值传递和引用传递
在java中实质只存在一种传递方式：即值传递。 而引用传递是对C/C++而言的。结论：
对于基本类型，在方法体内对方法参数进行重新赋值，并不会改变原有变量的值。
对于引用类型，在方法体内对方法参数进行重新赋予引用，并不会改变原有变量所持有的引用。
方法体内对参数进行运算，不影响原有变量的值。
方法体内对参数所指向对象的属性进行运算，将改变原有变量所指向对象的属性值。
在JAVA中变量有以下两种：
基本类型变量，包括char、byte、short、int、long、float、double、boolean。
引用类型变量，包括类、接口、数组（基本类型数组和对象数组）。
最后我们得出如下的结论：
基本类型和基本类型变量被当作参数传递给方法时，是值传递。在方法实体中，无法给原变量重新赋值，也无法改变它的值。
对象和引用型变量被当作参数传递给方法时，在方法实体中，无法给原变量重新赋值，但是可以改变它所指向对象的属性。
2 public class Person
3 {
4 private String name = null;
5 private int age = 0;
6
7 public Person(String name, int age) {
8 this.name = name;
9 this.age = age;
10 }
11
12 public void changeName(String newName) {
13 this.name = newName;
14 }
15
16 public void changeAge(int newAge) {
17 this.age = newAge;
18 }
19
20 public String toString() {
21 return "Person - Name: " + name + ", Age: " + age;
22 }
23 }
24
25 public class PassByValue
26 {
27 public static void main(String[] args) {
28 // one example
29 String value1 = "value1: aaa";
30 String value2 = "value2: bbb";
31
32 swapValue(value1, value2);
33
34 System.out.println(value1);
35 System.out.println(value2);
36
37 // another example
38 Person person = new Person("张三", 20);
39
40 System.out.println(person.toString());
41
42 changePerson(person);
43
44 System.out.println(person.toString());
45 }
46
47 public static void swapValue(String tmpValue1, String tmpValue2) {
48 String tempStr = tmpValue1;
49 tmpValue1 = tmpValue2;
50 tmpValue2 = tempStr;
51 }
52
53 public static void changePerson(Person tmpPerson) {
54 tmpPerson.changeName("李四");
55 tmpPerson.changeAge(30);
56 }
57 }
58
考虑一下上面这段代码的输出结果是什么？
我相信绝大多数人都能快速的给出正确答案：
value1: aaa value2: bbb Person - Name: 张三, Age: 20 Person - Name: 李四, Age: 30
答案为什么是这样的呢？那么 Java 到底是按值传递还是按引用传递参数的呢？
例 子1中的 swapValue() 方法不起作用，看起来是按值传递参数的，例子2中的 changeName() 和 changeAge() 方法起作用，从表面上看起来好像又是按引用传递参数的。呵呵，别急，先让我们来弄清楚一个重要的概念，就是 Java 操纵对象都是通过引用来实现的，所有的对象变量都是所指向的对象的引用，那也就是说所有对象变量的值并不是它所指向的对象本身，而是该对象的引用，这句话听起来可能有点让人犯晕，举个例子来说，String str = new String("aaa"); 这条语句的意思是 new 了一个值为 "aaa" 的字符串对象，然后用一个名字叫 str 的对象变量来指向它，保存它的引用，假设在内存中 new 出来的这个字符串对象的实际地址为 0xA242, 那么 str 的值实际上是 0xA242。
现在来看看例子1，假设
String value1 = "value1: aaa" 的地址为 0xA242， String value2 = "value2: bbb" 的地址为 0xA24B,
然 我们来看看 swapValue(value1, value2); 语句 Java 是怎么执行的。当程序执行到这句语句的时候，JVM 在栈中创建两个临时变量 tmpValue1,tmpValue2，用来接收传进来的 value1 和 value2 的值，而 value1 和 value2 的值分别为 0xA242 和 0xA24B，所以那两个临时变量 tmpValue1和tmpValue2 的值也被赋值为 0xA242 和 0xA24B，此时接下来的三条语句分别是
tempStr = 0xA242, tmpValue1 = 0xA24B, tmpValue2 = 0xA242;
此时 tmpValue1 的值是指向 "value2: bbb" 这个对象的，
tmpValue2 的值是指向 "value1: aaa" 这个对象，你可以在 swapValue() 方法内部打印 tmpValue1 和 tmpValue2 的值验证一下。
不 过这里的 tmpValue1和tmpValue2 是在栈中的临时变量，而并不是 main 方法中 value1和value2，main 方法中 value1和value2 的值并没有改变，那这里所谓的值传递是指什么呢？因为 value1 的值是 0xA242（"value1: aaa" 这个对象的引用），而并不是 "value1: aaa" 这个对象，传递参数时传递的是 "value1: aaa" 这个对象的引用也就是 value1 的值，所以是值传递。
例子2也是一样的，假设
Person person = new Person("张三", 20) 的地址为 0xA242，
那 么 person 的值是 0xA242, 那么通过调用 changePerson(person); 将 0xA242 传递给 tmpPerson, 也就是 tmpPerson 此时的值为 0xA242 也就是 new Person("张三", 20) 这个对象的引用，也就是说 tmpPerson 指向 new Person("张三", 20) 对象，对 tmpPerson 引用对象的修改实际上就是对 person 引用对象的修改，这看起来很想引用传递的概念，但实际上是值传递，为什么呢？因为在参数传递过程中传递的是 person 的值，也就是 0xA242。
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Даже и придраться не к чему, хотя я люблю это дело ))
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