在Java中将int转换为枚举

Question

I'm writing a polynomial calculator for a course assignment and I want to use enum in the switch case.

this is the enum method I wrote:

enum Options{
    ADDITION(1),
    MULTIPLICATION(2),
    EVALUATION(3),
    DERIVIATE(4),
    EXIT(5);

    public final int value;

     Options(int value){
         this.value = value;
     }

     public int getValue() {
         return value;
     }

     public static Options convert(int n) {
         for(Options o : values())
             if(o.getValue() == n)
                 return o;
         return null;
     }

}

this switch case in the program:

choice = mysc.nextInt();
Options o = Options.convert(choice);
switch(o)
{
    case ADDITION: Add(isRational);break;
    case MULTIPLICATION: Mul(isRational);break;
    case EVALUATION: Evaluate(isRational);break;
    case DERIVIATE: Derivative(isRational);break;
    case EXIT: break;
}

I want to use enum in the switch case but I want the input from the user to be an int, because it's a requirement for the assignment.

I want to convert the int I get from the user to the corresponding enum and use it in the switch case.

is there a way to convert it to the correct enum without using the convert method inside the enum?

Answer 1

Change your Enum to start at a 0 based index and use Enum.values()[index] . Here is a simple example of the code and you'll probably want to introduce some sort of validation for invalid enum values:

import java.io.BufferedInputStream;
import java.util.Scanner;

class Scratch {
    public static void main(String[] args) {
        Scanner input = new Scanner(new BufferedInputStream(System.in));
        System.out.println("Enter Value");
        int choice = Integer.parseInt(input.nextLine());
        System.out.println(choice);
        Options enumValue = Options.values()[choice];
        System.out.println(enumValue);
    }
}

enum Options{
    ADDITION(0),
    MULTIPLICATION(1),
    EVALUATION(2),
    DERIVIATE(3),
    EXIT(4);

    public final int value;

    Options(int value){
        this.value = value;
    }

    public int getValue() {
        return value;
    }
}

Output

Also FYI, Enum.values()[index] can be a bit expensive so keep that in mind if performance is an issue. As this is for an assignment, I doubt that is a concern but something to keep in mind for your own understanding.

Answer 2

The mapping between integers and enum values is not really part of the API that uses the enum values. Right now, you're asking the user to input an integer. But what if you later write a GUI and present the names of the enum values in a drop down? Or write a parser to parse arithmetic expressions? Those integers will no longer be relevant. They exist for the menu, so they belong to the part of the program that displays the menu.

You could introduce a Map<Integer,Options> that would map user input to enum values. But if you added an new value to the enum, you'd have to remember to also add it to your Map .

I'd use the array returned by Options.values() . Iterate over it to display your menu, and use it to look up values by index. There are also caveats to this approach. If you rearrange the values in your code, their order will change in the array as well. The order of menu options would change, possibly confusing the user. But your program would still run. The real problem with using enum ordinal values is when you serialize them (write them to persistent storage, like a file) and then try to read them back after changing the order. As long as you're not serializing your enum constants, it's okay to use their ordinal values.