String 对象是可变的！

public class T {

/**
* @param args
*/
public static void main(String[] args)throws Exception {
// TODO Auto-generated method stub
String s = "1";
System.out.println(s);
c(s);
System.out.println(s);

}

public static void c(String s)throws Exception{
Field f = s.getClass().getDeclaredField("value");
f.setAccessible(true);
Object o = f.get(s);
char[] c = (char[]) o;
f.set(s, "5".toCharArray());
//System.out.println(s);
}
}