How to use enum with grouping and subgrouping hierarchy/nesting

问题

I have one enum 'class' called Example as follows:

enum Example {
//enums belonging to group A:
   enumA1,
   enumA2,
   enumA3,
//enums belonging to group B:
   enumB1,
   enumB2,
   enumB3,
//enums belonging to group C:
   enumC1,
   enumC2,
   enumC3;
}

It's important for my project they all enums I work with belong to Example (since this is an argument in a constructor of a class).

How do I use enum hierarchy/nesting in order to achieve the following:

A method that tests whether an enum is of group A, B or C. For example, something like Example.enumA1.isGroupBelonging(Group.A) or isGroupBelonging(Example.enumA1,Group.A) would be a public method that returns true.
Be able to do the same thing with sub-groups of group A, B and C. For example, group A might have subgroups a, b and c. I then want a method that does something such as Example.enumA1.isSubGroupBelonging(SubGroup.a) which is a public boolean.
A way to do all this without needing to have some elaborate enum name that clogs up my code. For example, it would be nice to just be able to refer to Example.enumA1 throughout my other classes without needing to refer to it using something like Example.enumA1(Group.A,SubGroup.a) or Example.enumA1.Group.A.SubGroup.a

回答1:

I would use a very simple enum constructor which associates the corresponding group with the enum value:

public enum Example {

    ENUM_A1 (Group.A),
    ENUM_A2 (Group.A),
    ENUM_A3 (Group.A),

    ENUM_B1 (Group.B),
    ENUM_B2 (Group.B),
    ENUM_B3 (Group.B),

    ENUM_C1 (Group.C),
    ENUM_C2 (Group.C),
    ENUM_C3 (Group.C);

    private Group group;

    Example(Group group) {
        this.group = group;
    }

    public boolean isInGroup(Group group) {
        return this.group == group;
    }

    public enum Group {
        A,
        B,
        C;
    }
}

Usage:

import static Example.*;
import Example.Group;
...

ENUM_A1.isInGroup(Group.A);  // true
ENUM_A1.isInGroup(Group.B);  // false

To do the subgroups you can do a similar kind of structure for Group as for Example, using Group(SubGroup ... subgroups) as a constructor and an EnumSet<SubGroup> to contain the subgroups.

回答2:

You can use EnumSet to group various enums without creating a separate Enum class:

public enum Example {

    ENUM_A1, ENUM_A2, ENUM_A3,
    ENUM_B1, ENUM_B2, ENUM_B3,
    ENUM_C1, ENUM_C2, ENUM_C3;

    public static EnumSet<Example> groupA = EnumSet.of(ENUM_A1, ENUM_A2, ENUM_A3);
    public static EnumSet<Example> groupB = EnumSet.of(ENUM_B1, ENUM_B2, ENUM_B3);
    public static EnumSet<Example> groupC = EnumSet.of(ENUM_C1, ENUM_C2, ENUM_C3);

}   

public static void main(String[] args){
    if(Example.groupA.contains(Example.ENUM_A1)){
       System.out.println("Group A contains ENUM A1");
    }
}

回答3:

Small addition:

I do like both answers from @ELEVATE and @Wendel. And it depends (as always) on the scenario when to use which solution. If each entry must be part of a single group I prefer ELEVATE's solution, because it forces me to add this single group. However if only some enum need to be grouped or they are not distinct I go with Wendel's solution, because it is more flexible (but more error prone)

来源：https://stackoverflow.com/questions/19680418/how-to-use-enum-with-grouping-and-subgrouping-hierarchy-nesting

标签

java

enums

nested

grouping

hierarchy