Java - 数组中的 NullPointerException [英] Java - NullPointerException in Array

问题描述

我遇到了以下问题:我有一个带有私有成员的 java 类，如下所示:

I have encountered the following problem: I have a java class with a private member like so:

private Arcs[] arcs;

这不是在构造函数中初始化，因为我还不知道向量的长度，但它是在 read 函数中初始化的，我从文件中读取信息.在这个函数中，我执行以下操作:

This is not initialised in the constructor because I don't know the length of my vector yet, but it is initialised in the read function, where I read the info from a file. In this function I do the following:

arcs = new Arcs[n]; //n is a number read from file

然后有一个while循环，我从文件中读取其他内容，我有类似的东西:

Then there is a while cycle in which I read other stuff from the file and I have something like:

while(condition){
...
arcs[i].add(blah); //i is a valid number, smaller than n, and the add function is also correct
...
}

但是在这里我有一个错误说 NullPointerException 并且我不明白为什么.如果有人向我解释发生了什么，我将不胜感激.

But here I have an error saying NullPointerException and I don't understand why. I would appreciate it, if someone would explain to me what's happening.

推荐答案

您是否真的在 arcs[i] 中存储了 Arcs 对象?如果不是，arcs[] 的所有元素都将被初始化为 null.(因此是 NPE)

Are you actually ever storing an Arcs object in arcs[i]? If not, all elements of arcs[] will be initialized to null. (Hence the NPE)

做这样的事情:

while(condition){
    // ...
    arcs[i] = new Arcs();
    arcs[i].add(blah); 
    // ...
}

参考:

• Java 教程:数组

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