什么更有效：System.arraycopy vs Arrays.copyOf？ [英] What is more efficient: System.arraycopy vs Arrays.copyOf?

问题描述

ArrayList中的toArray方法，Bloch使用System.arraycopy和Arrays.copyOf来复制数组。

The toArray method in ArrayList , Bloch uses both System.arraycopy and Arrays.copyOf to copy an array .

public <T> T[] toArray(T[] a) {
  if (a.length < size)
        // Make a new array of a's runtime type, but my contents:
        return (T[]) Arrays.copyOf(elementData, size, a.getClass());
    System.arraycopy(elementData, 0, a, 0, size);
    if (a.length > size)
        a[size] = null;
    return a;
}

如何比较这两种复制方法，何时使用哪种？

How to compare these two copy methods , when to use which?

推荐答案

区别在于 Arrays.copyOf 不仅复制元素，它还创建一个新数组。 System.arrayCopy 复制到现有数组中。

The difference is that Arrays.copyOf does not only copy elements, it also creates a new array. System.arrayCopy copies into an existing array.

以下是数组的来源。 copyOf ，你可以看到它在内部使用 System.arraycopy 来填充新数组。

Here is the source for Arrays.copyOf, as you can see it uses System.arraycopy internally to fill up the new array.

  public static <T,U> T[] copyOf(U[] original, int newLength, Class<? extends T[]> newType) {
    T[] copy = ((Object)newType == (Object)Object[].class)
        ? (T[]) new Object[newLength]
        : (T[]) Array.newInstance(newType.getComponentType(), newLength);
    System.arraycopy(original, 0, copy, 0,
                     Math.min(original.length, newLength));
    return copy;
}

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