JAVA编程作业，求大神解答！

import java.io.BufferedReader;
import java.io.InputStream;
import java.io.InputStreamReader;


public class Test {
	public static void main(String[] args) throws Exception {
		
		String str = "请输入一个字符(如果想退出程序输入exit)：";
		String temp = "";
		int num = 0;
		
		InputStream in = System.in;
		InputStreamReader ir = new InputStreamReader(in);
		
		while(true){
			System.out.println(str);
			BufferedReader read = new BufferedReader(ir);
			temp = read.readLine();
			if("exit".equals(temp)){
				System.out.println("再见");break;
			}
			System.out.println("输入的数据："+temp);
			
			char[] cs = temp.toCharArray();
			temp = "";
			for(int i : cs){
				if(i>=65&&i<=90){//大写变小写
					num = num + 1;
					i = i + 32;
				}else if(i>=97&&i<=122){//小写变大写
					num = num + 1;
					i = i - 32;
				}
				
				temp = temp + (char)i;
			}
			
			System.out.println("要修改的字符的个数："+num);	
			System.out.println("修改后的数据："+temp);
		}
	
	}
}

public static int count = 0;
    public static void main(String[] args){

        Scanner sc = new Scanner(System.in);
        String str = "";
        while(!(str=sc.next()).equals("!")){
            String reStr = exChange(str);
            System.out.println(str);
            System.out.println(reStr);
            System.out.println(count);
        }
        System.out.println("over");
    }
    public static String exChange(String str){
        count = 0;
        StringBuffer sb = new StringBuffer();
        if(str!=null){
            for(int i=0;i                char c = str.charAt(i);
                if(Character.isUpperCase(c)){
                    sb.append(Character.toLowerCase(c));
                    count++;
                }else if(Character.isLowerCase(c)){
                    sb.append(Character.toUpperCase(c)); 
                    count++;
                }else{
                    sb.append(c);
                }
            }
        }
        
        return sb.toString();
    }