由于Java泛型的实现,你不能有这样的代码:
public class GenSet<E> {
private E a[];
public GenSet() {
a = new E[INITIAL_ARRAY_LENGTH]; // error: generic array creation
}
}
如何在保持类型安全的同时实现这一点?
我在Java论坛上看到一个解决方案是这样的:
import java.lang.reflect.Array;
class Stack<T> {
public Stack(Class<T> clazz, int capacity) {
array = (T[])Array.newInstance(clazz, capacity);
}
private final T[] array;
}
但我真的不明白这是怎么回事。
我必须反过来问一个问题:你的发电机组是“检查”还是“未检查”?
这是什么意思?
Checked: strong typing. GenSet knows explicitly what type of objects it contains (i.e. its constructor was explicitly called with a Class<E> argument, and methods will throw an exception when they are passed arguments that are not of type E. See Collections.checkedCollection.
-> in that case, you should write:
public class GenSet<E> {
private E[] a;
public GenSet(Class<E> c, int s) {
// Use Array native method to create array
// of a type only known at run time
@SuppressWarnings("unchecked")
final E[] a = (E[]) Array.newInstance(c, s);
this.a = a;
}
E get(int i) {
return a[i];
}
}
Unchecked: weak typing. No type checking is actually done on any of the objects passed as argument.
-> in that case, you should write
public class GenSet<E> {
private Object[] a;
public GenSet(int s) {
a = new Object[s];
}
E get(int i) {
@SuppressWarnings("unchecked")
final E e = (E) a[i];
return e;
}
}
Note that the component type of the array should be the erasure of the type parameter:
public class GenSet<E extends Foo> { // E has an upper bound of Foo
private Foo[] a; // E erases to Foo, so use Foo[]
public GenSet(int s) {
a = new Foo[s];
}
...
}
所有这些都是由于Java中泛型的一个已知且刻意的弱点:它是使用擦除来实现的,因此“泛型”类不知道它们在运行时是用什么类型参数创建的,因此不能提供类型安全,除非实现了一些显式机制(类型检查)。
要扩展到更多维度,只需将[]和维度参数添加到newInstance() (T是类型参数,cls是Class<T>, d1到d5是整数):
T[] array = (T[])Array.newInstance(cls, d1);
T[][] array = (T[][])Array.newInstance(cls, d1, d2);
T[][][] array = (T[][][])Array.newInstance(cls, d1, d2, d3);
T[][][][] array = (T[][][][])Array.newInstance(cls, d1, d2, d3, d4);
T[][][][][] array = (T[][][][][])Array.newInstance(cls, d1, d2, d3, d4, d5);
详情请参阅Array.newInstance()。
如果你真的想包装一个固定大小的通用数组,你将有一个方法来添加数据到该数组,因此你可以正确地初始化数组,这样做:
import java.lang.reflect.Array;
class Stack<T> {
private T[] array = null;
private final int capacity = 10; // fixed or pass it in the constructor
private int pos = 0;
public void push(T value) {
if (value == null)
throw new IllegalArgumentException("Stack does not accept nulls");
if (array == null)
array = (T[]) Array.newInstance(value.getClass(), capacity);
// put logic: e.g.
if(pos == capacity)
throw new IllegalStateException("push on full stack");
array[pos++] = value;
}
public T pop() throws IllegalStateException {
if (pos == 0)
throw new IllegalStateException("pop on empty stack");
return array[--pos];
}
}
在这种情况下,您使用java.lang.reflect.Array.newInstance来创建数组,它将不是Object[],而是一个真正的T[]。
您不应该担心它不是最终的,因为它是在类中管理的。
注意,在push()上需要一个非空对象才能获得要使用的类型,因此我添加了一个对您所推送的数据的检查,并在那里抛出异常。
不过,这有点没有意义:通过push存储数据,方法的签名保证只有T个元素可以进入。所以数组是Object[]还是T[]或多或少是无关紧要的。