Kalkulator

package naya;
import java.util.*;
public class Kalkulator {
    public static void main(String[] args) {
        char lanjut;
        Scanner in = new Scanner(System.in);
        double output = 0;
        int operasi = 0;
        opening();
        for (int i = 0; i < 6; i++) {
            if (operasi == 0){
                System.out.print("Masukkan bilangan pertama: ");
                double input1 = in.nextDouble();
                System.out.print("Masukkan bilangan kedua: ");
                double input2 = in.nextDouble();
                System.out.println("Pilihan operasi:\n1. Penjumlahan\n2. Pengurangan\n3. Perkalian\n4. Pembagian");
                output = operasi(input1, input2);
                System.out.println("Hasil operasi: " + output);
            } else {
                System.out.println("Bilangan pertama adalah " + output);
                System.out.print("Masukkan bilangan kedua: ");
                double input2 = in.nextDouble();
                System.out.println("Pilihan operasi:\n1. Penjumlahan\n2. Pengurangan\n3. Perkalian\n4. Pembagian");
                output = operasi(output, input2);
                System.out.println("Hasil operasi: " + output);
            }
            operasi++;
        }
        do {
            System.out.println("Bilangan pertama adalah " + output);
            System.out.print("Masukkan bilangan kedua: ");
            double input2 = in.nextDouble();
            System.out.println("Pilihan operasi:\n1. Penjumlahan\n2. Pengurangan\n3. Perkalian\n4. Pembagian");
            output = operasi(output, input2);
            System.out.println("Hasil operasi: " + output);
            System.out.print("Lakukan lagi?(y/t): ");
            lanjut = in.next().charAt(0);
            operasi++;
        } while (lanjut == 'y');
        System.out.println("Terima kasih");
    }
    public static double jumlah (double a, double b){
        return a + b;
    }
    public static double kurang (double a, double b){
        return a - b;
    }
    public static double kali (double a, double b){
        return a * b;
    }
    public static double bagi (double a, double b){
        return a / b;
    }
    public static double operasi (double a, double b){
        char lanjut; int operasi; double out = 0;
        Scanner in = new Scanner(System.in);
        System.out.print("Pilih operasi: ");
        operasi = in.nextInt();
        switch(operasi){
            case 1 :
                out = jumlah(a,b);
                break;
            case 2 :
                out = kurang(a,b);
                break;
            case 3 :
                out = kali(a,b);
                break;
            case 4 :
                out = bagi(a,b);
                break;
        }
        return out;
    }
    public static void opening(){
        System.out.println("***************");
        System.out.println("Kalkulator v1.1");
        System.out.println("***************");
        System.out.println();
    }
}