CODEWITHSUNDEEP
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Kuan
Kuan

Reputation: 11389

Explain of a ENUM definition in JAVA

All:

I am on first day reading team's code(the one wrote this left...):

There is one enum definition confused me so much:

/**
 * Enum defines the processing stages and the order
 *
 */
public enum ProcessStage {

    /*
     * Individual stages in the process.
     * Order of processing is based on the order of listing.
     */
    EXTRACT("Extraction", "EXTRACTED", "EXTRACTION_FAILED"), 
    ROUTE("Routing", "ROUTED", "ROUTE_FAILED"), 
    PUBLISH("Publishing", "PUBLISHED", "PUBLISH_FAILED");

    private String detailedName;
    private String successState;
    private String failedState;

    private ProcessStage(String detailedName, String successState, String failedState) {
        this.detailedName = detailedName;
        this.successState = successState;
        this.failedState = failedState;
    }
    public String getSuccessState() {
        return successState;
    }
    public String getFailedState() {
        return failedState;
    }

    /**
     * Factory method to provide the ProcessStage from its success or failed state value stored in DB.
     * @param state
     * @return ProcessStage
     */
    public static ProcessStage getProcessStage(String state) {
        for(ProcessStage ps: ProcessStage.values()) {
            if(ps.getSuccessState().equals(state) || ps.getFailedState().equals(state)) {
                return ps;
            }
        }

        return null;
    }

    public String toString() {
        return detailedName;
    }

}

I wonder if anyone give me some simple introduction about how to read this(like what kinda syntax it uses)? The most confused part is:

    EXTRACT("Extraction", "EXTRACTED", "EXTRACTION_FAILED"), 
    ROUTE("Routing", "ROUTED", "ROUTE_FAILED"), 
    PUBLISH("Publishing", "PUBLISHED", "PUBLISH_FAILED");

I do not quite understand what this means and how to use this.

And why there are a lot of methods defined inside it and how to use method with a enum variable?

Thanks

Upvotes: 0

Views: 63

Answers (2)

rajuGT
rajuGT

Reputation: 6404

Enum

The enum declaration defines a class (called an enum type). The enum class body can include methods and other fields. The compiler automatically adds some special methods when it creates an enum.

enums are special type of class. Instead of creating singleton pattern using regular class or to create constants, like WeekDays, we can use enum in such places. Here

EXTRACT("Extraction", "EXTRACTED", "EXTRACTION_FAILED"),

Here EXTRACT is an enum constant meaning it is an instance of the classProcessStage and also all other enum constants(ROUTE, PUBLISH). All costants of enum are unique objects, meaning they are singleton instance created in the jvm and enum makes sure the instances are unique. You need not to put additional effort to create singleton pattern.

The above code is not only declaration, it is also calling the constructor with three String parameters to create the instance.

  private ProcessStage(String detailedName, String successState, String failedState) {
        this.detailedName = detailedName;
        this.successState = successState;
        this.failedState = failedState;
    }

why there are a lot of methods defined inside it?

Since it is also a class, it can have methods like any other classes. But the restriction is, it cannot be inherited, because internally enum extens the class Enum<E extends Enum<E>> class.

how to use method with a enum variable?

EXTRACT.getFailedState() //returns "EXTRACTION_FAILED"

Upvotes: 2

Brandon Laidig
Brandon Laidig

Reputation: 420

Keep in mind, without seeing more of the code, I can't be exactly sure what this particular enum is being used for. So, Let's say we have a method somewhere, where a process is passed through. 

public void doSomething(Process process) {}

Now, let's assume that the purpose of this method is to check the status of the process and then do some logic based upon that result. This would entail doing something like the following

public void doSomething(Process process) {
    if(ProcessStage.EXTRACT.equals(process.getStage()) {
        //do something here...you will have access to the methods within
        //the enum
    }
}

Without knowing more, this is all I can give you. I hope this gives you a slightly better understanding of what that enum is doing

Upvotes: 1

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