爪哇 - NullPointerException异常的Array [英] Java - NullPointerException in Array

问题描述

我也遇到了以下问题：我有一个Java类，象这样一个私有成员：

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

private Arcs[] arcs;

这是不是在构造函数初始化，因为我不知道我的向量的长度还，但它是在读的功能，在这里我从文件中读取的信息初始化。
在这个函数中我做到以下几点：

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

再有就是在我从文件中读取其他的东西了一会儿周期和我有类似：

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
...
}

但在这里我有一个错误，说NullPointerException异常，我不明白为什么。我想AP preciate它，如果有人向我解释发生了什么。

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

您确实曾经存放了弧对象弧线[我] ？如果不是的所有元素弧[] 将被初始化为null。 （因此，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：阵列

• Java Tutorial: Arrays

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