Add concept enums with concept exercise logs-logs-logs (#2496)

* Add `enums` concept

* Add concept exercise `logs-logs-logs`

* Add enums to practice exercise configs

Author: sanderploegsma

concepts/enums/.meta/config.json

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+{
+  "blurb": "Enums are useful to create a predefined set of constants.",
+  "authors": ["sanderploegsma"],
+  "contributors": []
+}

concepts/enums/about.md

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+# About
+
+An _enum type_ is a special data type that enables for a variable to be a set of predefined constants.
+The variable must be equal to one of the values that have been predefined for it.
+Common examples include compass directions (values of `NORTH`, `SOUTH`, `EAST`, and `WEST`) and the days of the week.
+
+Because they are constants, the names of an enum type's fields are in uppercase letters.
+
+## Defining an enum type
+
+In the Java programming language, you define an enum type by using the `enum` keyword.
+For example, you would specify a days-of-the-week enum type as:
+
+```java
+public enum DayOfWeek {
+    SUNDAY,
+    MONDAY,
+    TUESDAY,
+    WEDNESDAY,
+    THURSDAY,
+    FRIDAY,
+    SATURDAY
+}
+```
+
+You should use enum types any time you need to represent a fixed set of constants.
+That includes natural enum types such as the planets in our solar system and data sets where you know all possible values at compile time - for example, the choices on a menu, command line flags, and so on.
+
+## Using an enum type
+
+Here is some code that shows you how to use the `DayOfWeek` enum defined above:
+
+```java
+public class Shop {
+    public String getOpeningHours(DayOfWeek dayOfWeek) {
+        switch (dayOfWeek) {
+            case MONDAY:
+            case TUESDAY:
+            case WEDNESDAY:
+            case THURSDAY:
+            case FRIDAY:
+                return "9am - 5pm";
+            case SATURDAY:
+                return "10am - 4pm"
+            case SUNDAY:
+                return "Closed.";
+        }
+    }
+}
+```
+
+```java
+var shop = new Shop();
+shop.getOpeningHours(DayOfWeek.WEDNESDAY);
+// => "9am - 5pm"
+```
+
+## Adding methods and fields
+
+Java programming language enum types are much more powerful than their counterparts in other languages.
+The `enum` declaration defines a _class_ (called an _enum type_).
+The enum class body can include methods and other fields:
+
+```java
+public enum Rating {
+    GREAT(5),
+    GOOD(4),
+    OK(3),
+    BAD(2),
+    TERRIBLE(1);
+
+    private final int numberOfStars;
+
+    Rating(int numberOfStars) {
+        this.numberOfStars = numberOfStars;
+    }
+
+    public int getNumberOfStars() {
+        return this.numberOfStars;
+    }
+}
+```
+
+Calling the `getNumberOfStars` method on a member of the `Rating` enum type:
+
+```java
+Rating.GOOD.getNumberOfStars();
+// => 4
+```
+
+## Built-in methods
+
+The compiler automatically adds some special methods when it creates an enum.
+
+For example, they have a static `values` method that returns an array containing all of the values of the enum in the order they are declared:
+
+```java
+for (DayOfWeek dayOfWeek : DayOfWeek.values()) {
+    System.out.println(dayOfWeek.toString());
+}
+```
+
+The snippet above would print the following:
+
+```text
+SUNDAY
+MONDAY
+TUESDAY
+WEDNESDAY
+THURSDAY
+FRIDAY
+SATURDAY
+```
+
+The compiler also adds a static `valueOf` method that returns an enum member based on its name:
+
+```java
+DayOfWeek.valueOf("SUNDAY");
+// => DayOfWeek.SUNDAY
+```

concepts/enums/introduction.md

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+# Introduction
+
+An _enum type_ is a special data type that enables for a variable to be a set of predefined constants.
+The variable must be equal to one of the values that have been predefined for it.
+Common examples include compass directions (values of `NORTH`, `SOUTH`, `EAST`, and `WEST`) and the days of the week.
+
+Because they are constants, the names of an enum type's fields are in uppercase letters.
+
+## Defining an enum type
+
+In the Java programming language, you define an enum type by using the `enum` keyword.
+For example, you would specify a days-of-the-week enum type as:
+
+```java
+public enum DayOfWeek {
+    SUNDAY,
+    MONDAY,
+    TUESDAY,
+    WEDNESDAY,
+    THURSDAY,
+    FRIDAY,
+    SATURDAY
+}
+```
+
+You should use enum types any time you need to represent a fixed set of constants.
+That includes natural enum types such as the planets in our solar system and data sets where you know all possible values at compile time - for example, the choices on a menu, command line flags, and so on.
+
+## Using an enum type
+
+Here is some code that shows you how to use the `DayOfWeek` enum defined above:
+
+```java
+public class Shop {
+    public String getOpeningHours(DayOfWeek dayOfWeek) {
+        switch (dayOfWeek) {
+            case MONDAY:
+            case TUESDAY:
+            case WEDNESDAY:
+            case THURSDAY:
+            case FRIDAY:
+                return "9am - 5pm";
+            case SATURDAY:
+                return "10am - 4pm"
+            case SUNDAY:
+                return "Closed.";
+        }
+    }
+}
+```
+
+```java
+var shop = new Shop();
+shop.getOpeningHours(DayOfWeek.WEDNESDAY);
+// => "9am - 5pm"
+```
+
+## Adding methods and fields
+
+Java programming language enum types are much more powerful than their counterparts in other languages.
+The `enum` declaration defines a _class_ (called an _enum type_).
+The enum class body can include methods and other fields:
+
+```java
+public enum Rating {
+    GREAT(5),
+    GOOD(4),
+    OK(3),
+    BAD(2),
+    TERRIBLE(1);
+
+    private final int numberOfStars;
+
+    Rating(int numberOfStars) {
+        this.numberOfStars = numberOfStars;
+    }
+
+    public int getNumberOfStars() {
+        return this.numberOfStars;
+    }
+}
+```
+
+Calling the `getNumberOfStars` method on a member of the `Rating` enum type:
+
+```java
+Rating.GOOD.getNumberOfStars();
+// => 4
+```

concepts/enums/links.json

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+[
+  {
+    "url": "https://docs.oracle.com/javase/tutorial/java/javaOO/enum.html",
+    "description": "Java Tuturial on Enum Types"
+  }
+]