JAVA期末考试题，一点也不明白在干嘛

package com;

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.File;
import java.io.FileReader;
import java.io.FileWriter;

public class BufferedReaderDemo {

    public static void main(String[] args) throws Exception {
        
        int cnt1=0,cnt2=0;
        BufferedReader br = new BufferedReader(new FileReader(new File("c:\\test.txt")));
        String str="";
        
        while((str = br.readLine())!=null){
            for(int i=0;i<str.length();i++) {
                if(str.charAt(i)=='A')
                    cnt1++;
                if(str.charAt(i)=='a')
                    cnt2++;
            }
        }
        br.close();
        String s = "大写字母A的次数是："+cnt1+",小写字母a的次数是:"+cnt2;
        System.out.println(s);
        BufferedWriter bw = new BufferedWriter(new FileWriter(new File("d:\\result.txt")));
        bw.write(s, 0, s.length());
        bw.close();
    }

}