如何将对象数组转换为任意内容类型T（Object [] - > T []）

Question

I wanna make my own Java streams to easily parse some strings. But I can't convert an object array to a T-array.

What I have already tried:

//  T[]  <-       Object[] 
    arr = (T[]) cache.toArray();

and

    T[] a = new T[6]; // Cannot create a generic array of T
    int index = 0;
    for (T i : arr) {
     a[++index] = i;
    }

Code:

public StreamParser<T> forEach(Consumer<? super T> action) {
        ArrayList<T> cache = new ArrayList<T>();
        for (T i : arr) {
            action.accept(i);
            cache.add(i);
        }
        System.out.println(arr instanceof String[]);
        arr = (T[]) cache.toArray();
        System.out.println(arr instanceof String[]);
        return this;
    }

Output:

true
false

Answer 1

ArrayList.toArray() always returns exactly Object[] , never some subtype.

For this to work, you need to pass in a T[] or IntFunction<T[]> as a parameter (either to the constructor of StreamParser , or to the method), and use this in the toArray() call:

arr = cache.toArray(someIntFn.apply(0 /* or cache.size() */));
// Or
arr = cache.toArray(someArray);

Answer 2

You should use the second method toArray taking an extra parameter. Here are the javadoc link: https://docs.oracle.com/javase/7/docs/api/java/util/ArrayList.html#toArray%28java.lang.Object%5B%5D%29

Regarding the creating of a generic array, here is the trick required to do it:

public static <T> T[] genArray(Class<T> clazz, int size){
    return (T[]) Array.newInstance(clazz, size);
}

that means an extra step to perform for which you need to have the actual class matching that T parameter