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    <p>Enum instances are created magically by the C-like syntax <code>public enum E { A, B, C }</code> and you cannot add more instances, even if they would be known at compile time.</p> <p>Depending on how much logic you have to attach to each enum instance and how many different "sub-enum" categories you have, you can try one of these approaches:</p> <h3>Single enum class with parameters</h3> <pre class="lang-java prettyprint-override"><code>public enum Day { MONDAY(false), TUESDAY(false), WEDNESDAY(false), THURSDAY(false), FRIDAY(false), SATURDAY(true), SUNDAY(true); private final boolean weekend; private Day(boolean weekend) { this.weekend = weekend; } public boolean isWeekend() { return weekend; } public void doSomething() { if (isWeekend()) { // have fun } else { // do some work } } } </code></pre> <p>Pros: this approach allows you to have a single enum class (so for example you can iterate easily on all instances), is simple, and adds no boilerplate.</p> <p>Cons: if you test often for category membership your code will become cluttered with <code>if</code>s; you cannot enforce at compile time that a variable refers to an instance of a specific category.</p> <h3>Enums implementing interfaces</h3> <pre class="lang-java prettyprint-override"><code>public interface Day { public void doSomething(); } public enum WorkDay implements Day { MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY; @Override public void doSomething() { // do some work } } public enum WeekEnd implements Day { SATURDAY, SUNDAY; @Override public void doSomething() { // have fun } } </code></pre> <p>Pros: you can have type safe references to instances of a single category; you can isolate category specific code in methods, reducing the need for membership tests; can be extended without modifying the original code.</p> <p>Cons: requires some boilerplate (maybe too much for simple cases); you do not get an automatically generated collection of all instances.</p> <p>See this answer for more examples of <a href="https://stackoverflow.com/a/15147361/2093341">enums implementing interfaces</a>.</p>
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