Introduction

That post shows a simple way of how if-else statements can be changed with enums in Java. On that tutorial, we will use a simple application which was written solely using if-else statements, and remove all of them by creating an enum with constructor.

The example program

The Java program we are going to use for this tutorial is a simple command-line application that returns a HTTP status code after the user provides a required message.

For example, if we execute it with the argument "SUCCESS", then its return is "200".

The old code

Our application is composed of several if-else statements. Therefore, basing on user's message, it is tested on four conditions to define the corresponding HTTP status code and return it to the user.

public class Application {

    public static void main(String[] args) {

        if (args.length < 1) {
            System.out.println("No args provided!");
            System.exit(0);
        }

        String providedMessage = args[0];
        int returnCode = 0;

        if ("SUCCESS".equalsIgnoreCase(providedMessage)) {
            returnCode = 200;
        } else if ("CREATED".equalsIgnoreCase(providedMessage)) {
            returnCode = 201;
        } else if ("MOVED".equalsIgnoreCase(providedMessage)) {
            returnCode = 301;
        } else if ("UNAUTHORIZED".equalsIgnoreCase(providedMessage)) {
            returnCode = 401;
        }

        System.out.println("Your HTTP code is: " + returnCode);
    }
}

Running that Java program:

marcos@Desktop: javac Application.java
marcos@Desktop: java Application success
Your HTTP code is: 200.

The refactored code

To make that possible, we can use a property of java ENUMS: the constructors. Using it, for each value provided for our ENUM, we can define a default return value.

Writing our ENUM

The first change to be done is define our enum class.

enum HttpCode {

    SUCCESS,
    CREATED,
    MOVED,
    UNAUTHORIZED;

}

As much as the class is declared, we are not allowed to use it for replace our if-else statements, since Java will not know the code that needs to be returned after the instantiation of our enum HttpCode.

This can be ajusted very simply, by defining a constructor for HttpCode.

enum HttpCode {

    SUCCESS(200),
    CREATED(201),
    MOVED(301),
    UNAUTHORIZED(401);

    private int code;

    HttpCode(int code) {
        this.code = code;
    }

    public int getCode(){
        return this.code;
    }
}

Notice that a field code was created inside our enum class. Since each options of HttpCode has a default status defined for the contructor, after an instantiation using the method HttpCode.valueOf(String message), one of the default values will be stored at the field code, which can be obtained by the method HttpCode.getCode().

Adjusting the main method

Now that we have an enum class, we just need to change our original main code to use it.

public static void main(String[] args) {

    if (args.length < 1) {
        System.out.println("No args provided!");
        System.exit(0);
    }

    String providedMessage = args[0];
    HttpCode httpStatusCode = HttpCode.valueOf(providedMessage.toUpperCase());

    System.out.println("Your HTTP code is: " + httpStatusCode.getCode());
}

Done! Now, our application can return for user a HTTP status code for a provided message. The following code shows our entire class, already refactored.

public class Application {

    public static void main(String[] args) {

        if (args.length < 1) {
            System.out.println("No args provided!");
            System.exit(0);
        }

        String providedMessage = args[0];
        HttpCode httpStatusCode = HttpCode.valueOf(providedMessage.toUpperCase());

        System.out.println("Your HTTP code is: " + httpStatusCode.getCode());
    }
}

enum HttpCode {

    SUCCESS(200),
    CREATED(201),
    MOVED(301),
    UNAUTHORIZED(401);

    private int code;

    HttpCode(int code) {
        this.code = code;
    }

    public int getCode() {
        return this.code;
    }
}

Running that Java program:

marcos@Desktop: javac Application.java
marcos@Desktop: java Application UNAUTHORIZED
Your HTTP code is: 401.

The first thing we can think on that case is: "Okay, I removed my statements, but I also improved the quantity of lines of my code!". In the truth, it seems that our application is much bigger, but on real cases, that can decrease the quantity of lines, since the ENUM can be reused on another parts of our code.

Also, makes the code more readable, since the if-else structures, with several validations, ceases to exist, and Java starts to do this verification for us.

Another good thing is that we can add more options at enums's class without the need of change at all occurrences of status code validations, giving it only exists in the enum that was created.

Conclusion

This is a simple, and not that much applicable, example. However, it can demonstrate a better way to use enums. It's also important to note: do not change every if-else statement you see without think on consequences. This is an idea of how to do things on other way, every scenario is different, you must check, agreed ;D?

That's all!
Please, leave your suggestions, corrections and tests on comments. Thank you for reading :D.