对于每个循环无法初始化数组中的对象 [英] for each loop cannot initialize objects in array

问题描述

我将迅速解决这个问题.我有一堂简单的课

I will dive quick to the problem. I have a straightforward class

class Vector{
    float x, y;
}

另一个类将这些对象的数组作为其成员

and another class has an array of these objects as its member

Vector[] buffer;

我这样初始化它:

buffer = new Vector[8];
for(Vector v: buffer)
    v = new Vector();

但是当我尝试访问此数组中该对象的成员时，我得到一个NullPointerException直接指向我的堆栈跟踪.也就是说，尚未构造数组的对象.另一方面，这种更为传统的代码非常完美:

but when I try to access this objects' members in this array I get a NullPointerException straight to my stack trace. That is, objects of array have not been constructed. On the other hand this more traditional code works just perfect:

buffer = new Vector[8];
for(int i = 0; i<8; i++)
    buffer[i] = new Vector;

正如此讨论所指出的，两者都应编译后保持不变.

As this discussion points it out, both should be the same after compilation.

我的问题是，为什么每个循环都无法从item数组初始化/构造对象?

My question is, why for each loop fails to initialize/construct objects from the item array?

推荐答案

在每个示例中，您将覆盖循环的局部变量，该局部变量不会保存回数组中.这类似于您的第二个循环:

In your for-each example you are overwriting the local variable of the loop which does not get saved back into the array. It is similar to in your second loop going:

for(int i = 0; i < buffer.length; i++){
    Vector v = buffer[i];
    v = new Vector();
}

查看了解Java中的每个循环，了解基本相同的问题

Check out Understanding for each loop in Java for basically the same issue.

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