本文共 3713 字,大约阅读时间需要 12 分钟。
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
snull
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.println(str.toString()); //输出空字符串 setValue2(str); System.out.println(str.toString()); //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}. ** 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}. *
* Examples: *
* * @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; }* "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) *
引用类型会引起浅拷贝和深拷贝现象。