In this article, we provide several examples of the usage of Enums in Java.

Enum Overview

Enum is a Java type/class that holds a fixed set of related constants. It is a replacement for the traditional definition of multiple static final variables in Java. It is mostly recommended when you define a method that accepts a constant argument. This way you force the method callers to commit to the predefined constants and prevent them from passing random constant values.

A Typical Enum

public enum ErrorCodes {
    BUSINESS_ERROR(100), SERVER_ERROR(500), NETWORK_ERROR(1000);

    private int errorCode;

    private ErrorCodes(int errorCode) {
    this.errorCode = errorCode;
    }

    public int getErrorCode() {
    return errorCode;
    }
}

The above enum can be represented traditionally as:

public static final int BUSINESS_ERROR = 100;
public static final int SERVER_ERROR = 500;
public static final int NETWORK_ERROR = 1000;

Some points to consider when defining an enum:

1. Enum implicitly extends java.lang.Enum, so it can't extend other classes.
2. Enum constructors can never be invoked in the code — they are always called automatically when an enum is initialized.
3. You can't create an instance of Enum using new operators. It should have a private constructor and is normally initialized as: ErrorCodes error = ErrorCodes.BUSSINESS_ERROR
4. Each constant in the enum has only one reference, which is created when it is first called or referenced in the code.

Common Operations of Enum

The following are common operations of Enum:

Instantiate Enum

private static void instantiateEnum() {
    ErrorCodes businessError = ErrorCodes.BUSINESS_ERROR;
    System.out.println(businessError);
}

Output:

BUSINESS_ERROR

Convert String to Enum

private static void convertStringToEnum() {
    ErrorCodes businessError = ErrorCodes.valueOf("BUSINESS_ERROR");
    System.out.println(businessError);
    System.out.println(businessError.getErrorCode());
}

Output:

BUSINESS_ERROR
100

Compare Enums

To compare enums, you can use: ==, equals() or switch() block.

private static void checkEnumForEquality() {
    ErrorCodes businessError = ErrorCodes.BUSINESS_ERROR;

    if(businessError == ErrorCodes.BUSINESS_ERROR)
    {
    System.out.println("You can check enum for equality using == operator.");
    }

    if(businessError.equals(ErrorCodes.BUSINESS_ERROR))
    {
    System.out.println("You can check enum for equality using equals() method.");
    }

    switch(businessError)
    {
        case BUSINESS_ERROR: 
        {
            System.out.println("You can check enum for equality using switch block.");
            break;
        }
        default: System.out.println("Non-business error.");
    }
}

Output:

You can check enum for equality using == operator.
You can check enum for equality using equals() method.
You can check enum for equality using switch block.

Iterate over Enum values

private static void iterateEnumValues() {
    System.out.println("Iterating over ErrorCodes enum");
    for(ErrorCodes errorCode : ErrorCodes.values())
    {
        System.out.println("Enum key = " + errorCode);
        System.out.println("Enum value = " + errorCode.getErrorCode());
    }
}

Output:

Iterating over ErrorCodes enum
Enum key = BUSINESS_ERROR
Enum value = 100
Enum key = SERVER_ERROR
Enum value = 500
Enum key = NETWORK_ERROR
Enum value = 1000

Retrieve Enum by Value

By default, Java doesn't provide any way to retrieve an Enum instance by its value. To do so, update ErrorCodes enum in order to expose a hashmap, which maps each value to its corresponding Enum instance. The hashmap is filled and exposed as the following:

public enum ErrorCodes {
    BUSINESS_ERROR(100), SERVER_ERROR(500), NETWORK_ERROR(1000);

    private int errorCode;
    private static Map<Integer, ErrorCodes> errorCodeByErrorNumber = new HashMap<Integer, ErrorCodes>();

    static {
        for (ErrorCodes errorCode : ErrorCodes.values()) {
        errorCodeByErrorNumber.put(errorCode.getErrorCode(), errorCode);
        }
    }

    private ErrorCodes(int errorCode) {
    this.errorCode = errorCode;
    }

    public int getErrorCode() {
    return errorCode;
    }

    public static ErrorCodes getErrorCodeByNumber(Integer errorNumber) {
    return errorCodeByErrorNumber.get(errorNumber);
    }
}

Now, you can retrieve an enum instance of a numeric value as the following:

private static void retrieveEnumByValue() {
    ErrorCodes businessError = ErrorCodes.getErrorCodeByNumber(100);
    System.out.println(businessError);
}

Output:

BUSINESS_ERROR

That's it!