概要

用数组对象的getClass方法获取数据的类类型，在通过Class.getComponentType获取数组元素类型。

  1.     public static <T> T[] toArray(T...a) {
  3.         List<T> list = new ArrayList<>();
  4.         Collections.addAll(list, a);
  6.         T[] r =  (T[])java.lang.reflect.Array
  7.                 .newInstance(a.getClass().getComponentType(), 0);
  9.         return list.toArray(r);
  10.     }

JDK Class.getComponent方法签名如下

  1.     /**
  2.      * Returns the {@code Class} representing the component type of an
  3.      * array.  If this class does not represent an array class this method
  4.      * returns null.
  5.      *
  6.      * @return the {@code Class} representing the component type of this
  7.      * class if this class is an array
  8.      * @see     java.lang.reflect.Array
  9.      * @since JDK1.1
  10.      */
  11.     public native Class<?> getComponentType();

JDK ArrayList.toArray方法签名如下：

  1. // 因此调用List.toArray方法时，传入一个数组size=0的对象即可 
  2. // 如list.toArray(new String[0]);
  3. public <T> T[] toArray(T[] a) {
  4.         if (a.length < size)
  5.             // Make a new array of a's runtime type, but my contents:
  6.             return (T[]) Arrays.copyOf(elementData, size, a.getClass());
  7.         System.arraycopy(elementData, 0, a, 0, size);
  8.         if (a.length > size)
  9.             a[size] = null;
  10.         return a;
  11.     }

Arrays.copyOf方法签名如下：

  1.   public static <T,U> T[] copyOf(U[] original, int newLength, Class<? extends T[]> newType) {
  2.         @SuppressWarnings("unchecked")
  3.         T[] copy = ((Object)newType == (Object)Object[].class)
  4.             ? (T[]) new Object[newLength]
  5.             : (T[]) Array.newInstance(newType.getComponentType(), newLength);
  6.         System.arraycopy(original, 0, copy, 0,
  7.                          Math.min(original.length, newLength));
  8.         return copy;
  9.     }

List.toArray() VS List.toArray(T[])

list.toArray是直接把集合转换数组，返回Object类型数组，会导致泛型丢失，导致后续可能需要类型转换
list.toArray(T) 返回泛型数组，入参的数组大小建议为原始List的Size(实际测试下来，性能最佳）