Question

我正在做一个创建链表以形成两个多项式的项目，然后为用户提供了对列表进行数学运算的选项。我已经到了我正在阅读我的列表的位置，并使用堆栈来正确订购它。然后我想确保我的列表被正确读取并且到目前为止这么好，除了我的尾部没有被读入。或者也许它是我的while循环中的语法？

import java.util.*;

public class IntegerNode {

    private int coefficient;
    private int exponent;
    private IntegerNode next;

    public IntegerNode() {
        coefficient = 0;
        exponent = 1;
        next = null;
    }

    public IntegerNode(int c, int e) {
        coefficient = c;
        exponent = e;
        next = null;
    }

    public IntegerNode(int c, int e, IntegerNode n) {
        coefficient = c;
        exponent = e;
        next = n;
    }

    public void setCoefficient(int newCoefficient) {
        coefficient = newCoefficient;
    }

    public void setExponent (int newExponent) {
        exponent = newExponent;
    }

    public int getExponent() {
        return exponent;
    }//end getItem

    public int getCoefficient() {
        return coefficient;
    }//end getItem

    public void SetNext (IntegerNode nextNode) {
        next = nextNode;
    }//end setNext

    public IntegerNode getNext () {
        return next;
    }//end getNext

}//end class IntegerNode

class Driver {

public static void main(String[] args) {

    //First polynomial:
    Scanner sc = new Scanner(System.in);
    System.out.println("Enter coefficient: ");
    int coeff = sc.nextInt();
    System.out.println("Enter exponent: ");
    int exp = sc.nextInt();
    Stack<IntegerNode> p1 = new Stack<IntegerNode>();
    IntegerNode n = new IntegerNode(coeff, exp);
    p1.push(n);
    while (exp != 0){
        System.out.println("Enter coefficient: ");
        coeff = sc.nextInt();
        System.out.println("Enter exponent: ");
        exp = sc.nextInt();
        IntegerNode m = new IntegerNode(coeff, exp);
        p1.push(m);           
    }

    IntegerNode tail = p1.pop();
    IntegerNode head = null;
    while(!p1.empty()){
        head = p1.pop();
        head.SetNext(tail);
        System.out.println(head.getCoefficient() + " to " + tail.getCoefficient());
        tail = head;
    }

    IntegerNode cur = tail;
    while(!(cur.getNext()).equals(null)){
        System.out.println(cur.getCoefficient()+" "+cur.getExponent());
        System.out.println("NEXTNODE: " + cur.getNext().getCoefficient());
        cur = cur.getNext();
    }
    //Read in two integers, check each time if exponent is zero
    //create linked list in reverse order of input
    //Use stack to create LIFO list



    //Repeat above, but for second polynomial

    //Prompt user to choose which operation
    //Prompt user to specify what order if subtracted

    //iterate through p1
    //for every element in p1:
    //iterate through p2 and see if that element matches the current element
    //in p1. If so, then add their coefficients
    //Store results as you go
    //Output the results and indicate what operation was done


    // TODO Auto-generated method stub

}

这是错误：

Exception in thread "main" java.lang.NullPointerException
    at Driver.main(IntegerNode.java:85)

那将是我最后一次循环。有什么想法吗？

Answer 1

while(!(cur.getNext()).equals(null))

而不是上面尝试一次：

while(!(cur.getNext())==null)

永远不能在对象引用上调用.equals（）方法。这是一个方法，就像其他方法一样，所以如果对象引用真的为null，它只会抛出一个NullPointerException。

试试这个：

do{

} while(((cur.getNext())!=null))

Answer 2

首先，当你这样做时：

IntegerNode tail = p1.pop();

......你应该确保有东西可以弹出。

第二，这：

while(!(cur.getNext()).equals(null))

...可能导致您的问题。您正在与null对象进行比较。不能这样做。使用此：

while(cur.getNext() != null)

就打印你的最后一个值而言，你得到了你的最后一个节点，然后你就离开了它们，所以你永远不会打印最后一个节点上的内容。我会使用do{}while()。

无法在链表中打印我的最后一个节点

2 个答案: