将Java Enum值链接到动态实例化的对象

Question

I've got a code smell in my Java 8 application which I would like to resolve by some redesign.

I have declared an Enum DataType which contains not just enum values, but also an ordered list of these enum values based on some logic. To summarize, every enum value declares a set of dependencies on other enum values (of the same type) and the list is a topologically sorted list of these enum values. Every enum value also has a Calculator associated with it.

public Enum DataType
{
    A(EnumSet.noneOf(DataType.class), new CalculatorA()),
    B(EnumSet.of(A), new CalculatorB()),
    C(EnumSet.noneOf(DataType.class), new CalculatorC()),
    D(EnumSet.Of(A,B,C), new CalculatorD()),
    ... more ...

    ...constructor and attributes...

    public static final List<DataType> DATA_TYPES_SORTED_BY_DEPENDENCIES;

    static
    {
        DATA_TYPES_SORTED_BY_DEPENDENCIES = ...
    }
}

I know enums are naturally ordered (ordinal) and thus by declaring them in the correct order there would no need to do any kind of sorting. However, the amount of enum values is large and can change over time. I don't want to be burdened with maintaining the correct order myself, I want the program to do it for me if I just declare the dependencies. It is also self explaining code, because it is expliticly mentioned that cetain dependencies exist and that they are sorted.

The goal of my program is to run the calculators on a set of records in the correct order, hence why the enum is ordered:

for(Record r : records)
{
    for(DataType type : DataType.DATA_TYPES_SORTED_BY_DEPENDENCIES)
    {
        Integer result = type.getCalculator().calculate(r);
        r.getDataMap().put(type, result);
    }
} 

This setup was running ok until a requirement came that the calculators, which used to be statically instantiated within the enum class, should be parameterizable. The parameters will come from a webservice or database call. I have to instatiate the Calculator classes outside of the enum, and I have now lost the link between enum types and calculators.

To remedy that, I created a HashMap for easy retrieval:

    ...get some parameters from another service...

    Map<DataType, Calculator> calculatorsByTypeMap = new HashMap<>(400);

    calculatorsByTypeMap.put(DataType.A, new CalculatorA(someParameterA));
    calculatorsByTypeMap.put(DataType.B, new CalculatorB());
    calculatorsByTypeMap.put(DataType.C, new CalculatorC(someParameterC1, somereParameterC2));
    calculatorsByTypeMap.put(DataType.D, new CalculatorD(someParameterD));
    ...

    return calculatorsByTypeMap;

This bothers me because I have conceptually declared my (rather large) enum twice: once for the ordering logic, and once for the linking with the calculators. Is there a way to prevent this?

Answer 1

Build the enum with a null calculator field. At runtime, do a topological sort and build the sorted list. Get the new calculators and set the calculator value in the enum.