What is Enum in Java?

0 votes

I thought I understood Java generics pretty well, but then I came across the following in java.lang.Enum:

class Enum<E extends Enum<E>>

Could someone explain how to interpret this type parameter? Bonus points for providing other examples of where a similar type parameter could be used.

Nov 13, 2018 in Java by Neha
• 6,280 points
67 views

1 answer to this question.

0 votes

 It means that the type argument for enum has to derive from an enum which itself has the same type argument. How can this happen? By making the type argument the new type itself. So if I've got an enum called StatusCode, it would be equivalent to:

public class StatusCode extends Enum<StatusCode>

Now if you check the constraints, we've got Enum<StatusCode> - so E=StatusCode. Let's check: does E extend Enum<StatusCode>? Yes! We're okay.

You may well be asking yourself what the point of this is :) Well, it means that the API for Enum can refer to itself - for instance, being able to say that Enum<E> implements Comparable<E>. The base class is able to do the comparisons (in the case of enums) but it can make sure that it only compares the right kind of enums with each other. (EDIT: Well, nearly - see the edit at the bottom.)

I've used something similar in my C# port of ProtocolBuffers. There are "messages" (immutable) and "builders" (mutable, used to build a message) - and they come as pairs of types. The interfaces involved are:

public interface IBuilder<TMessage, TBuilder>
  where TMessage : IMessage<TMessage, TBuilder> 
  where TBuilder : IBuilder<TMessage, TBuilder>

public interface IMessage<TMessage, TBuilder>
  where TMessage : IMessage<TMessage, TBuilder> 
  where TBuilder : IBuilder<TMessage, TBuilder>

This means that from a message you can get an appropriate builder (e.g. to take a copy of a message and change some bits) and from a builder you can get an appropriate message when you've finished building it. It's a good job users of the API don't need to actually care about this though - it's horrendously complicated, and took several iterations to get to where it is.

answered Nov 13, 2018 by Frankie
• 9,810 points

Related Questions In Java

+1 vote
3 answers

What is the syntax to declare and initialize an array in java?

You can use this method: String[] strs = ...READ MORE

answered Jul 25, 2018 in Java by samarth295
• 2,190 points
570 views
0 votes
2 answers

What is the use of toString method in Java and how can I use it ?

Whenever you require to explore the constructor ...READ MORE

answered Aug 23, 2018 in Java by Daisy
• 8,050 points
798 views
0 votes
2 answers

What is the difference between Set and List in java?

LIST SET Duplicates YES NO Order Ordered Depends on Implementation Positional Access YES NO List is an ordered sequence ...READ MORE

answered Aug 14 in Java by Sirajul
• 36,620 points
140 views
0 votes
2 answers

What is the use of @Override annotation in Java ? When do we use it ?

@Override annotation is used when we override ...READ MORE

answered Aug 14 in Java by Sirajul
• 36,620 points
111 views
0 votes
1 answer

Are arrays equivalent to objects in Java ?

Yes; the Java Language Specification writes: In the Java ...READ MORE

answered May 10, 2018 in Java by Rishabh
• 3,540 points
76 views
0 votes
2 answers

Java enum lookup using a String

Check this...Java Enum in details READ MORE

answered Dec 4, 2018 in Java by allenvarna
• 520 points
77 views
0 votes
1 answer

What is the difference == and equals() in Java?

Both the answers (Syntaxes) are correct. If ...READ MORE

answered Jun 5, 2018 in Java by Akrati
• 3,150 points
42 views
0 votes
1 answer

Remove objects from an array in Java?

We can use external libraries: org.apache.commons.lang.ArrayUtils.remove(java.lang.Object[] array, int ...READ MORE

answered Jun 26, 2018 in Java by scarlett
• 1,280 points
65 views
+1 vote
1 answer

What is a simple way to repeat a string in java?

From Java 11 on, there's a method String::repeat that does ...READ MORE

answered Aug 28, 2018 in Java by Frankie
• 9,810 points
788 views
0 votes
2 answers

In Java, what is the best way to determine the size of an object?

I happened to find a java class "jdk.nashorn.internal.ir.debug.ObjectSizeCalculator", ...READ MORE

answered Aug 19 in Java by Sirajul
• 36,620 points
165 views