受到here问题的启发,我正在搞一个实验性的收集:
/**
* Pretends to be a Collection of samples from the items.
*
* @param <T>
*/
class Samples<T> extends AbstractCollection<T[]> implements Collection<T[]> {
private final int of;
private final T[] items;
public Samples(int of, T... items) {
this.of = of;
this.items = items;
}
@Override
public int size() {
// I know this is wrong.
return items.length * of;
}
@Override
public Iterator<T[]> iterator() {
// Make the iterator on the fly.
return new Iterator<T[]>() {
// Start at the beginning.
int which = 0;
@Override
public boolean hasNext() {
// That's how many there are.
return which < size();
}
@Override
public T[] next() {
// Make my new one by cloning the original.
T[] next = Arrays.copyOf(items, of);
// Pick the items with reference to which.
int count = which;
for (int i = 0; i < of; i++) {
// count mod length is the next one to use.
next[i] = items[count % items.length];
// Used that now.
count /= items.length;
}
// Consumed that one.
which += 1;
return next;
}
};
}
}
public void test() {
Samples<String> samples = new Samples(4, "A", "B", "C", "D", "E");
// Walk it with an iterator.
Iterator<String[]> i = samples.iterator();
while (i.hasNext()) {
System.out.println(Arrays.toString(i.next()));
}
// Walk it using enhanced for loop.
for (String[] s : samples) { // Line 91 - error thrown here.
System.out.println(Arrays.toString(s));
}
}
并且发现如果我拉出iterator
并且走路一切正常但是如果我尝试使用增强的for
循环则会出错:
java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to [Ljava.lang.String;
有什么我想念的东西 - 咖啡可能不够吗?
请忽略不正确的size
方法 - 我确信这不是问题的原因。
PS:jdk = jdk1.8.0_11但jdk1.7.0_65仍然失败
答案 0 :(得分:3)
您实例化原始类型Samples
,因此Samples.items
类型为Object[]
,而不是String[]
。在test()
方法中:
Samples<String> samples = new Samples(4, "A", "B", "C", "D", "E");
将其更改为:
// Note the diamond operator: <>
Samples<String> samples = new Samples<>(4, "A", "B", "C", "D", "E");
作为旁注:在Samples
课程中,您不需要声明实施Collection
,因为AbstractCollection
已经这样做了。