不变和传递价值 [英] Immutable and pass by value
问题描述
我有以下代码，其中包含
a mutable Person类，String和修改String和Person实例的方法
I have the following code which has
a mutable Person class, String and a method to modify the instances of String and Person
class Person{
int a = 8;
public int getA() {
return a;
}
public void setA(int a) {
this.a = a;
}
@Override
public String toString() {
return "Person [a=" + a + "]";
}
}
-
public class TestMutable {
public static void main(String[] args)
{
Person p = new Person();
p.setA(34);
String s = "bar";
modifyObject(s, p); //Call to modify objects
System.out.println(s);
System.out.println(p);
}
private static void modifyObject(String str, Person p)
{
str = "foo";
p.setA(45);
}
}
输出是正如所料。打印
bar
Person [a=45]
现在，我的问题是
在你说str =的地方发生了什么foo？
最初我们假设s ='bar'且数据位于0x100内存
Initially let's assume that s='bar' and the data resides in 0x100 memory
现在将字符串的引用传递给另一个方法，另一个方法尝试使用s =foo将内存位置（0x100）的内容更改为foo。这是发生了什么，或者'foo'是在不同的内存位置创建的？
Now the reference of string is passed to another method, the other method tries to change the contents of the memory location(0x100) to 'foo' using s="foo". Is this what is happening, or is 'foo' is created in differennt memory location ?
java是否按值传递引用？
推荐答案
Java总是按值而不是通过引用传递参数。
Java always passes arguments by value NOT by reference.
让我通过示例：
public class Main
{
public static void main(String[] args)
{
Foo f = new Foo("f");
changeReference(f); // It won't change the reference!
modifyReference(f); // It will change the object that the reference variable "f" refers to!
}
public static void changeReference(Foo a)
{
Foo b = new Foo("b");
a = b;
}
public static void modifyReference(Foo c)
{
c.setAttribute("c");
}
}
我将分步说明：
1-声明类型 Foo 的名为 f 的引用并指定它到 Foo 类型的新对象，其属性为f。
1- Declaring a reference named f of type Foo and assign it to a new object of type Foo with an attribute "f".
Foo f = new Foo("f");
2-从方法方面，类型为 Foo 的引用声明名称 a ，并且它最初被分配给 null 。
2- From the method side, a reference of type Foo with a name a is declared and it's initially assigned to null.
public static void changeReference(Foo a)
< img src =https://i.stack.imgur.com/k2LBD.pngalt =在此处输入图像说明>
3-当你打电话给方法 changeReference ，引用 a 将被分配给作为参数传递的对象。
3- As you call the method changeReference, the reference a will be assigned to the object which is passed as an argument.
changeReference(f);
4-声明类型为<$ c $的名为 b 的引用c> Foo 并将其分配给类型为 Foo 的新对象，其属性为b。
4- Declaring a reference named b of type Foo and assign it to a new object of type Foo with an attribute "b".
Foo b = new Foo("b");
5- a = b 重新分配参考 a NOT f 指向其属性为b的对象 。
5- a = b is re-assigning the reference a NOT f to the object whose its attribute is "b".
6-当您调用 modifyReference（Foo c）方法时，引用 c 已创建并分配给属性为f的对象。
6- As you call modifyReference(Foo c) method, a reference c is created and assigned to the object with attribute "f".
7 - c.setAttribute（c）; 将更改引用 c 的对象的属性指向它，和引用 f 指向它的对象相同。
7- c.setAttribute("c"); will change the attribute of the object that reference c points to it, and it's same object that reference f points to it.
我希望您现在了解如何将对象作为参数传递在Java中：）
I hope you understand now how passing objects as arguments works in Java :)
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