java 当整数值是更改数组值之类的参数时，如何更改它？
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How can I change Integer value when it is an argument like change array's value?
提问by cow12331
public static void main(String[] args) {
Integer i = new Integer(0);
int[] arr = {1};
p1(i);
p2(arr);
System.out.println(i);
System.out.println(arr[0]);
}
public static void p1(Integer i) {
i = 2;
}
public static void p2(int[] i) {
i[0] = 2;
}
//output: 0, 2
//输出：0, 2
How can I change the value of i like I change the value of arr?
如何更改 i 的值，就像更改 arr 的值一样？
回答by Jon Skeet
You can't change the value of the variable iin mainfrom within the p1method, because the argument is passed by value: the parameter iin p1is entirely separate from the ivariable, it's just that they have the same value at the start of the method. Java alwaysuses pass-by-value semantics - but when the parameter type is a class, it's a referencethat is passed by value.
你不能在方法内部改变变量iin的值，因为参数是按值传递的：参数in与变量完全分开，只是它们在方法开始时具有相同的值。Java总是使用按值传递语义——但是当参数类型是一个类时，它是一个按值传递的引用。mainp1ip1i
In fact, you're not changing the value of arr, either - it's a reference to the same array as before, but the value inthe array has been changed. And that's what you can't do with Integer, because Integeris an immutable type.
事实上，你不改变的价值arr，无论是-这是同一个数组作为前一个参考，但该值在数组已经改变。这就是你不能做的事情Integer，因为Integer是不可变的类型。
If you want a mutable class like Integer, you could use AtomicIntegerinstead:
如果你想要一个像 一样的可变类Integer，你可以使用AtomicInteger：
public static void main(String[] args) {
AtomicInteger i = new AtomicInteger(0);
modify(i);
System.out.println(i);
}
private static void modify(AtomicInteger x) {
x.set(2);
}
I would usually notdo this, however - I usually try not to modify the objects that method parameters refer to. Instead, I write methods which compute a single result, and return that.
但是，我通常不会这样做 - 我通常会尽量不修改方法参数引用的对象。相反，我编写了计算单个结果并返回该结果的方法。
回答by Smutje
Simply put: You can't, because Integeris immutable and you only get the object address by value, so swapping the whole object is not possible because after the method finished, the old object gets reassigned.
简单地说：你不能，因为它Integer是不可变的，你只能通过值获取对象地址，所以交换整个对象是不可能的，因为在方法完成后，旧对象被重新分配。
回答by Dmitry Ginzburg
You can use AtomicInteger, which allows changing, instead of Integer:
您可以使用AtomicInteger，它允许更改，而不是Integer：
public static void main(String[] args) {
AtomicInteger i = new AtomicInteger(0);
p1(i);
System.out.println(i);
}
public static void p1(AtomicInteger i) {
i.set(2);
}
回答by walkeros
Only by using some "hacks". You can do this like this:
只有通过使用一些“黑客”。你可以这样做：
public static void p1(Integer curInt) {
Field field = curInt.getClass().getDeclaredField("value"); // Integer stores the real value in private field "value"
field.setAccessible(true);
field.set(curInt, 2);
}
