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Manish Kumar
Manish Kumar

Reputation: 10502

Call parametrised constructor in enum singleton?

I have this enum singleton class:

enum C1 {
    INSTANCE("");

    C1(String s) {
        System.out.println("with param = " +s);
    }
    C1() {
        System.out.println("without param");
    }   
    public void g() {
        System.out.println("inside g");
    }
}

public class Main {
    public static void main(String s[]) {
        C1.INSTANCE.g();
        C1.INSTANCE.g();

    }
}

How can i call C1(String s) constructor using INSTANCE by passing custom parameter ?

Upvotes: 0

Views: 547

Answers (3)

Level_Up
Level_Up

Reputation: 824

Generally you can use class type, not enum type. Enums are intended to be static and final. The idea of the enum constructor is to encapsulate several values so you can use them after by setting only one type of enum.

public enum Vehicle {
    CAR(4, 2),
    TRUCK(6, 20);

    private Integer numOfTyres;
    private Integer maxWeight;

    Vehicle(Integer numOfTyres, Integer maxWeight) {
        this.numOfTyres = numOfTyres;
        this.maxWeight = maxWeight;
        System.out.println("Number of Tyres = " + numOfTyres);
        System.out.println("Max weight = " + maxWeight);
    }


    public Integer getMaxWeight() {
         return maxWeight;
    }

    public Integer getNumberOfTyres() {
        return numOfTyres;
    }

}


public class Main {
    public static void main(String s[]) {
        Vehicle.CAR.getNumberOfTyres();

        for (Vehicle v : Vehicle.values()) {
            System.out.println();

            StringBuilder sb = new StringBuilder();
            sb.append("Vehicle ");
            sb.append(v.name());
            sb.append(" has ");
            sb.append(v.getNumberOfTyres());
            sb.append(" number of tyres and ");
            sb.append(v.getMaxWeight());
            sb.append(" max weight ");


            System.out.println("-----------------");
            System.out.println(sb.toString());
            System.out.println("-----------------");
        }

    }

}

Upvotes: 1

ikos23
ikos23

Reputation: 5354

As you probably know - the constructor for an enum type must be package-private or private access.

It is called automatically and creates the constants that are defined at the beginning of the enum body.

You cannot invoke an enum constructor yourself.

Here are some examples that might be helpful. 

public enum MySingleton {
    // if you have multiple constants here - it's not Singleton
    INSTANCE;

    // if you need to provide some info to your INSTANCE,
    // you can add params to its methods
    public void someMethod(String value) {
        System.out.println("inside someMethod, value is " +value);
    }
}

If you need the instance to have some state you can add fields and a constructor:

public enum MySingleton {
    INSTANCE("Hello");

    private String message;

    MySingleton(String msg) {
        this.message = msg;
    }

    // you can even have getter/setter for it
    public String getMessage() {
        return message;
    }

    public void setMessage(String message) {
        this.message = message;
    }
}

Main method sample:

public static void main(String[] args) {
    System.out.println(MySingleton.INSTANCE.getMessage()); // prints Hello

    MySingleton.INSTANCE.setMessage("Bye");

    System.out.println(MySingleton .INSTANCE.getMessage()); // prints Bye
}

Upvotes: 0

Alexander Pavlov
Alexander Pavlov

Reputation: 2220

You can have someting like that

    enum C1 {
        WITH_PARAM("value"),
        EMPTY();

        private String value;
        C1(String s) {
            System.out.println("with param = " +s);
            value=s;
        }
        C1() {
            System.out.println("without param");
        }
        public void g() {
            System.out.println("inside g, value is "+value);
        }
    }

        public static void main(String s[]) {
            C1.EMPTY.g();
            C1.WITH_PARAM.g();

        }

Upvotes: 2

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