Kalkulator

package naya;
import java.util.*;
public class Kalkulator {
    public static void main(String[] args) {
        char lanjut;
        Scanner in = new Scanner(System.in);
        String output;
        do {
            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);
            hasil(output);
            System.out.print("Lakukan lagi?(y/t): ");
            lanjut = in.next().charAt(0);
        } 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 String operasi (double a, double b){
        char lanjut; int operasi; String out = "";
        Scanner in = new Scanner(System.in);
        do{
            System.out.print("Pilih operasi: ");
            operasi = in.nextInt();
            switch(operasi){
                case 1 :
                    out += Double.toString(jumlah(a,b)) + " ";
                    break;
                case 2 :
                    out += Double.toString(kurang(a,b)) + " ";
                    break;
                case 3 :
                    out += Double.toString(kali(a,b)) + " ";
                    break;
                case 4 :
                    out += Double.toString(bagi(a,b)) + " ";
                    break;
            }
            System.out.print("Lanjut?(y/t): ");
            lanjut = in.next().charAt(0);
        } while (lanjut == 'y');
        return out;
    }
    public static void hasil(String a){
        String[] output = a.split(" ");
        double[] hasil = new double[output.length];
        for (int i = 0; i < output.length; i++) {
            hasil[i] = Double.parseDouble(output[i]);
            System.out.println("Hasil operasi ke-" + (i+1) + ": " + hasil[i]);
        }
    }
}