Java - implementation instances inside an interface

Question

In my project I need to create objects for each kind of Java Math Operator like "Add", "Substraction", "Multiplication", etc. And these operators should be singletons.

So here is what I am going to do. I define the Math Operator as an interface and I put those implementations inside it as I don't want to define singleton classes for each operator.

public interface MathOperator {


double operate(double a, double b);

MathOperator ADD = new MathOperator(){

    @Override
    public double operate(double a, double b) {
        return a + b;
    }

};

MathOperator SUBSTRACT = new MathOperator(){

    @Override
    public double operate(double a, double b) {
        return a - b;
    }

};  

}

I don't see much of such usage when I Google this. So I wonder if this is a good practice and if there are better and more graceful approaches?

Stan Kurilin · Accepted Answer · 2012-02-07 09:23:17Z

5

I would do smt like

1) Define interface

interface MathOperator {
    double operate(double a, double b);
}

2) Than have some common implementation in enum (less code)

enum MathOperators implements MathOperator {
    ADD {
        @Override
        public double operate(double a, double b) {
            return a + b;
        }
    },

    SUBTRACT {
        @Override
        public double operate(double a, double b) {
            return a - b;
        }
    }
}

3) Or public static members (more clean solution).

class MathOperators {
    public static MathOperator ADD = new MathOperator() {
        @Override
        public double operate(double a, double b) {
            return a + b;
        }
    };
    public static MathOperator SUBTRACT = new MathOperator() {
        @Override
        public double operate(double a, double b) {
            return a - b;
        }
    };
}

can create new MathOperator without changing MathOperators
have nice API for common operations
shouldn't write singletons
have nice clean interface

edited Feb 7, 2012 at 9:23

answered Feb 7, 2012 at 9:07

Stan Kurilin

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2 Comments

Peter Lawrey Over a year ago

You need to make MathOperators implement MathOperator otherwise you are not @Override-ing anything.

Jay Over a year ago

Yep, this looks better I will take the 'enum implements interface' solution. Thanks.

NPE · Accepted Answer · 2012-02-07 08:29:48Z

5

One idiom that I've seen used in precisely these circumstances is the use of enum:

public enum MathOperator {

    ADD {
        @Override
        public double operate(double a, double b) {
            return a + b;
        }
    },

    SUBTRACT {
        @Override
        public double operate(double a, double b) {
            return a - b;
        }
    };

    public abstract double operate(double a, double b);

}

answered Feb 7, 2012 at 8:29

NPE

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3 Comments

Bohemian Over a year ago

enums are OK when you have a finite number of possible impls, but when you just want some common ones handy, the interface constants rule. What you could do though is have enum MathOperatorImpl implements MathOperator! I like that even better!! :D

Jay Over a year ago

Thanks @Bohemian, I will take the 'enum implements interface' solution.

Malcolm Over a year ago

@Bohemian I remember such a recommendation in Effective Java, to use interfaces for emulating extensible enums. I think Jay is absoultely right to use this solution.

Bohemian · Accepted Answer · 2012-02-07 08:26:24Z

4

I use this pattern often, especially for specific implementations of generic interfaces. I find it works really well for me.

I like the way it puts the implementations where you can find them. I do it slightly differently - I make them static (it's a style thing, so the interface impls look more like class impls):

public interface MathOperator {

    double operate(double a, double b);

    public static MathOperator ADD = new MathOperator() {
        @Override
        public double operate(double a, double b) {
            return a + b;
        }
    };

    public static MathOperator SUBSTRACT = new MathOperator() {
        @Override
        public double operate(double a, double b) {
            return a - b;
        }
    };

}

answered Feb 7, 2012 at 8:26

Bohemian♦

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1 Comment

Bohemian Over a year ago

yes, but for abstract classes I use static, so the code style stays consistent

Lukas Eder · Accepted Answer · 2012-02-07 08:28:19Z

2

I don't see any problem with it. Take java.lang.String.CASE_INSENSITIVE_ORDER for instance. It is almost the same, except that

String is not an interface but a final class
The Comparator is not declared using an anonymous class, but using a static inner class, which is essentially the same

answered Feb 7, 2012 at 8:28

Lukas Eder

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Comments

casablanca · Accepted Answer · 2012-02-07 08:31:56Z

1

Personally, I don't like putting implementations within interfaces. I would either:

make MathOperator an enum
keep the interface but have a factory or a static class (say MathOperators) with the implementations

answered Feb 7, 2012 at 8:31

casablanca

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Comments

adranale · Accepted Answer · 2012-02-07 09:21:40Z

0

Why don't you define each implementation in its own class/file? This would make it clearer and would leave the interface clean.

answered Feb 7, 2012 at 9:21

adranale

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