问个java关于字符串赋值的问题

public class Test
{
public static void main(String[] args)
{
String str1 = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
String str2 = "";
for(int i = str1.length() - 1; i >= 0; i--) {
str2 += String.valueOf(str1.charAt(i));
}
System.out.println(str2);
}
}

import java.util.Scanner;

public class abc {

/**
* @param args
*/
public static void main(String[] args) {
// TODO 自动生成方法存根
int ri, repeat, i;
char ch;
String str1, str2="";
Scanner in = new Scanner(System.in);
repeat = Integer.parseInt(in.nextLine());
for (ri = 1; ri <= repeat; ri++) {
str1 = in.nextLine();// 每行输入一个字符串

/*---------*/
char ch2;
// str2=str1;
for (i = 0; i < str1.length(); i++) {
ch = str1.charAt(i);
ch2 = ch;

if (('A' <= ch) & (ch <= 'Z')) {
ch2 = (char) ('A' + 'Z' - ch);

}
str2 += String.valueOf(ch2);
}

/*---------*/
System.out.println(str2);
}
}

}