List list = new ArrayList<Integer>();
ListIterator<Integer> litr = null;
list.add("A");
list.add("1");
list.add(5);
litr = list.listIterator();
while(litr.hasNext()){
System.out.println("UIterating " + litr.next());
}
我希望它抛出一个ClassCastException,而是将它写入控制台
A 1 5
这看起来很奇怪.我试过的时候：
List<Integer> list = new ArrayList<Integer>();
我有一个编译时错误.
如果有人能解释如何将String对象添加到ArrayList,我将不胜感激
解决方法
在任何情况下,这都不会产生ClassCastException,泛型只会影响编译.在运行时
将类型放在list变量上的情况：
List<Integer> list = new ArrayList<Integer>();
它是首选,它应该生成一个编译器错误,告诉你在集合中输入了错误的类型.
在this article中描述了遗留,非通用代码和通用代码如何互操作：
In proper generic code,Collection would always be accompanied by a type parameter. When a generic type like Collection is used without a type parameter,it’s called a raw type.
Most people’s first instinct is that Collection really means
Collection<Object>. However,as we saw earlier,it isn’t safe to pass aCollection<Part>in a place where aCollection<Object>is required. It’s more accurate to say that the type Collection denotes a collection of some unknown type,just likeCollection<?>.But wait,that can’t be right either! Consider the call to
getParts(),which returns a Collection. This is then assigned to k,which is aCollection<Part>. If the result of the call is aCollection<?>,the assignment would be an error.In reality,the assignment is legal,but it generates an unchecked warning. The warning is needed,because the fact is that the compiler can’t guarantee its correctness. We have no way of checking the legacy code in
getAssembly()to ensure that indeed the collection being returned is a collection of Parts. The type used in the code is Collection,and one could legally insert all kinds of objects into such a collection.So,shouldn’t this be an error? Theoretically speaking,yes; but practically speaking,if generic code is going to call legacy code,this has to be allowed. It’s up to you,the programmer,to satisfy yourself that in this case,the assignment is safe because the contract of
getAssembly()says it returns a collection of Parts,even though the type signature doesn’t show this.
