[英]Problems with Java generics and collections
I have a class that represents a tree-like structure, the essential bits look like this: 我有一个表示树状结构的类,基本位如下所示:
public Node<T> placeAll(Collection<T> elements){
for (T e : elements)
addElement(e);
// LOG/DEBUG etc
return root;
}
public void addElement(T el) {
Node<T> node = new Node<T>(el);
addElement(root, node);
}
private void addElement(Node<T> parent, Node<T> child) {
// .... PLACE THE NODE
}
Now this works perfectly fine when I place the nodes one by one in a test case: 现在,当我将节点逐个放在测试用例中时,这种方法非常好用:
public void test() {
List<Integer> s1 = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11);
// 13 more lists
List<Integer> s15 = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 221, 251);
Hypergraph<Object> hg = new Hypergraph<>(...);
hg.addElement(s1);
System.out.println(hg.getRoot().toStringTree());
System.out.println();
.
.
.
hg.addElement(s15);
System.out.println(hg.getRoot().toStringTree());
System.out.println();
}
If I add the following line hg.placeAll(Arrays.asList(s1,s2,s3,s4,s5,s6,s7,s8,s9,s10,s11,s12,s13,s14,s15));
如果我添加以下行
hg.placeAll(Arrays.asList(s1,s2,s3,s4,s5,s6,s7,s8,s9,s10,s11,s12,s13,s14,s15));
to my test case, I get an error regarding the use of generics: 对于我的测试用例,我收到有关泛型使用的错误:
The method placeAll(Collection<Object>) in the type Hypergraph<Object> is not applicable for the arguments (List<List<Integer>>)
I don't quite understand this... If addElement(T el)
works fine when I call it with T
resolved to List<Integer>
, why does List<List<Integer>>
comply to placeAll(Collection<T> c)
? 我不太明白这一点...如果我用
T
解析为List<Integer>
调用addElement(T el)
工作正常,为什么List<List<Integer>>
符合placeAll(Collection<T> c)
? Considering that List<T>
is a Collection<T>
I can't make sense out of this.. 考虑到
List<T>
是Collection<T>
我无法理解这一点..
The problem is that the method expects a Collection<Object>
(as T
seems to be Object
in your example), but you are passing a Collection<List<Integer>>
. 问题是该方法需要
Collection<Object>
(在您的示例中T
似乎是Object
),但是您传递的是Collection<List<Integer>>
。 And while a List<Integer>
is an Object
, a Collection<List<Integer>>
is not a subclass of a Collection<Object>
. 虽然
List<Integer>
是一个Object
,但Collection<List<Integer>>
不是 Collection<Object>
的子类。
Change the method signature to accept a Collection<? extends T>
更改方法签名以接受
Collection<? extends T>
Collection<? extends T>
, then it should work. Collection<? extends T>
,然后它应该工作。
public Node<T> placeAll(Collection<? extends T> elements) {
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