嘗試添加多項時遇到NullPointerException

Question

我需要添加兩個多項式，它由兩個整數組成。 例如，系數和指數3x ^ 2將使用3和2作為參數構造。 我得到一個NullPointerException但我無法弄清楚為什么。 任何幫助，將不勝感激！

public class Polynomial {

private Node poly;

public Polynomial() {
}

private Polynomial(Node p) {
    poly = p;
}

private class Term {

    int coefficient;
    int exponent;

    private Term(int coefficient, int exponent) {
        this.coefficient = coefficient;
        this.exponent = exponent;
    }
}

private class Node {

    private Term data;
    private Node next;

    private Node(Term data, Node next) {
        this.data = data;
        this.next = next;
    }
}

public void addTerm(int coeff, int exp) {
    Node pointer = poly;
    if (pointer.next == null) {
        poly.next = new Node(new Term(coeff, exp), null);
    } else {
        while (pointer.next != null) {
            if (pointer.next.data.exponent < exp) {
                Node temp = new Node(new Term(coeff, exp), pointer.next.next);
                pointer.next = temp;
                return;
            }

            pointer = pointer.next;
        }
        pointer.next = new Node(new Term(coeff, exp), null);
    }

}

public Polynomial polyAdd(Polynomial p) {
    return new Polynomial(polyAdd(this.poly, p.poly));

}

private Node polyAdd(Node p1, Node p2) {
    if (p1 == p2) {
        Term adding = new Term(p1.data.coefficient + p2.data.coefficient,
                p1.data.exponent);
        p1 = p1.next;
        p2 = p2.next;
        return new Node(adding, null);
    }
    if (p1.data.exponent > p2.data.exponent) {
        p2 = p2.next;

    }
    if (p1.data.exponent < p2.data.exponent) {
        p1 = p1.next;

    }
    if (p1.next != null && p2.next != null) {
        return polyAdd(p1, p2);


    }
    return new Node(null, null);
}
}

import javax.swing.JOptionPane;

/ ** * * @author Bill Kraynek * / public class Polynomials {

/**
 * @param args the command line arguments
 */
public static void main(String[] args) {
    Polynomial p1 = new Polynomial();
    Polynomial p2 = new Polynomial();
    Polynomial p0 = new Polynomial();
    p1.addTerm(5, 2);
    p1.addTerm(4, 5);
    p1.addTerm(3, 3);
    p1.addTerm(1, 2);
    p1.addTerm(5, 6);
    p2.addTerm(3, 8);
    p2.addTerm(2, 5);
    p2.addTerm(1, 3);
    Polynomial p3 = p1.polyAdd(p2);
    JOptionPane.showMessageDialog(null, "p1 is " + p1 + "\np2 is " + p2 + "\np1+p2 is " + p3);
 // Polynomial p4 = p1.polyMultiply(p2);
//    JOptionPane.showMessageDialog(null, "p1 is " + p1 + "\np2 is " + p2 + "\np1*p2 is " + p4);
//    Polynomial p5 = p2.polyMultiply(p2);
    //JOptionPane.showMessageDialog(null, "p2 is " + p2 + "\np2*p2 is " + p5);
  //  Polynomial p6 = p0.polyMultiply(p2);
    //JOptionPane.showMessageDialog(null, "p0 is " + p0 + "\n" + "p2 is " + p2 + "\np0*p2 is " + p6);
    Polynomial p7 = p0.polyAdd(p2);
    JOptionPane.showMessageDialog(null, "p0 is " + p0 + "\n" + "p2 is " + p2  + "\np0+p2 is " + p7);
    p1 = p1.polyAdd(p2);
    JOptionPane.showMessageDialog(null, "After p1 = p1+p2  p1 is " + p1);
  //  p2 = p2.polyMultiply(p2);
    JOptionPane.showMessageDialog(null,"After p2 = p2*p2  p2 is " + p2);
}

有些行是//因為我還沒有第二種方法

http://users.cis.fiu.edu/~kraynek/COP3337-assignments/Spring-2012/AssignmentFive-Polynomials-Spring-2012.html

Answer 1

節點指針= poly;

由於poly在調用默認的Polynomial構造函數時未實例化，因此無法訪問它。 但你的下一行是：

if（pointer.next == null）{

哪個拋出空指針。 所以你應該檢查那里或禁用默認構造函數。

但這只是一個存在空指針異常的情況。 如果你能提供你的主要課程，那將會非常有幫助。

//編輯：如果你將非默認構造函數設為public並使用那個，你的代碼可以這樣使用：

  Polynomial p = new Polynomial(new Node(new Term(3, 2), null)); Polynomial p2 = new Polynomial(new Node(new Term(3, 2), null)); p.polyAdd(p2); 

沒有任何例外。 但我會考慮重新設計它，因為它是不必要的復雜。