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‎app/data/authors.yaml‎

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photo_url: https://danthe1st.github.io/img/Daniel.jpg
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website: https://danthe1st.github.io/
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description: |
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Daniel is a Java Developer from Austria who is also managing an online Java User Group.
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Daniel is a Java Developer from Austria who is also managing the <a href="https://discordjug.net/">Discord Java Community</a>.

‎app/pages/learn/01_tutorial/03_getting-to-know-the-language/04_classes_objects/01_enums.md‎

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layout: learn/tutorial-group.html
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subheader_select: tutorials
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main_css_id: learn
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toc:
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- What are enums? {intro}
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- Accessing and comparing enums {accessing}
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- Adding members to enums {members}
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- Special methods {functionality}
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- Using enums for singletons {singletons}
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- Abstract methods in enums {abstract}
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description: "Working with enums."
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last_update: 2023-09-28
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author: ["DanielSchmid"]
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---
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<a id="intro">&nbsp;</a>
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## What are enums?
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Enums are classes where all instances are known to the compiler.
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They are used for creating types that can only have few possible values.
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Enums can be created similar to classes but use the `enum` keyword instead of `class`.
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In the body, there is a list of instances of the enum named enum constants which are seperated by `,`.
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In the body, there is a list of instances of the enum called enum constants which are seperated by `,`.
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No instances of the enum can be created outside of enum constants.
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```java
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public enum DayOfWeek {
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MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY
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// enum constant are listed here:
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MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY
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}
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```
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All enums implicitely extend [`java.lang.Enum`](javadoc:Enum) and cannot have any subclasses.
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<a id="accessing">&nbsp;</a>
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## Accessing and comparing enums
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The values of an enum can be used as constants.
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In order to check whether two instances of an enum are the same, the `==` operator can be used.
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```java
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DayOfWeek weekStart = DayOfWeek.MONDAY;
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if (weekStart == DayOfWeek.MONDAY) {
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System.out.println("The week starts on Monday.");
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System.out.println("The week starts on Monday.");
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}
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```
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DayOfWeek someDay = DayOfWeek.FRIDAY;
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switch (someDay) {
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case MONDAY:
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System.out.println("The week just started.");
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break;
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case TUESDAY:
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case WEDNESDAY:
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case THURSDAY:
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System.out.println("We are somewhere in the middle of the week.");
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break;
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case FRIDAY:
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System.out.println("The weekend is near.");
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break;
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case SATURDAY:
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case SUNDAY:
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System.out.println("Weekend");
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break;
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default:
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throw new AssertionError("Should not happen");
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case MONDAY:
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System.out.println("The week just started.");
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break;
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case TUESDAY:
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case WEDNESDAY:
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case THURSDAY:
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System.out.println("We are somewhere in the middle of the week.");
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break;
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case FRIDAY:
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System.out.println("The weekend is near.");
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break;
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case SATURDAY:
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case SUNDAY:
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System.out.println("Weekend");
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break;
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default:
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throw new AssertionError("Should not happen");
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}
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```
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DayOfWeek someDay = DayOfWeek.FRIDAY;
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String text = switch (someDay) {
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case MONDAY -> "The week just started.";
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case TUESDAY, WEDNESDAY, THURSDAY -> "We are somewhere in the middle of the week.";
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case FRIDAY -> "The weekend is near.";
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case SATURDAY, SUNDAY -> "Weekend";
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case MONDAY -> "The week just started.";
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case TUESDAY, WEDNESDAY, THURSDAY -> "We are somewhere in the middle of the week.";
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case FRIDAY -> "The weekend is near.";
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case SATURDAY, SUNDAY -> "Weekend";
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};
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System.out.println(text);
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```
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<a id="members">&nbsp;</a>
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## Adding members to enums
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Just like classes, enums can have constructors, methods and fields.
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```java
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public enum DayOfWeek {
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MONDAY("MON"), TUESDAY("TUE"), WEDNESDAY("WED"), THURSDAY("THU"), FRIDAY("FRI"), SATURDAY("SAT"), SUNDAY("SUN");
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private final String abbreviation;
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DayOfWeek(String abbreviation) {
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this.abbreviation = abbreviation;
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}
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public String getAbbreviation() {
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return abbreviation;
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}
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MONDAY("MON"), TUESDAY("TUE"), WEDNESDAY("WED"), THURSDAY("THU"), FRIDAY("FRI"), SATURDAY("SAT"), SUNDAY("SUN");
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private final String abbreviation;
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DayOfWeek(String abbreviation) {
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this.abbreviation = abbreviation;
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}
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public String getAbbreviation() {
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return abbreviation;
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}
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}
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```
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<a id="functionality">&nbsp;</a>
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## Special methods
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All enums have a few methods that are added implicitely.
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For example, the method `name()` is present in all enum instances and can be used to get the name of the enum constant.
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Similarly, a method named `ordinal()` returns the position of the enum constant in the declaration.
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```java
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System.out.println(DayOfWeek.MONDAY.name());//MONDAY
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System.out.println(DayOfWeek.MONDAY.ordinal());//0 because MONDAY is the first constant in the DayOfWeek enum
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System.out.println(DayOfWeek.MONDAY.name()); // prints "MONDAY"
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System.out.println(DayOfWeek.MONDAY.ordinal()); // prints "0" because MONDAY is the first constant in the DayOfWeek enum
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```
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Aside from instance methods, there are also static methods added to all enums.
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The method `values()` returns an array containing all instances of the enum and the method `valueOf(String)` can be used to get a specific instance by its name.
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```java
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DayOfWeek[] days = DayOfWeek.values();//all days of the week
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DayOfWeek[] days = DayOfWeek.values(); // all days of the week
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DayOfWeek monday = DayOfWeek.valueOf("MONDAY");
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```
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<a id="singletons">&nbsp;</a>
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## Using enums for singletons
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Since enums can only have a specific number of instances, it is possible to create a singleton by creating an enum with only a single enum constant.
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```java
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public enum SomeSingleton {
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INSTANCE;
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//fields, methods, etc.
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INSTANCE;
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//fields, methods, etc.
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}
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```
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<a id="abstract">&nbsp;</a>
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## Abstract methods in enums
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Even though enums cannot be extended, they can still have `abstract` methods. In that case, an implementation must be present in each enum constant.
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```java
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enum MyEnum {
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A() {
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@Override
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void doSomething() {
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System.out.println("a");
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}
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},
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B() {
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@Override
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void doSomething() {
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System.out.println("b");
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}
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};
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abstract void doSomething();
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A() {
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@Override
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void doSomething() {
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System.out.println("a");
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}
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},
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B() {
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@Override
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void doSomething() {
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System.out.println("b");
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}
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};
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abstract void doSomething();
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}
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```

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