System.arrayCopy（）复制对象或对象的引用？ [英] System.arrayCopy() copies object or reference to object?

问题描述

我有一个最终的课程 NameAndValue 。我使用 System.arrayCopy（）复制了一个 NameAndValue 对象的数组，当我更改时NameAndValue 复制数组中的对象，它会反映在原始数组中。

I am having a final class NameAndValue. I copied an array of NameAndValue objects using System.arrayCopy() and when i changed a NameAndValue object in copied array, it gets reflected in the original array.

public final class NameAndValue
{
    public String name;
    public String value;

    public NameAndValue()
    {

    }

    public NameAndValue(String name,String value)
    {
        this.name = name;
        this.value = value;
    }
}

public class Main
{
    public static void main(String[] args)
    {
        NameAndValue[] nv = new NameAndValue[4];
        nv[0] = new NameAndValue("A", "1");
        nv[1] = new NameAndValue("B", "2");
        nv[2] = new NameAndValue("C", "3");
        nv[3] = new NameAndValue("D", "4");

        NameAndValue[] nv2 = new NameAndValue[4];
        System.arraycopy(nv, 0, nv2, 0, 2);
        nv2[2] = new NameAndValue("Y","25");
        nv2[3] = new NameAndValue("Z","26");

        for (int i = 0; i < 2; i++)
        {
            NameAndValue[] nv3 = new NameAndValue[4];
            System.arraycopy(nv2, 0, nv3, 0, 4);
            nv3[2].value = String.valueOf(i);
            nv3[3].value = String.valueOf(i+1);

            System.out.println(nv2[2].value);
            System.out.println(nv2[3].value);
            System.out.println("-----------------------");
        }
    }
}

我得到输出为，

0
1
-----------------------
1
2
-----------------------

在所有情况下，我的输出应该是25和26，对吧？为什么会改变？

I should have got output as 25 and 26 in all the cases,right?? why does it get changed??

推荐答案

System.arrayCopy（）复制对象或对象的引用？

System.arrayCopy() copies object or reference to object?

参考，它是浅副本。令人惊讶的是，文档没有明确说明，只是含蓄地，因为他们只是谈论复制数组元素，而不是递归复制他们引用的东西。

Reference, it's a shallow copy. Surprisingly, the docs don't say that explicitly, just implicitly as they only talk about copying array elements, not recursively copying the things they reference.

这与你有这个完全相同：

It's exactly the same as if you had this:

NameAndValue nv1 = new NameAndValue("A", "1");
NameAndValue nv2 = nv1;
nv2.value = "4";
System.out.println(nv1.value); // 4

每个数组元素都像 nv1 和 nv2 上面的变量。就像 nv1 和 nv2 引用（指向）相同的底层对象一样，数组条目也是如此，包括那些条目从一个数组复制到另一个数组。

Each array element is like the nv1 and nv2 vars above. Just as nv1 and nv2 reference (point to) the same underlying object, so do the array entries, including when those entries are copied from one array to another.

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