import java.io.BufferedInputStream;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.Scanner;


public class lap3_54122420214 {

    /**
     * @param args
     * @throws IOException
     */
    public static void main(String[] args) throws IOException {
        // TODO Auto-generated method stub
        System.out.println("\tเครื่องคิดเลข");
        System.out.println("=========================");
        System.out.println("*    1.บวก        *");
        System.out.println("*    2.ลบ        *");
        System.out.println("*    3.คูณ        *");
        System.out.println("*    4.หาร        *");
        System.out.println("*    5.จบหารทำงาน    *");
        System.out.println("=========================");
        
          BufferedReader stadin=new BufferedReader(new InputStreamReader(System.in));
          String input="";
          System.out.print("เลือกรูปแบบการคำนวณ 1-5: ");
          input=stadin.readLine();
          int number = Integer.parseInt(input);
          if(number==1){
              System.out.println("\tบวก");
              Scanner cl1 = new Scanner(System.in);
              System.out.print("Enter first nuber : ");
              int clt1 = cl1.nextInt();              
              Scanner cl2 = new Scanner(System.in);
              System.out.print("Enter second number : ");
              int clt2 = cl2.nextInt();
              
              
              int sum = clt1+clt2;
              System.out.println("คำตอบ  = "+sum);
              System.out.println("กลับสู่เมนูหลักหรือจบการทำงาน (Y / N ): ");
          }
              else if (number==2) {
              System.out.println("\tลบ");
              Scanner bl1 = new Scanner(System.in);
              System.out.print("Enter first nuber : ");
              int blt1 = bl1.nextInt();              
              Scanner bl2 = new Scanner(System.in);
              System.out.print("Enter second number : ");
              int blt2 = bl2.nextInt();
              
              
              int sub = blt1-blt2;
              System.out.println("คำตอบ  = "+sub);
              System.out.println("กลับสู่เมนูหลักหรือจบการทำงาน (Y / N ): ");
              }
              
              else if (number==3) {
                  System.out.println("\tคูณ");
                  Scanner vl1 = new Scanner(System.in);
                  System.out.print("Enter first nuber : ");
                  int vlt1 = vl1.nextInt();              
                  Scanner vl2 = new Scanner(System.in);
                  System.out.println("Enter second number : ");
                  int vlt2 = vl2.nextInt();
                  
                  
                  int multiply = vlt1*vlt2;
                  System.out.println("คำตอบ  = "+multiply);
                  System.out.println("กลับสู่เมนูหลักหรือจบการทำงาน (Y / N ): ");
              }
                  
                  else if (number==4) {
                      System.out.println("\tหาร");
                      Scanner ql1 = new Scanner(System.in);
                      System.out.print("Enter first nuber : ");
                      int qlt1 = ql1.nextInt();              
                      Scanner ql2 = new Scanner(System.in);
                      System.out.print("Enter second number : ");
                      int qlt2 = ql2.nextInt();
            
                      
                      int divide = qlt1/qlt2;
                      System.out.println("คำตอบ  = "+divide);
                      System.out.println("กลับสู่เมนูหลักหรือจบการทำงาน (Y / N ): ");
                  }

                    }
                }


คือผมเขียนเหลือคำสั่งหนึ่งคำสั่งคือ ทำให้มันย้อนกลับเมนูหลัก / กับ จบการทำงาน ซึ่งตอนนี้งงมากเลยอยากรบกวน ผู้รู้ชี้แนะผมหน่อยครับ