java - 为什么从基元创建数组会复制它们？

Question

在程序中

public static void main(String[] args){
    int x = 1;
    int y = 2;
    int[] z = new int[]{x, y};
    z[0] = 3;
    System.out.println(x); //prints 1
}

我预计会打印 3。但是1是。为什么？我认为 Java 只复制引用，即当我们将引用传递给方法时，该方法将使用对同一对象的另一个引用进行操作。这就像 C++ 指针和原始类型。

于是尝试咨询JLS 10.6并没有发现任何有用的东西。也许我对 Java 中的原始类型有一些误解。你能澄清一下吗？

Answer 1

Why does creating an array from primitives copy them?

出于完全相同的原因:

int a = 5;
int b = a;

...复制 a 的值进入b ，而不在 a 之间创建任何类型的链接和 b : 值 被复制，而不是对变量的某种引用。

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回复你的评论:

But when we operate on reference types, the reference copies, doesn't it?

是的，确实如此，就像在您的 int 中复制的数字一样场景。

b = a (或你的数组初始值设定项)工作完全相同的方式无论是否a和 b是原始类型变量或引用类型变量:a 中的值被复制到b , 然后 a 之间没有持续的链接和 b .

唯一的区别是，当您处理引用类型时，变量中的值不是实际的东西，它是对实际的东西的引用。因此，将该引用从一个变量复制到另一个变量只会使这两个变量引用同一事物，就像我上面的代码使 a 一样。和 b两者的值为 5。

考虑:让我们创建一个列表:

List<String> a = new ArrayList<String>();

这给了我们这样的内存:

              +----------+a(55465)----->| the list |              +----------+

The variable a contains a value which is a reference to an object. I've depicted that value above as 55465, but we never actually see the raw value in our Java code (and it changes as garbage collection is done). It's just a value, like any other value, which tells the JVM where to find the object in memory. You can think of it like a long containing a memory address (that's not what it really is, but it works conceptually).

Now we do this:

List<String> b = a;

现在我们在内存中有这样的东西:

a(55465)--+          |    +----------+          +--->| the list |          |    +----------+b(55465)--+

Both a and b contain a value which refers to the list.

You can see how that's exactly like:

int a = 5

给我们

a(5)

and then

int b = a;

给我们

a(5)b(5)

Values are copied between variables, passed into functions, etc. The only difference with reference types is what that value is used for, how it's interpreted.

If so, I would say that java is pass by value in the sense of copying values of primitives and the references of reference types. Right?

No. "Pass by value" and "pass by reference" have a specific meaning in computing: It's about having a reference to a variable, not an object. This is what pass-by-reference looks like:

// NOT JAVA; Java doesn't have this
void foo(int &a) { // Fake pass-by-reference thing
    a = 3;
}

int a = 5;
foo(&a); // Fake pass-by-reference operator
System.out.println(a); // 3

按引用传递与对象引用无关。他们唯一的共同点是“引用”这个词。 Java 是一种纯粹的按值传递语言。