了解值传递和引用传递

Java中，String、Date、基本类型和基本类型的封装类都是以值的方式参数传递，而其他类型都是以引用的方式参数传递。
值参数传递示例：
//基本类型
public static void main(String[] args) {
int i = 5;
counter(i);
System.out.println(i); //5
}
static void counter(int t) {
t = 23;
}

//基本类型封装类
public static void main(String[] args) {
Integer age = new Integer(12);
counter(age);
System.out.println(age); //12
}
static void counter(Integer age) {
age = 27;
}

//String类
public static void main(String[] args) {
String str = new String("First");
counter(str);
System.out.println(str); //First
}
static void counter(String str) {
str = "Second";
}

//Date类
public static void main(String[] args) {
Date now = new Date();
counter(now);
System.out.println(now); //Fri Mar 06 22:15:14 CST 2009
}
static void counter(Date date) {
date = null;
}

引用参数传递示例：
//StringBuffer类
public static void main(String[] args) {
StringBuffer sb = new StringBuffer("First");
counter(sb);
System.out.println(sb); //FirstSecond
}
static void counter(StringBuffer sb) {
sb.append("Second");
}

//String数组
public static void main(String[] args) {
String[] sa = new String[1];
sa[0] = "A";
counter(sa);
System.out.println(sa[0]); //[B]
}
static void counter(String[] alias) {
alias[0] = "B";
}

//List类
public static void main(String[] args) {
List list = new ArrayList();
list.add("A");
counter(list);
System.out.println(list); //[A, B]
}
static void counter(List alias) {
alias.add("B");
}