Java - 如何将类型集合转换为 ArrayList？
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Java - How to convert type collection into ArrayList?
提问by user236691
public class MyGraph<V,E> extends SparseMultigraph<V,E>{
private ArrayList<MyNode> myNodeList;
public MyNode getNode(int nodeId){
myNodeList = new ArrayList<MyNode>();
myNodeList = (ArrayList<MyNode>)this.getVertices();
int i;
The following are the error msg:
以下是错误信息：
Exception in thread "main" java.lang.ClassCastException: java.util.Collections$UnmodifiableCollection cannot be cast to java.util.ArrayList...
线程“main”中的异常 java.lang.ClassCastException: java.util.Collections$UnmodifiableCollection 不能转换为 java.util.ArrayList...
Can anyone help?
任何人都可以帮忙吗？
采纳答案by Chad Okere
As other people have mentioned, ArrayList has a constructor that takes a collection of items, and adds all of them. Here's the documentation:
正如其他人所提到的，ArrayList 有一个构造函数，它接受一组项目，然后添加所有项目。这是文档：
http://java.sun.com/javase/6/docs/api/java/util/ArrayList.html#ArrayList%28java.util.Collection%29
http://java.sun.com/javase/6/docs/api/java/util/ArrayList.html#ArrayList%28java.util.Collection%29
So you need to do:
所以你需要这样做：
ArrayList<MyNode> myNodeList = new ArrayList<MyNode>(this.getVertices());
However, in another comment you said that was giving you a compiler error. It looks like your class MyGraph is a genericclass. And so getVertices() actually returns type V, not type myNode.
但是，在另一条评论中，您说这会给您带来编译器错误。看起来您的类 MyGraph 是一个通用类。因此 getVertices() 实际上返回类型 V，而不是类型 myNode。
I think your code should look like this:
我认为你的代码应该是这样的：
public V getNode(int nodeId){
ArrayList<V> myNodeList = new ArrayList<V>(this.getVertices());
return myNodeList(nodeId);
}
But, that said it's a very inefficient way to extract a node. What you might want to do is store the nodes in a binary tree, then when you get a request for the nth node, you do a binary search.
但是，这表示这是一种非常低效的提取节点的方式。您可能想要做的是将节点存储在二叉树中，然后当您收到第 n 个节点的请求时，您会进行二分查找。
回答by sleske
More information needed for a definitive answer, but this code
确定答案需要更多信息，但此代码
myNodeList = (ArrayList<MyNode>)this.getVertices();
will onlywork if this.getVertices()returns a (subtype of) List<MyNode>. If it is a different collection (like your Exception seems to indicate), you want to use
将只有当工作this.getVertices()回报（亚型）List<MyNode>。如果它是一个不同的集合（就像你的 Exception 似乎表明的那样），你想使用
new ArrayList<MyNode>(this.getVertices())
This will work as long as a Collection type is returned by getVertices.
只要 getVertices 返回 Collection 类型，这就会起作用。
回答by Dwivedi Ji
Try this code
试试这个代码
Convert ArrayList to Collection
将 ArrayList 转换为集合
ArrayList<User> usersArrayList = new ArrayList<User>();
Collection<User> userCollection = new HashSet<User>(usersArrayList);
Convert Collection to ArrayList
将集合转换为 ArrayList
Collection<User> userCollection = new HashSet<User>(usersArrayList);
List<User> userList = new ArrayList<User>(userCollection );
回答by Sandeep Bhardwaj
public <E> List<E> collectionToList(Collection<E> collection)
{
return (collection instanceof List) ? (List<E>) collection : new ArrayList<E>(collection);
}
Use the above method for converting the collection to list
使用上述方法将集合转换为列表
回答by Mircea Stanciu
The following code will fail:
以下代码将失败：
List<String> will_fail = (List<String>)Collections.unmodifiableCollection(new ArrayList<String>());
This instead will work:
这将起作用：
List<String> will_work = new ArrayList<String>(Collections.unmodifiableCollection(new ArrayList<String>()));
