为什么Java不能转换ArrayList< TreeSet< Integer>>到列表< Set< Object>> ;? [英] Why can't Java convert an ArrayList<TreeSet<Integer>> into a List<Set<Object>>?
问题描述
我试图做一些看起来很简单的事情:总结一组列表的大小。 Netbeans给出了以下警告/错误:
pre $ 实际参数java.util.ArrayList< java.util.TreeSet< java.lang。整数>>不能转换成java.util.List< java.util.Set< java.lang.Object>>通过方法调用转换
获得以下两段代码:
/ **
*汇总列表中所有组的大小。请注意,尽管在单个集合中将有
*没有重复元素,但姐妹集合可能包含
*元素,所以返回的值不是**等于唯一$ b的数目所有集合中的$ b *元素。
* @param list,集合列表
* @return所有集合中包含的元素数量
* /
public static int sizeOfListOfSets(List< Set< Object>> ; list){
int size = 0;
for(Set< Object> set:list){
size + = set.size();
}
返回大小;
}
然后用以下方式调用它:
ArrayList< TreeSet< Integer>> testList = new ArrayList< TreeSet< Integer>>();
TreeSet<整数>测试集;
int size = 0;
testSet = new TreeSet< Integer>();
testSet.add(new Integer(++ size));
testSet.add(new Integer(++ size));
testList.add(testSet);
testSet = new TreeSet< Integer>();
testSet.add(new Integer(++ size));
testList.add(testSet);
int expResult = size;
int result = Helpers.sizeOfListOfSets(testList);
最后一行给出编译错误:
错误:方法sizeOfListOfSets类Helpers不能应用于给定的类型;
1错误
所以,为什么java.lang.Integer不能转换为java .lang.Object?
A 列表< Integer>
a List< Object>
。如果 Java
允许,那么你可以用 List< String>
调用该方法,这样你就会被破坏。正如 Jeremy Heiler
指出的,您可以使用 List<扩展对象>
,你会没事的。这意味着允许扩展 Object
的每种类型。 ?
在通用术语中称为通配符
。
I'm attempting to do something that seems rather simple: sum the sizes of a list of sets. Netbeans gives the following warning/error:
actual argument java.util.ArrayList<java.util.TreeSet<java.lang.Integer>> cannot be converted to java.util.List<java.util.Set<java.lang.Object>> by method invocation conversion
for the following two pieces of code:
/**
* Sums the sizes of all sets in a list. Note that while there will be
* no duplicate elements in a single set, "sister" sets may contain
* elements, so the value returned is **not** equal to the number of unique
* elements in all sets.
* @param list, a List of Sets
* @return the number of elements contained in all sets
*/
public static int sizeOfListOfSets(List<Set<Object>> list) {
int size = 0;
for (Set<Object> set : list) {
size += set.size();
}
return size;
}
and then calling it with the following:
ArrayList<TreeSet<Integer>> testList = new ArrayList<TreeSet<Integer>>();
TreeSet<Integer> testSet;
int size = 0;
testSet = new TreeSet<Integer>();
testSet.add(new Integer(++size));
testSet.add(new Integer(++size));
testList.add(testSet);
testSet = new TreeSet<Integer>();
testSet.add(new Integer(++size));
testList.add(testSet);
int expResult = size;
int result = Helpers.sizeOfListOfSets(testList);
the last line gives the compilation error:
error: method sizeOfListOfSets in class Helpers cannot be applied to given types;
1 error
So, why can't java.lang.Integer be converted to java.lang.Object?
A List<Integer>
is not a List<Object>
. If Java
allowed that, then you could call the method with List<String>
and you will be broken. As Jeremy Heiler
pointed out you can use List<? extends Object>
and you will be fine. This means every type which extends Object
is allowed. ?
is called a wildcard
in generic jargon.
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