从对象向下转换为整数运行时错误：java.lang.ClassCastException
运行时异常 - java.lang.ClassCastingException...
Integer intArr[] = new Integer[arrList.size()];
ArrayList <Integer> arrList =new ArrayList();
intArr=(Integer[])arrList.toArray(); // returns Object class which is downcaste to Integer;
我知道向下转换并不安全，但为什么会发生这种情况？ 我还尝试将 ArrayList 转换为 String 到 Integer 到 int，但我得到了同样的错误。
Run time exception-- java.lang.ClassCastingException...
Integer intArr[] = new Integer[arrList.size()];
ArrayList <Integer> arrList =new ArrayList();
intArr=(Integer[])arrList.toArray(); // returns Object class which is downcaste to Integer;
I understand down-casting is not safe but why is this happening?
I also tried to converting ArrayList to String to Integer to int, but I get the same error.
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尝试这样做，
您得到的是一个类型化的
Integer数组，而不是一个对象数组。Try to do this
What you get is a typed
IntegerArray and not a Object array.首先，这不会将 ArrayList 绑定到 Integer 类型。
相反，这就是发生的情况，
arrList被分配给原始类型的ArrayList，但这不是问题。问题在于，
由于
arrList是原始类型（由于赋值，它被编译器分配为new ArrayList尝试将
arrList分配给new ArrayList() 并执行以下操作：First of all, this doesn't bind the ArrayList to type
Integer.Instead, this is what happens,
arrListis assigned to anArrayListof raw type, but that isn't a problem.The problem lies in,
since
arrListis a raw-type (due to the assignment, it gets assigned asnew ArrayList<Object>()by the compiler), you're effectively getting anObject[]instead.Try assigning
arrListtonew ArrayList<Integer>()and do this:这里的问题是您正在尝试将对象数组转换为整数数组。 Array 本身就是一个对象，
Integer[]不是ArrayList的子类，反之亦然。在您的情况下，您需要做的是转换单个项目，如下所示：当然，如果数组列表中的单个元素不是 Integer 类型，您可能会得到 ClassCastException。
The problem here is that you are trying to convert an array of objects to an array of integers. Array is an object in itself and
Integer[]is not a sub-class ofArrayList, nor vice versa. What you have to do in your case is cast individual items, something like this:Naturally, you may get ClassCastException if individual elements in the array list are not of type Integer.
toArray(T[] a) 接受一个参数：
“a - 要存储列表元素的数组（如果足够大）；否则，是相同 runt 的新数组”
toArray(T[] a)takes a paramter:"a - the array into which the elements of the list are to be stored, if it is big enough; otherwise, a new array of the same runt"