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public class MyFunction extends AbstractFunction<Double> {

    public static final SDFDatatype[][] accTypes = new SDFDatatype[][] {{ SDFDatatype.DOUBLE },
                                                                        { SDFDatatype.DOUBLE }};

    public MyFunction() {
        super("myFunction", 2, accTypes, SDFDatatype.DOUBLE);
    }

    @Override
    public Double getValue() {
        double a = (double) this.getInputValue(0);
        double b = this.getNumericalInputValue(1);

        return a  +b;
    }

    @Override
    final public int getTimeComplexity() {
        return 3;
    }

    @Override
    final public int getSpaceComplexity() {
        return 5;
    }
 }

Access to function attributes

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To access the additional content of an incoming streaming object you can use the getAdditionalContents method to access all contents. If you only want to access a special field you can issue the getAdditionalContent(fieldName) method.

Support for optimization

To support the optimization of predicates, the methods getTimeComplexity and getSpaceComplexity can be implemented. Both functions should return a value between 0-9 depending on their average expected complexity. Depending on the returned value, the MEP function will be placed differently in the resulting optimized predicate. A rule of thumb should be, MEP functions with logarithmic complexity should return a value between 0-3,  linear complexity a value between 4-6, and exponential complexity a value between 7-9. However, this is just a first draft. The basic idea is to evaluate cheap functions first and avoid expensive functions if possible.