我正在嘗試做一些看起來相當簡單的事情:總結一組列表的大小。 Netbeans中給出瞭如下警告/錯誤:爲什麼Java不能將ArrayList <TreeSet <Integer>>轉換爲列表<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
以下兩件代碼:
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);
最後一行給出了:
/**
* 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;
}
,然後用下面的調用它編譯錯誤:
error: method sizeOfListOfSets in class Helpers cannot be applied to given types;
1 error
那麼,爲什麼不能將java.lang.Integer轉換爲java.lang.Object?
正確的問題的標題是, 「爲什麼不能轉換成Java的一個'的ArrayList>'入'名單>'?」 –