System.arraycopy(...)的时间复杂度？

Question

System.arraycopy(Object src, int srcPos, Object dest, int destPos, int length) 是一种本地方法.

这种方法的时间复杂度是多少？

Answer 1

它必须遍历数组中的所有元素才能执行此操作.Array是一种独特的数据结构,您必须在初始化时指定大小.顺序是源数组的大小,或者在大O项中是O(长度).

事实上,这发生在ArrayList内部.ArrayList包装一个数组.虽然ArrayList看起来像一个动态增长的集合,但在内部它必须扩展时才进行arrycopy.

Answer 2

我做了一些调查，后来决定写一个测试代码，这就是我所拥有的。

我的测试代码如下：

import org.junit.Test;

public class ArrayCopyTest {

  @Test
  public void testCopy() {
    for (int count = 0; count < 3; count++) {
      int size = 0x00ffffff;
      long start, end;
      Integer[] integers = new Integer[size];
      Integer[] loopCopy = new Integer[size];
      Integer[] systemCopy = new Integer[size];

      for (int i = 0; i < size; i++) {
        integers[i] = i;
      }

      start = System.currentTimeMillis();
      for (int i = 0; i < size; i++) {
        loopCopy[i] = integers[i];
      }
      end = System.currentTimeMillis();
      System.out.println("for loop: " + (end - start));

      start = System.currentTimeMillis();
      System.arraycopy(integers, 0, systemCopy, 0, size);
      end = System.currentTimeMillis();
      System.out.println("System.arrayCopy: " + (end - start));
    }
  }

}

它产生如下所示的结果

for loop: 47
System.arrayCopy: 24

for loop: 31
System.arrayCopy: 22

for loop: 36
System.arrayCopy: 22

所以，Bragboy 是正确的。

Answer 3

以下是 OpenJDK 8 (openjdk-8-src-b132-03_mar_2014) 的一些相关源代码。我在Java 本机方法源代码的帮助下找到了它（注意：那里的说明令人困惑；我只是在源代码中搜索了相关标识符）。我认为福特船长的评论是正确的；即，在（许多）情况下不需要迭代每个元素。请注意，不迭代每个元素并不一定意味着O(1)，它只是意味着“更快”。我认为，无论如何，数组副本本质上必须是 O(x)，即使 x 不是数组中的项目数；也就是说，无论你怎么做，数组中的元素越多，复制的成本就会越高，并且如果你有一个非常大的数组，则将花费线性的长时间。警告：我不确定这是否是您正在运行的 Java 的实际源代码；只是这是我在 OpenJDK 8 源代码中找到的唯一实现。我认为这是一个跨平台的实现，但我可能是错的——我肯定还没有弄清楚如何构建这个代码。另请参阅： Oracle JDK 和 Open JDK 之间的差异。以下来自：/openjdk/hotspot/src/share/vm/oops/objArrayKlass.cpp

// Either oop or narrowOop depending on UseCompressedOops.
template <class T> void ObjArrayKlass::do_copy(arrayOop s, T* src,
                               arrayOop d, T* dst, int length, TRAPS) {

  BarrierSet* bs = Universe::heap()->barrier_set();
  // For performance reasons, we assume we are that the write barrier we
  // are using has optimized modes for arrays of references.  At least one
  // of the asserts below will fail if this is not the case.
  assert(bs->has_write_ref_array_opt(), "Barrier set must have ref array opt");
  assert(bs->has_write_ref_array_pre_opt(), "For pre-barrier as well.");

  if (s == d) {
    // since source and destination are equal we do not need conversion checks.
    assert(length > 0, "sanity check");
    bs->write_ref_array_pre(dst, length);
    Copy::conjoint_oops_atomic(src, dst, length);
  } else {
    // We have to make sure all elements conform to the destination array
    Klass* bound = ObjArrayKlass::cast(d->klass())->element_klass();
    Klass* stype = ObjArrayKlass::cast(s->klass())->element_klass();
    if (stype == bound || stype->is_subtype_of(bound)) {
      // elements are guaranteed to be subtypes, so no check necessary
      bs->write_ref_array_pre(dst, length);
      Copy::conjoint_oops_atomic(src, dst, length);
    } else {
      // slow case: need individual subtype checks
      // note: don't use obj_at_put below because it includes a redundant store check
      T* from = src;
      T* end = from + length;
      for (T* p = dst; from < end; from++, p++) {
        // XXX this is going to be slow.
        T element = *from;
        // even slower now
        bool element_is_null = oopDesc::is_null(element);
        oop new_val = element_is_null ? oop(NULL)
                                      : oopDesc::decode_heap_oop_not_null(element);
        if (element_is_null ||
            (new_val->klass())->is_subtype_of(bound)) {
          bs->write_ref_field_pre(p, new_val);
          *p = *from;
        } else {
          // We must do a barrier to cover the partial copy.
          const size_t pd = pointer_delta(p, dst, (size_t)heapOopSize);
          // pointer delta is scaled to number of elements (length field in
          // objArrayOop) which we assume is 32 bit.
          assert(pd == (size_t)(int)pd, "length field overflow");
          bs->write_ref_array((HeapWord*)dst, pd);
          THROW(vmSymbols::java_lang_ArrayStoreException());
          return;
        }
      }
    }
  }
  bs->write_ref_array((HeapWord*)dst, length);
}