Java 数组 NullPointerException [英] Java array NullPointerException

问题描述

代码块如下:

public static Vertex[] computeSubGraph(Vertex[] AdjList, int[] retiming)
{
    Vertex[] subGraph = new Vertex[AdjList.length];
    for (int i = 0; i < AdjList.length; i++) {
        System.out.println(i);
        subGraph[i].nodeDelay = AdjList[i].nodeDelay;
        subGraph[i].predecessor = AdjList[i].predecessor;
        subGraph[i].mark = AdjList[i].mark;
        subGraph[i].starTime = AdjList[i].starTime;
        subGraph[i].finishTime = AdjList[i].finishTime;
        for (int j = 0; j < AdjList[i].inArcList.size(); j++) {
            ArcNode old = AdjList[i].inArcList.get(j);
            ArcNode newNode = new ArcNode(old.adjVex, old.arcWeight);
            subGraph[i].outArcList.add(newNode);
            subGraph[old.adjVex].inArcList.add(newNode);
        }
    }
    return subGraph;
}

这是顶点类:

public class Vertex implements Comparable<Vertex> {
    public int arcWeight;               
    public int preDelay;                
    public boolean infinite = true;
    public int nodeDelay = 0;
    public Vertex predecessor = null;
    public ArcNode firstArc = null;
    public int mark = 0;
    public int starTime;    
    public int finishTime;
    public ArrayList<ArcNode> inArcList = new ArrayList<ArcNode>();
    public ArrayList<ArcNode> outArcList = new ArrayList<ArcNode>();
}

实际上，我只想将AdjList 中的元素复制到一个新数组子图.但是错误信息显示java.lang.NullPointerException"并显示问题出在subGraph[i].nodeDelay = AdjList[i].nodeDelay;"线.

Actually, I just want to copy the element in AdjList to a new array subgraph. But the error message shows that "java.lang.NullPointerException" and shows the problem lies in "subGraph[i].nodeDelay = AdjList[i].nodeDelay;" line.

我通过打印到控制台进行了测试.并发现 AdjList.length 为 8，问题出现在第一轮；即使我只写subGraph[i].nodeDelay;"没有为其分配任何值，它也会显示错误消息.对此有什么想法吗?提前致谢.

I tested by printing to the console. And found the AdjList.length is 8 and the problem occurs in the very first round; And even when I only write "subGraph[i].nodeDelay;" without assigning any value to it, it also shows the wrong message. Any idea on this? Thanks in advance.

推荐答案

将其添加到循环中:

subGraph[i] = new Vertex();

在访问它之前，您首先需要实例化一个对象(在您的情况下为 subGraph[i]).

You first need to instantiate an object (subGraph[i] in your case) before accessing it.

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