Java：如何根据对象的类型动态创建指定类型的数组？
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Java: How can I dynamically create an array of a specified type based on the type of an object?
提问by JeffV
I would like to take a passed List that I know is homogeneous and from it create an array of the same type as the elements within it.
我想获取一个我知道是同构的传递列表，并从中创建一个与其中的元素类型相同的数组。
Something like...
就像是...
List<Object> lst = new ArrayList<Object>;
lst.add(new Integer(3));
/// somewhere else ...
assert(my_array instanceof Integer[]);
回答by Bozho
The conversion would happen runtime, while the type is lost at compile time. So you should do something like:
转换将在运行时发生，而类型在编译时丢失。所以你应该做这样的事情：
public <T> T[] toArray(List<T> list) {
Class clazz = list.get(0).getClass(); // check for size and null before
T[] array = (T[]) java.lang.reflect.Array.newInstance(clazz, list.size());
return list.toArray(array);
}
But beware that the 3rd line above may throw an exception - it's not typesafe.
但请注意，上面的第三行可能会引发异常 - 它不是类型安全的。
回答by erickson
This method is type safe, and handles some nulls (at least one element must be non-null).
此方法是类型安全的，并处理一些空值（至少一个元素必须为非空值）。
public static Object[] toArray(Collection<?> c)
{
Iterator<?> i = c.iterator();
for (int idx = 0; i.hasNext(); ++idx) {
Object o = i.next();
if (o != null) {
/* Create an array of the type of the first non-null element. */
Class<?> type = o.getClass();
Object[] arr = (Object[]) Array.newInstance(type, c.size());
arr[idx++] = o;
while (i.hasNext()) {
/* Make sure collection is really homogenous with cast() */
arr[idx++] = type.cast(i.next());
}
return arr;
}
}
/* Collection is empty or holds only nulls. */
throw new IllegalArgumentException("Unspecified type.");
}
回答by irreputable
java.lang.reflect.Array.newInstance(Class<?> componentType, int length)
回答by Steven Schlansker
If you need to dynamically create an array based on a type known only at runtime (say you're doing reflection or generics) you'll probably want Array.newInstance
如果您需要根据仅在运行时已知的类型动态创建数组（例如您正在执行反射或泛型），您可能需要Array.newInstance
