.clear();
System.out.println("After clearing the stack...");
CollectionTest.print(stk);
System.out.println();
System.out.println("Trying to get the top element...");
try {
System.out.println("The top element is " + stk.top());
}
catch (EmptyCollectionException e) {
System.out.println("Error: " + e.getMessage());
}
}
}
任何帮助是极大的赞赏！
编辑：
节点.java
package datastructuresPart2;
public class Node<T> {
T data;
Node<T> next;
public Node(T data, Node<T> next) {
this.data = data;
this.next = next;
}
public T getData() { return data; }
public void setData(T data) { this.data = data; }
public Node<T> getNext() { return next; }
public void setNext(Node<T> next) { this.next = next; }
@Override
public String toString() {
return data + "--->" + next; //recursion implicita
}
}
AbstractCollection.java
package datastructuresPart2;
public abstract class AbstractCollection<E> implements Collection<E> {
protected int size = 0;
@Override
public int size() { return size; }
@Override
public void clear() { size = 0; }
@Override
public boolean isEmpty() { return (size == 0); }
@Override public abstract boolean contains(E element);
@Override public abstract String toString();
@Override public abstract boolean equals(Object obj);
}
编辑：和 Collection.java ...不妨把整个事情大声笑。
package datastructuresPart2;
public interface Collection<E> {
int size();
boolean isEmpty();
boolean contains(E element);
void clear();
}