如何通过引用正确传递 Integer 类? [英] How can I pass an Integer class correctly by reference?

问题描述

我希望有人可以为我澄清这里发生的事情.我在整数类中研究了一会儿，但因为整数 覆盖 + 运算符我无法弄清楚出了什么问题.我的问题在于这一行:

整数 i = 0;我 = 我 + 1;//← 我认为这是在以某种方式创建一个新对象！

这是我的推理:我知道 java 是按值传递的(或按引用值传递)，所以我认为在以下示例中，整数对象应每次递增.

公共类 PassByReference {公共静态整数公司(整数 i){我=我+1；//我认为这一定是**偷偷**创建一个新整数...System.out.println("公司:"+i);返回我；}公共静态无效主要(字符串[]参数){整数整数 = 新整数(0)；for (int i =0; i<10; i++){公司(整数)；System.out.println("main:"+integer);}}}

这是我的预期输出:

<上一页>公司:1主要:1公司:2主要:2公司:3主要:3公司:4主要:4公司:5主要:5公司:6主要:6...

这是实际输出.

<上一页>公司:1主要:0公司:1主要:0公司:1主要:0...

为什么会这样?

解决方案

有两个问题:

1. 整数是按值传递，而不是按引用传递.更改方法内的引用不会反映到调用方法中传入的引用中.
2. 整数是不可变的.没有像 Integer#set(i) 这样的方法.否则，您可以直接使用它.

要让它工作，你需要重新分配 inc() 方法的返回值.

integer = inc(integer);

<小时>

要了解有关按值传递的更多信息，这里有另一个示例:

public static void main(String...args) {字符串[] 字符串 = 新字符串 [] { "foo", "bar" };更改参考(字符串)；System.out.println(Arrays.toString(strings));//仍然是 [foo, bar]更改值(字符串)；System.out.println(Arrays.toString(strings));//[foo, foo]}公共静态无效changeReference(字符串[]字符串){字符串 = 新字符串 [] { "foo", "foo" };}公共静态无效changeValue(字符串[]字符串){字符串[1] = "foo";}

I am hoping that someone can clarify what is happening here for me. I dug around in the integer class for a bit but because integer is overriding the + operator I could not figure out what was going wrong. My problem is with this line:

Integer i = 0;
i = i + 1;  // ← I think that this is somehow creating a new object!

Here is my reasoning: I know that java is pass by value (or pass by value of reference), so I think that in the following example the integer object should be incremented each time.

public class PassByReference {

    public static Integer inc(Integer i) {
        i = i+1;    // I think that this must be **sneakally** creating a new integer...  
        System.out.println("Inc: "+i);
        return i;
    }

    public static void main(String[] args) {
        Integer integer = new Integer(0);
        for (int i =0; i<10; i++){
            inc(integer);
            System.out.println("main: "+integer);
        }
    }
}

This is my expected output:

Inc: 1
main: 1
Inc: 2
main: 2
Inc: 3
main: 3
Inc: 4
main: 4
Inc: 5
main: 5
Inc: 6
main: 6
...

This is the actual output.

Inc: 1
main: 0
Inc: 1
main: 0
Inc: 1
main: 0
...

Why is it behaving like this?

解决方案

There are two problems:

1. Integer is pass by value, not by reference. Changing the reference inside a method won't be reflected into the passed-in reference in the calling method.
2. Integer is immutable. There's no such method like Integer#set(i). You could otherwise just make use of it.

To get it to work, you need to reassign the return value of the inc() method.

integer = inc(integer);

---

To learn a bit more about passing by value, here's another example:

public static void main(String... args) {
    String[] strings = new String[] { "foo", "bar" };
    changeReference(strings);
    System.out.println(Arrays.toString(strings)); // still [foo, bar]
    changeValue(strings);
    System.out.println(Arrays.toString(strings)); // [foo, foo]
}
public static void changeReference(String[] strings) {
    strings = new String[] { "foo", "foo" };
}
public static void changeValue(String[] strings) {
    strings[1] = "foo";
}

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