Java 数组 NullPointerException

Question

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>();
}

其实我只是想将AdjList中的元素复制到一个新的数组subgraph中。但错误消息显示“java.lang.NullPointerException”并显示问题出在“subGraph[i].nodeDelay = AdjList[i].nodeDelay;”线。

我通过打印到控制台进行测试。发现AdjList.length为8，问题出现在第一轮；即使我只写“subGraph[i].nodeDelay;”如果不给它分配任何值，它也会显示错误的消息。对此有什么想法吗？提前致谢。

Answer 1

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])。