CODEWITHSUNDEEP
javaenumsinstance
florin flo
florin flo

Reputation: 119

Java enum instatiate using constructor parameter

I have this enum:

    public enum Operation {
        ADD {
            public double apply(double a, double b) {
                return a + b;
            }
        },
        SUBTRACT {
            public double apply(double a, double b) {
                return a - b;
            }
        }
        } ;

        public abstract double apply(double a, double b);
`}`

I want to instantiate this like:

Operation op=new Operation("+");
op.aply(2,3);//now use ADD

Is it possible to write a constructor with string parameter that tells to enum which operation to aply?

Upvotes: 0

Views: 306

Answers (5)

Mark Rotteveel
Mark Rotteveel

Reputation: 109233

You can define a constructor in an enum, but you cannot use it outside of the enum definition. The available enum values are fixed at compile time and you can't create more at runtime by using new. If you want to look up the enum by symbol, you need to define a lookup in the code.

For example by switching on the symbol:

public enum Operation {

    ADD {/*...*/};

    // ...

    public static Operation forSymbol(String symbol) {
        switch(symbol) {
        case "+":
            return ADD;
        //...
        default:
            throw new IllegalArgumentException("Unsupported symbol: " + symbol);
        }
    }
}

Or by defining the symbol together with the enum, and using that in the look up:

public enum Operation {

    ADD("+") {/*...*/};

    private final String symbol;

    private Operation(String symbol) {
        this.symbol = symbol;
    }

    // ...

    public static Operation forSymbol(String symbol) {
        for (Operation operation : Operation.values()) {
            if (operation.symbol.equals(symbol)) return operation
        }
        throw new IllegalArgumentException("Unsupported symbol: " + symbol);
    }
}

Upvotes: 1

user3838784
user3838784

Reputation:

Unfortunately, no. You can create a method with a String parameter that returns the specific Operation depending on what is the parameter. Example :

public enum Operation {
    ADD("+") {
        public double apply(double a, double b) {
            return a + b;
        }
    },
    SUBTRACT("-") {
        public double apply(double a, double b) {
            return a - b;
        }
    };

    private String operationChar;

    private Operation(String s) {
        this.operationChar = s;
    }

    public static Operation getOperation(String s) {
    for (Operation o : Operation.values()) {
            if (o.operationChar.equals(s))
                return o;
        }
        return null;
    }

    public abstract double apply(double a, double b);
}

You can get the instance like this :

Operation op = Operation.getOperation("+");
op.apply(2,3); // ADD will be used here

(You can use a char instead of a String, since most operations only have one char (1+1, 1-1, 1*1, 1/1))

Upvotes: 2

Mena
Mena

Reputation: 48444

You can try the following example.

It's a bit over-engineered (you could ditch the whole "+" or "-" and call elements by name) but it works. 

public enum Operation {
    ADD("+") {
        public double apply(double a, double b) {
            return a + b;
        }
    },
    SUBTRACT("-") {
        public double apply(double a, double b) {
            return a - b;
        }
    };
    public abstract double apply(double a, double b);
    String type;
    Operation(String type) {
        this.type = type;
    }
    public String getType() {
        return type;
    }
    static Operation get(String type) {
        for (Operation o: Operation.values()) {
            if (o.getType().equals(type)) {
                return o;
            }
        }
        throw new NoSuchElementException();
    }
    public static void main(String[] args) throws Exception {
        System.out.println(Operation.get("+").apply(1, 2));
    }
}

Output

3.0

Upvotes: 1

nickb
nickb

Reputation: 59709

You can define a public static method to get the correct operation based on a String:

public static Operation get(String input) {
    if(input.equals("+")) {
        return ADD;
    }
    if(input.equals("-")) {
        return SUBTRACT;
    }
    throw new IllegalArgumentException();
}

Then you would call it like:

Operation op = Operation.get("+");
op.apply(2, 3);

Upvotes: 4

Pphoenix
Pphoenix

Reputation: 1473

It is not possible, due to the fact that enums cannot have public constructors (more information in this question). To use an Operation you will have to manually tell the compiler which Operation to use. This can be done by: 

Operation op = ADD;
double x = op.apply(2,3);

which uses ADD as the Operation.

Upvotes: 0

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