java 如何创建 List<Integer> 类型的对象
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How to create an Object of List<Integer> type
提问by Ian Brandon Anderson
I am having trouble creating and using objects to type List<Integer>. When I run the following code, I get a NullPointerExceptionbecause the Object isn't initialized.
我在创建和使用对象键入时遇到问题List<Integer>。当我运行以下代码时，我得到一个NullPointerException因为对象未初始化。
import java.util.List;
public class RBTree {
public static class Tree {
public List<Integer> parent;
public List<Integer> right;
public List<Integer> left;
public List<Integer> data;
public List<Boolean> black;
}
public static void main (String[] args){
Tree rb =new Tree();
rb.data.add(-1);
rb.left.add(-1);
rb.right.add(-1);
rb.parent.add(-1);
rb.black.add(Boolean.TRUE);
}
}
The compiler also gives me errors unless I add staticto the public static class Treeline, but I don't want Treeto be statici.e. immutable. I need to be able to use a Tree more-or-less like a structin C.
除非我添加static到public static class Tree行中，否则编译器也会给我错误，但我不想Tree成为static不可变的。我需要能够或多或少地像structC 中的a 一样使用 Tree 。
回答by Oliver Charlesworth
So far, you've only created a reference, there's no underlying object. Try the following:
到目前为止，您只创建了一个引用，没有底层对象。请尝试以下操作：
public List<Integer> parent = new ArrayList<Integer>();
// etc.
回答by dasblinkenlight
staticin public static classnested type declaration means that Treeobjects can be created outside the context of the RBTreeinstance. Your errors have nothing to do with static: you get NPEs because your lists are not initialized. You can add initialization in the constructor of the Tree, or add an initializer.
staticinpublic static class嵌套类型声明意味着Tree可以在RBTree实例的上下文之外创建对象。您的错误与static以下内容无关：您得到 NPE，因为您的列表未初始化。您可以在 的构造函数中添加初始化Tree，或添加初始化程序。
回答by Adi
you forgot to create the actual list objects:
你忘了创建实际的列表对象：
public List<Integer> parent = new ArrayList<Integer>();
public List<Integer> right = new ArrayList<Integer>();
public List<Integer> left = new ArrayList<Integer>();
public List<Integer> data = new ArrayList<Integer>();
public List<Boolean> black = new ArrayList<Boolean>();
if you don't do this, then your lists are null and access a property or method on someting which is not there produces a NullPointerException.
如果您不这样做，那么您的列表将为空，并且访问某个不存在的属性或方法会产生 NullPointerException。
