为什么我不能访问类数组索引(why can i not access a class array index)

为什么我不能访问类数组索引(why can i not access a class array index)

当我尝试访问我创建的数组的元素并且不知道原因时，我不断收到NullPointerException。 抛出的代码：

TreeNode[] list1; list1 = new TreeNode[1000]; list1[0].edges = new EdgeNode(1); //Throw line

但如果我这样做，它的工作原理：

TreeNode[] list1; list1 = new TreeNode[1000]; list1[0] = new TreeNode(); list1[0].edges = new EdgeNode(1);

而且我不知道为什么。 显然，我可以循环遍历整个数组并制作新的元素，但这不会打败新的点吗？ 另外，如果相关，我已经为TreeNode定义了默认构造函数。

I keep getting a NullPointerException when I try to access elements of an array I created and don't know why. The code that throws:

TreeNode[] list1; list1 = new TreeNode[1000]; list1[0].edges = new EdgeNode(1); //Throw line

but if I do this, it works:

TreeNode[] list1; list1 = new TreeNode[1000]; list1[0] = new TreeNode(); list1[0].edges = new EdgeNode(1);

and I don't know why. Obviously I could for loop through the whole array and make new elements but doesn't that defeat the point of new? Also, if relevant, I have defined the default constructor for TreeNode.

最满意答案

您实际上回答了自己的问题：应该单独初始化数组的元素。 默认情况下，它们对于对象数组为null 。 因此NullPointerException 。

You actually answered your own question: elements of an array should be individually initialized. By default they are null for array of objects. Hence NullPointerException.