请问字符串解码问题

[fivefouryuan]

String hello = "Hello world 世界你好";
hello = new String(hello.getBytes(), "GB2312");
System.out.println(hello.length());//输出16

byte[] b=hello.getBytes("gb2312");
System.out.println(b.length);//输出20

hello = new String(hello.getBytes(), "UTF-8");
System.out.println(hello.length());//输出18

byte[] b=hello.getBytes("UTF-8");
System.out.println(b.length);//输出24


请问这个长度是怎么计算出来的，


就是第三个不好理解，字符串的长度空间是怎样计算的
16：每个字符（包括中文）长度为1，gb2312,转换成byte时，英文占一个字节中文占两个
字节，UTF-8时，英文一个字节，中文三个字节，输出24,但是长度为什么是18呢？

谢谢

[fivefouryuan]

这个是源程序，还是搞不懂怎么得出来的
import java.io.UnsupportedEncodingException;
public class ZiFuChuan{
public static void main(String arg[]){
String hello = "Hello world 世界你好";
try{
//hello = new String(hello.getBytes(), "GB2312");
byte[] b=hello.getBytes("gb2312");
System.out.println(b.length);//输出20

String hello1 = new String(hello.getBytes(), "gb2312");
System.out.println(hello1.length());//输出16
//hello=new String(hello.getBytes(),"UTF-8");//----(1)
byte[] b1=hello.getBytes("UTF-8");
System.out.println(b1.length);//输出24,如果加上（1）输出28
String hello2=new String(hello.getBytes(),"UTF-8");
System.out.println(hello2.length());//输出18

}
catch(UnsupportedEncodingException e){}


}
}

[wbel]

import java.io.UnsupportedEncodingException;
public class ziFuChuan{
public static void main(String arg[]){
String hello = "Hello world 世界你好";
try{
hello = new String(hello.getBytes(), "GB2312");
byte[] b=hello.getBytes("gb2312");
System.out.println(b.length);//输出20

hello = new String(hello.getBytes(), "UTF-8");
System.out.println(hello.length());//输出18

byte[] b1=hello.getBytes("UTF-8");
System.out.println(b1.length);//输出28
System.out.println(hello.length());//输出18

}
catch(UnsupportedEncodingException e){}


}
}