我正在尝试做一些看起来相当简单的事情:总结一组列表的大小。 Netbeans中给出了如下警告/错误:为什么Java不能将ArrayList <TreeSet <Integer>>转换为列表<Set<Object>>?
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
以下两件代码:
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);
最后一行给出了:
/**
* 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;
}
,然后用下面的调用它编译错误:
error: method sizeOfListOfSets in class Helpers cannot be applied to given types;
1 error
那么,为什么不能将java.lang.Integer转换为java.lang.Object?
正确的问题的标题是, “为什么不能转换成Java的一个'的ArrayList>'入'名单>'?” –