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)。
我想知道这段代码是否会创建一个有效的泛型数组?
public T [] createArray(int desiredSize){
ArrayList<T> builder = new ArrayList<T>();
for(int x=0;x<desiredSize;x++){
builder.add(null);
}
return builder.toArray(zeroArray());
}
//zeroArray should, in theory, create a zero-sized array of T
//when it is not given any parameters.
private T [] zeroArray(T... i){
return i;
}
编辑:也许另一种创建这样一个数组的方法,如果你所需要的大小是已知的和小的,将是简单地提供所需的“null”的数量到zeroArray命令?
虽然这显然不如使用createArray代码通用。