为什么Java中“枚举”的实例字段是“枚举”？

Question

For the below TypeAndSize class,

class TypeAndSize {

  Species type;               // runType EMPTY, SHARK, or FISH
  int size;                   // Number of cells in the run for that runType.

  enum Species{EMPTY,SHARK,FISH}

  /**
   *  Constructor for a TypeAndSize of specified species and run length.
   *  @param species is Ocean.EMPTY, Ocean.SHARK, or Ocean.FISH.
   *  @param runLength is the number of identical cells in this run.
   *  @return the newly constructed Critter.
   */

  TypeAndSize(Species species, int runLength) {
    if (species == null)    {   
      System.out.println("TypeAndSize Error:  Illegal species.");
      System.exit(1);
    }
    if (runLength < 1) {
      System.out.println("TypeAndSize Error:  runLength must be at least 1.");
      System.exit(1);
    }
    this.type = species;
    this.size = runLength;
  }

}

with the usage of member fields of enum class type in below code,

class RunLengthEncoding {
    ...
    public RunLengthEncoding(int i, int j, int starveTime) {
          this.list = new DList2();
          this.list.insertFront(TypeAndSize.Species.EMPTY, i*j);
          ....
      }
    ...
}

let me ask this question.

My question: Why member field of enum class designed to be an instance of enum ? Because of the ease in passing on the parameters of enum class type which can be backward compatible with old concept of enum in C language which was a collection of constants? Is that the reason?

Answer 1

What you are talking about ( TypeAndSize.Species.EMPTY ) is not a member field of Spicies . When we talk about "member field", it normally implies instance variable (for which it is also possible to write in enum in Java).

In the aspect you are asking, you can simply interpret enum being a special shorthand of writing a special class with class constants:

enum Foo {
  A,
  B;
}

is similar to

class Foo {
    public static final Foo A = new Foo();
    public static final Foo B = new Foo();

    private Foo() {}
}

(There are still a lot of difference between enum and the handcrafted class, but they are not yet your concern)

Answer 2

with the usage of member fields of enum class type

TypeAndSize.Species.EMPTY is not using a member field. It is access the enum type defined in TypeAndSize.

Species type is defining a member field but it's type isn't enum, it's type is Species.

Enum is on the same level as class and interface. These are the building block types we can use. The reason that we want to use enums over integer constants can be read elsewhere .

Hopefully the link link helps answer the second part of your question.
I've tried my best to clear up the terms used in the question.