Java:如何为`Collection`实现`toArray` [英] Java: how to implement `toArray` for `Collection`
问题描述
目前,我有:
public< T& T [] toArray(T [] old){
T [] arr = Arrays.copyOf(old,old.length + size());
int i = old.length;
for(E obj:this){
arr [i] = old.getClass()。getComponentType()。cast(obj);
++ i;
}
return arr;
}
(请注意,这不遵循axtavt指出的合同。)
我收到此警告:
从捕获#2的未选中的投射?到T
这仍然是实现它的最好/最直接的方法吗?我能以某种方式编码它没有警告的方式吗?
编辑:我目前的解决方案。首先,我真的不想在 toArray
本身中有这样的警告。因此,我对这些小助手函数进行了编码(在此处阅读)讨论这些):
@SuppressWarnings(unchecked)static< T&类< ;?延伸T> classOf(T obj){
return(Class< ;? extends T>)obj.getClass();
}
@SuppressWarnings(unchecked)static< T>类< ;?扩展T> classOf(T [] array){
return(Class< ;? extends T>)array.getClass()。getComponentType();
}
@SuppressWarnings(unchecked)static< T> T [] newArray(Class< T> clazz,int size){
return(T [])Array.newInstance(clazz,size);
}
现在,我的 toArray
实现类似:
public< T& T [] toArray(T [] array){
int size = size();
if(array.length< size){
array = newArray(classOf(array),size);
} else if(array.length> size){
array [size] = null;
}
int i = 0;
for(E e:this){
array [i] = classOf(array).cast(e);
i ++;
}
return array;
}
这仍然是实现它的最好/最直接的方法吗?
这不是Josh Bloch的做法。请查看 AbstractCollection#toArray()
。下面是JDK 1.6.0_22的相关性摘要。
public< T& T [] toArray(T [] a){
//估计数组的大小;准备看到更多或更少的元素
int size = size();
T [] r = a.length> = size
? a
:(T [])Array.newInstance(a.getClass()。getComponentType(),size);
迭代器< E> it = iterator();
for(int i = 0; i if(!it.hasNext()){//少于预期的元素
if (a!= r)
return Arrays.copyOf(r,i);
r [i] = null; // null-terminate
return r;
}
r [i] =(T)it.next();
}
return it.hasNext()? finishToArray(r,it):r;
}
源代码位于 src.zip
文件的JDK。你可以将它集成到任何象Eclipse / IDEA / Netbeans这样的IDE,以便你可以看到它,当你打开 AbstractCollection
类。
我可以用某种方式编码它没有警告吗?
否。
如果是打扰您,请使用 @SuppressWarnings(unchecked)
扩展 AbstractCollection
而不是实现集合
(如果可能),以便至少已经为您实现的基本功能。 p>
Right now, I have:
public <T> T[] toArray(T[] old) {
T[] arr = Arrays.copyOf(old, old.length + size());
int i = old.length;
for(E obj : this) {
arr[i] = old.getClass().getComponentType().cast(obj);
++i;
}
return arr;
}
(Note that this does not follow the contract as it was pointed out by axtavt.)
where I get this warning:
Type safety: Unchecked cast from capture#2-of ? to T
Is this still the best / most straightforward way to implement it? Can I somehow code it in a way without that warning? How would I implement it otherwise?
Edit: My current solution. First, I really wanted to not have such a warning in toArray
itself. Therefore, I coded these little helper functions (read here for a further discussion about these):
@SuppressWarnings("unchecked") static <T> Class<? extends T> classOf(T obj) {
return (Class<? extends T>) obj.getClass();
}
@SuppressWarnings("unchecked") static <T> Class<? extends T> classOf(T[] array) {
return (Class<? extends T>) array.getClass().getComponentType();
}
@SuppressWarnings("unchecked") static <T> T[] newArray(Class<T> clazz, int size) {
return (T[]) Array.newInstance(clazz, size);
}
Now, my toArray
implementation looks like:
public <T> T[] toArray(T[] array) {
int size = size();
if (array.length < size) {
array = newArray(classOf(array), size);
} else if (array.length > size) {
array[size] = null;
}
int i = 0;
for (E e : this) {
array[i] = classOf(array).cast(e);
i++;
}
return array;
}
Is this still the best / most straightforward way to implement it? How would I implement it otherwise?
It's not how Josh Bloch did. Have a look at the source of AbstractCollection#toArray()
. Here's an extract of relevance from JDK 1.6.0_22.
public <T> T[] toArray(T[] a) {
// Estimate size of array; be prepared to see more or fewer elements
int size = size();
T[] r = a.length >= size
? a
: (T[]) Array.newInstance(a.getClass().getComponentType(), size);
Iterator<E> it = iterator();
for (int i = 0; i < r.length; i++) {
if (!it.hasNext()) { // fewer elements than expected
if (a != r)
return Arrays.copyOf(r, i);
r[i] = null; // null-terminate
return r;
}
r[i] = (T) it.next();
}
return it.hasNext() ? finishToArray(r, it) : r;
}
The source code is available in src.zip
file of the JDK. You can integrate it in any decent IDE like Eclipse/IDEA/Netbeans so that you can see it when you open the AbstractCollection
class.
Can I somehow code it in a way without that warning?
No. Use @SuppressWarnings("unchecked")
if it is bothering you.
That said, I'd suggest to extend AbstractCollection
instead of implementing Collection
if possible so that you have at least the basic features already implemented for you.
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