Repeat loop while user choose option to continue

Question

I am developing an application in Java and after calculating everything the user asked, I need it to inform the following message: "Deseja calcular novamente? S/N".

I’m using the do/while but I’m not getting it to work. Below is the code:

package beta;

import java.util.Scanner;

/**
 *
 * @author lab
 */
public class Beta {

    public static void main(String[] args) {

        Scanner obj = new Scanner(System.in);
        String escolha ;
        double raio , area , base , altura , raioM , diagonalm , diagonalM ;
        String volta;
        Scanner scanner = new Scanner(System.in);


        do{

            System.out.println("O que deseja Calcular? ");
                    System.out.println("1. Circulo");
                    System.out.println("2. Triângulo");
                    System.out.println("3. Trapézio");
                    System.out.println("4. Retângulo");
                    System.out.println("5. Corda Circular");
                    System.out.println("6. Diagonal do Losango");
                    System.out.println("7. Sair");
            escolha = obj.nextLine();


switch(escolha){

             case "1":
                System.out.println("Qual o raio do Círculo? ");
                raio = obj.nextDouble();
                area = 3.14 * (raio*raio);
                System.out.println("A área do Círculo é " + area);
             break;

             case "2":
                System.out.println("Qual a base do Triângulo: ");
                base = obj.nextDouble();
                System.out.println("Qual a Altura do Triângulo: ");
                altura = obj.nextDouble();
                area = (base*altura)/2;
                System.out.println("A área do Triângulo é " + area);
             break;

             case "3":
                System.out.println("Qual a base do Trapézio: ");
                base = obj.nextDouble();
                System.out.println("Qual a Altura do Trapézio: ");
                altura = obj.nextDouble();
                area = (base+base)*altura/2;
                System.out.println("A área do Trapézio é " + area);
             break;
             case "4":
                System.out.println("Qual a base do Retângulo: ");
                base = obj.nextDouble();
                System.out.println("Qual a Altura do Retângulo: ");
                altura = obj.nextDouble();
                area = base*altura;
                System.out.println("A área do Retângulo é " + area);
             break; 
             case "5":
                System.out.println("Qual o maior raio da Corda Circular: ");
                raio = obj.nextDouble();
                System.out.println("Qual o menor raio da Corda Circular: ");
                raioM = obj.nextDouble();
                area = raio*raio - raioM*raioM;
                System.out.println("A área da corda circular é Pi " + area);
             break;
             case "6":
                System.out.println("Qual a maior Diagonal do Losango: ");
                diagonalM = obj.nextDouble();
                System.out.println("Qual a menor Diagonal do Losango: ");
                diagonalm = obj.nextDouble();
                area = (diagonalM*diagonalm)/2;
                System.out.println("A área do Losango é " + area);
             break;                          

           }   
      escolha = scanner.next();     
    } while(escolha.equalsIgnoreCase("Deseja calcular novamente? S/N"));            

    } // fim main

} // fim classe

Answer 1

First, there’s no need to create two Scanner's, since both are reading from System.in. Using just one is enough (and calling one of them obj doesn’t help much, give better names for variables).

Another point is that the escolha should be compared with "s", and not with the text "Deseja calcular novamente? S/N".

Another detail is that methods like nextDouble() do not consume line break (in this case, the ENTER), then you should call nextLine() soon after, otherwise the next call from nextLine() will consume only the line break, leaving the escolha empty.

Missed also check if the option is "7", and exit the while in this case. I also removed the variable volta, because it is not used in the code.

Anyway, the code would look like this:

Scanner scanner = new Scanner(System.in);
String escolha;
double raio, area, base, altura, raioM, diagonalm, diagonalM;

do {
    System.out.println("O que deseja Calcular? ");
    System.out.println("1. Circulo");
    System.out.println("2. Triângulo");
    System.out.println("3. Trapézio");
    System.out.println("4. Retângulo");
    System.out.println("5. Corda Circular");
    System.out.println("6. Diagonal do Losango");
    System.out.println("7. Sair");
    escolha = scanner.nextLine();
    // se for 7, sai do while
    if ("7".equals(escolha))
        break;

    switch (escolha) {
        case "1":
            System.out.println("Qual o raio do Círculo? ");
            raio = scanner.nextDouble(); // use o "scanner", não mais o "obj"
         .... dentro do switch é igual
    }

    scanner.nextLine(); // consumir a quebra de linha, pois nextDouble() não a consome
    System.out.println("Deseja calcular novamente? S/N");
} while (scanner.nextLine().equalsIgnoreCase("s")); // nem precisa guardar o resultado de nextLine() em uma variável, pode comparar direto

---

But if the options can only be numbers, then why not use nextInt() to read them?

Scanner scanner = new Scanner(System.in);
int escolha;
double raio, area, base, altura, raioM, diagonalm, diagonalM;

do {
    System.out.println("O que deseja Calcular? ");
    System.out.println("1. Circulo");
    System.out.println("2. Triângulo");
    System.out.println("3. Trapézio");
    System.out.println("4. Retângulo");
    System.out.println("5. Corda Circular");
    System.out.println("6. Diagonal do Losango");
    System.out.println("7. Sair");
    escolha = scanner.nextInt();
    if (escolha == 7)
        break;

    switch (escolha) {
        case 1:
           ....

        case 2:
           ....

           ....etc

        default: // pequena "firula"
            System.out.printf("Opçao inválida: %d\n", escolha);
    }

    scanner.nextLine();
    System.out.println("Deseja calcular novamente? S/N");
} while (scanner.nextLine().equalsIgnoreCase("s"));

Answer 2

You can kill this charade by looking at the condition of the while, in your condition "(choice.equalsIgnoreCase("Do you want to calculate again? S/N");" you are comparing if the user wrote "Do you want to calculate again? S/N" ignoring the size of the letters, that is, to present the menu again at the end of one of its options (considering that the user has typed a number in the range [1,7]) the user would have to type "Want to calculate again? S/N".

Probably what you want can be done by swapping

escolha = scanner.next();     
} while(escolha.equalsIgnoreCase("Deseja calcular novamente? S/N")); 

for

System.out.println("Deseja calcular novamente? S/N");
escolha = scanner.next();  

} while(escolha.equalsIgnoreCase("s")); 

Obs: There are passages in your code that can be improved, I tried to focus only on the core of the question.