What does EnumSet really mean?

Question

I have the following example:

import java.util.EnumSet; import java.util.Iterator;  public class SizeSet {      public static void main(String[] args) {         EnumSet largeSize = EnumSet.of(Size.XL,Size.XXL,Size.XXXL);         for(Iterator it = largeSize.iterator();it.hasNext();){             Size size = (Size)it.next();             System.out.println(size);         }     } }   enum Size {   S, M, L, XL, XXL, XXXL;  } 

In this code I can understand that the Enum creates an Enum type of Sizes.

My question is: is largeSize an object of EnumSet type? What does it really mean? I really want to understand it better.

Answer 1

As for any variable, its type is found in its declaration:

EnumSet largeSize 

So yes, largeSize (which should be named largeSizes since it's a collection) is of type EnumSet. It should also be generified, and thus be declared as

EnumSet<Size> largeSizes 

What it means, is that largeSizes is of type EnumSet. An EnumSet is a Set which contains enum instance of a specific enum type, in a more efficient way than other Set implementations (like HashSet, TreeSet, etc.). To know what an EnumSet is, read its API.

Answer 2

A simple Enum is a list of values that you can only select one from at a time. Using your example, a size can be only one of S, M, L, etc for any given cloth. You could use simple constants instead of the Enum but this has its advantages of readability, easier maintenance and strict type checking.

An EnumSet will be used when you have the need for a variable to assume more than one Enum value at the same time. For instance, a font you write to screen with can be both bold and italic at the same time. An EnumSet will allow you to add the various values and to test whether one of those is actually set at any given time. If you have come to Java from other programming languages, this is the functionality usually called flags.

Compare the two:

enum Size { S, M, L, XL, XXL, XXXL } Size currentSize; ... currentSize = Size.S; ... if (currentSize == Size.S) ... 

defines, assigns and then checks for a single Enum value.

enum FontStyle { Bold, Italic, Underline, Strikethrough } EnumSet<FontStyle> currentStyle; ... currentStyle = EnumSet.of(FontStyle.Bold, FontStyle.Italic); ... if (currentStyle.contains(FontStyle.Italic)) ... 

defines, assigns two Enum values at the same time, then checks whether one of those is actually set or not.