import java.io.BufferedReader; 
 import java.io.BufferedWriter; 
 import java.io.FileInputStream; 
 import java.io.FileOutputStream; 
 import java.io.IOException; 
 import java.io.InputStreamReader; 
 import java.io.OutputStreamWriter; 
 import java.nio.charset.Charset;

public class Main {

public static void main(String[] args)throws IOException{  
BufferedReader br = new BufferedReader(new InputStreamReader  
    (new FileInputStream("calc.txt"), Charset.forName("UTF-16")));     
   BufferedWriter bw = new BufferedWriter(new OutputStreamWriter 
(new FileOutputStream("result.txt"), Charset.forName("UTF-8")));



  여기 가운데 무슨 코드가 들어가야 할지 모르겟어요ㅜㅜ



 switch(cmpStr[0]){ 

case "add": data += Double.parseDouble(cmpStr[i]); break; 
case "substract": data += Double.parseDouble(cmpStr[i]); break; 
case "multiply": data += Double.parseDouble(cmpStr[i]); break; 
case "divide": data += Double.parseDouble(cmpStr[i]); break; 

bw.write("결과 : "+ data.System.lineSeparator()); 
  }
    bw.flush(); 
    bw.close(); 
    br.close(); 
   }
}

결과는 이렇게 나옵니다!