我不确定是否已经回答过这个问题,因为我在“类似问题”下没有看到它,但是在这里...对于一个学校项目,我们必须创建一个由Assignment
和当我执行addAssignment(Assignment a)
函数时,出现了NullPointerException。
在我们的项目中,Assignment
包括作业名称String
,int
代表获得的分数和给出的分数,double
代表百分比等级和给定的字母等级为char
。 Node
包含一个名为“ assignment”,“ next”和“ prev” Assignment
引用的Node
。 DList
(我们的双向链接列表)在开始处由“ head”和“ tail” Node
组成。
相关代码如下。我添加了评论,详细说明了控制台在何处显示了错误消息。
在班级Assignment
:
/**
* setter
* @param n = the name of the assignment
*/
public void setName(String n) {
name = n;
}
在班级Node
:
/**
* default constructor
*/
public Node() {
assignment.setName("");
assignment.setEarnedPts(0);
assignment.setTotalPts(0);
next = prev = null;
}
/**
* n-argument constructor
* @param a = assignment
* @param e = # of earned points
* @param t = # of total possible points
* @param n = "next" node
* @param p = "previous" node
*/
public Node(String name, int e, int t, Node n, Node p) {
assignment.setName(name); // got an error here
assignment.setEarnedPts(e);
assignment.setTotalPts(t);
assignment.setGrade();
next = n;
prev = p;
}
最后,在DList
类中:
/**
* adds an assignment to the list
* @param a = the assignment
*/
public void addAssignment(Assignment a) {
// check to see if this is the first node in the list
Node temp = new Node(a.getName(), a.getEarnedPts(), a.getTotalPts(), tail, head); // got an error here
if (head == tail) {
head = temp;
}
else { // there are other node(s) in the list
Node p = head;
Node q = new Node();
while (p.getNext() != null) {
q = p; // set q equal to p, so that q will equal previous node
p = p.getNext(); // advance p by one node
}
p.setNext(temp);
p.setPrev(q); // ??? ask about this in class later
}
}
是否在其他情况下完成分配初始化?
不幸的是,我们没有完整的代码,但是我怀疑,在您的类Node中,您有一个像这样的字段:
class Node{
Assignment assignment;
如果是这种情况,这就是您的问题:您没有初始化赋值变量的任何值,因此它为Null。 Java不像C ++那样默认初始化。您将必须自己做,Node类的构造函数如下所示:
public Node() {
assignment = new Assignment();
assignment.setName("");
assignment.setEarnedPts(0);
assignment.setTotalPts(0);
next = prev = null;
}
public Node(String name, int e, int t, Node n, Node p) {
assignment = new Assignment();
assignment.setName(name); // got an error here
assignment.setEarnedPts(e);
assignment.setTotalPts(t);
assignment.setGrade();
next = n;
prev = p;
}