Java变量参数传⼊⽅法，修改后是否影响外⾯的值public class Test2 {
public static void setValue(String str){
str = "ss";
}
public static void setValue(Man str){
str = new Man("test");
}
public static class Man{
private String name;
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public Man(String name) {
this.name = name;
}
@Override
public String toString() {
return "Man{" +
"name='" + name + '\'' +
'}';
}
}
public static void main(String[] args) {
String str = "s";
setValue(str);
System.out.println(str);
Man man = null;
setValue(man);
System.out.println(man);
}
}
如上⾯代码实践，结果输出
s
null
原因是⽅法在执⾏的时候有栈帧的概念，⼊栈的时候只是压栈⽅法参数是传⼊参数的副本。
JVM⾼级特性
此时区分数据类型：基本类型和引⽤类型
基本类型：值存放在局部变量表中，⽆论如何修改只会修改当前栈帧的值，⽅法执⾏结束对⽅法外不会做任何改变；此时需要改变外层的变量，必须返回主动赋值。
引⽤数据类型：指针存放在局部变量表中，调⽤⽅法的时候，副本引⽤压栈，赋值仅改变副本的引⽤。但是如果直接改变副本引⽤的值，修改了引⽤地址的对象，此时⽅法以外的引⽤此地址对象当然被修改。（两个引⽤，同⼀个地址，任何修改⾏为2个引⽤同时⽣效）
⽐如
public static void setValue(StringBuilder str){
str = new StringBuilder("sss");
}
public static void setValue2(StringBuilder str){
str.append("sss");
}
public static void main(String[] args) {
StringBuilder str = new StringBuilder();
setValue(str);
System.out.String()); //输出空字符串
setValue2(str);
System.out.String()); //输出sss
}
关于String，本质是final类型char数组，不可修改，只能赋值，在做参数传⼊⽅法修改时，其实是新建对象，必须返回重新对外⾯的变量赋值才会对外⾯的String引⽤⽣效。
看String源码的任意⼀个⽅法即可明⽩
/**
* Returns a string resulting from replacing all occurrences of
* {@code oldChar} in this string with {@code newChar}.
java replace方法* <p>
* If the character {@code oldChar} does not occur in the
* character sequence represented by this {@code String} object,
* then a reference to this {@code String} object is returned.
* Otherwise, a {@code String} object is returned that
* represents a character sequence identical to the character sequence      * represented by this {@code String} object, except that every
* occurrence of {@code oldChar} is replaced by an occurrence
* of {@code newChar}.
* <p>
* Examples:
* <blockquote><pre>
* "mesquite in your cellar".replace('e', 'o')
*        returns "mosquito in your collar"
* "the war of baronets".replace('r', 'y')
*        returns "the way of bayonets"
* "sparring with a purple porpoise".replace('p', 't')
*        returns "starring with a turtle tortoise"
* "JonL".replace('q', 'x') returns "JonL" (no change)
* </pre></blockquote>
*
* @param  oldChar  the old character.
* @param  newChar  the new character.
* @return  a string derived from this string by replacing every
*          occurrence of {@code oldChar} with {@code newChar}.
*/
public String replace(char oldChar, char newChar) {
if (oldChar != newChar) {
int len = value.length;
int i = -1;
char[] val = value; /* avoid getfield opcode */
while (++i < len) {
if (val[i] == oldChar) {
break;
}
}
if (i < len) {
char buf[] = new char[len];
for (int j = 0; j < i; j++) {
buf[j] = val[j];
}
while (i < len) {
char c = val[i];
buf[i] = (c == oldChar) ? newChar : c;
i++;
}
//String的value 是final char[]，⼀旦创建不能改变
return new String(buf, true);
}
}
return this;
}
引⽤类型会引起浅拷贝和深拷贝现象。

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