Add code tabs for _tour/annotations (#2566)

* Add code tabs for _tour/annotations

* Update annotations.md

* Update _tour/annotations.md

Co-authored-by: Jamie Thompson <[email protected]>

* add code tabs for java annots

Co-authored-by: Jamie Thompson <[email protected]>

Author: flomebul

_tour/annotations.md

@@ -11,21 +11,39 @@ redirect_from: "/tutorials/tour/annotations.html"
 ---
 
 Annotations associate meta-information with definitions. For example, the annotation `@deprecated` before a method causes the compiler to print a warning if the method is used.
-```
+
+{% tabs annotations_1 class=tabs-scala-version %}
+{% tab 'Scala 2' for=annotations_1 %}
+```scala mdoc:fail
 object DeprecationDemo extends App {
   @deprecated("deprecation message", "release # which deprecates method")
   def hello = "hola"
 
-  hello  
+  hello
 }
 ```
+{% endtab %}
+{% tab 'Scala 3' for=annotations_1 %}
+```scala
+object DeprecationDemo extends App:
+  @deprecated("deprecation message", "release # which deprecates method")
+  def hello = "hola"
+
+  hello
+```
+{% endtab %}
+{% endtabs %}
+
 This will compile but the compiler will print a warning: "there was one deprecation warning".
 
 An annotation clause applies to the first definition or declaration following it. More than one annotation clause may precede a definition and declaration. The order in which these clauses are given does not matter.
 
 
 ## Annotations that ensure correctness of encodings
 Certain annotations will actually cause compilation to fail if a condition(s) is not met. For example, the annotation `@tailrec` ensures that a method is [tail-recursive](https://en.wikipedia.org/wiki/Tail_call). Tail-recursion can keep memory requirements constant. Here's how it's used in a method which calculates the factorial:
+
+{% tabs annotations_2 class=tabs-scala-version %}
+{% tab 'Scala 2' for=annotations_2 %}
 ```scala mdoc
 import scala.annotation.tailrec
 
@@ -38,8 +56,26 @@ def factorial(x: Int): Int = {
   factorialHelper(x, 1)
 }
 ```
-The `factorialHelper` method has the `@tailrec` which ensures the method is indeed tail-recursive. If we were to change the implementation of `factorialHelper` to the following, it would fail:
+{% endtab %}
+{% tab 'Scala 3' for=annotations_2 %}
+```scala
+import scala.annotation.tailrec
+
+def factorial(x: Int): Int =
+
+  @tailrec
+  def factorialHelper(x: Int, accumulator: Int): Int =
+    if x == 1 then accumulator else factorialHelper(x - 1, accumulator * x)
+  factorialHelper(x, 1)
 ```
+{% endtab %}
+{% endtabs %}
+
+The `factorialHelper` method has the `@tailrec` which ensures the method is indeed tail-recursive. If we were to change the implementation of `factorialHelper` to the following, it would fail:
+
+{% tabs annotations_3 class=tabs-scala-version %}
+{% tab 'Scala 2' for=annotations_3 %}
+```scala mdoc:fail
 import scala.annotation.tailrec
 
 def factorial(x: Int): Int = {
@@ -50,76 +86,134 @@ def factorial(x: Int): Int = {
   factorialHelper(x)
 }
 ```
-We would get the message "Recursive call not in tail position".
+{% endtab %}
+{% tab 'Scala 3' for=annotations_3 %}
+```scala
+import scala.annotation.tailrec
 
+def factorial(x: Int): Int =
+  @tailrec
+  def factorialHelper(x: Int): Int =
+    if x == 1 then 1 else x * factorialHelper(x - 1)
+  factorialHelper(x)
+```
+{% endtab %}
+{% endtabs %}
+
+We would get the message "Recursive call not in tail position".
 
 ## Annotations affecting code generation
+
+{% tabs annotations_4 class=tabs-scala-version %}
+{% tab 'Scala 2' for=annotations_4 %}
+
 Some annotations like `@inline` affect the generated code (i.e. your jar file might have different bytes than if you hadn't used the annotation). Inlining means inserting the code in a method's body at the call site. The resulting bytecode is longer, but hopefully runs faster. Using the annotation `@inline` does not ensure that a method will be inlined, but it will cause the compiler to do it if and only if some heuristics about the size of the generated code are met.
 
+{% endtab %}
+{% tab 'Scala 3' for=annotations_4 %}
+
+Some annotations like `@main` affect the generated code (i.e. your jar file might have different bytes than if you hadn't used the annotation). A `@main` annotation on a method generates an executable program that calls the method as an entry point.
+
+{% endtab %}
+{% endtabs %}
+
 ### Java Annotations ###
 When writing Scala code which interoperates with Java, there are a few differences in annotation syntax to note.
 **Note:** Make sure you use the `-target:jvm-1.8` option with Java annotations.
 
 Java has user-defined metadata in the form of [annotations](https://docs.oracle.com/javase/tutorial/java/annotations/). A key feature of annotations is that they rely on specifying name-value pairs to initialize their elements. For instance, if we need an annotation to track the source of some class we might define it as
 
-```
+{% tabs annotations_5 %}
+{% tab 'Java' for=annotations_5 %}
+```java
 @interface Source {
-  public String URL();
+  public String url();
   public String mail();
 }
 ```
+{% endtab %}
+{% endtabs %}
 
 And then apply it as follows
 
-```
-@Source(URL = "https://coders.com/",
+{% tabs annotations_6 %}
+{% tab 'Java' for=annotations_6 %}
+```java
+@Source(url = "https://coders.com/",
         mail = "[email protected]")
-public class MyClass extends TheirClass ...
+public class MyJavaClass extends TheirClass ...
 ```
+{% endtab %}
+{% endtabs %}
 
 An annotation application in Scala looks like a constructor invocation, for instantiating a Java annotation one has to use named arguments:
 
-```
-@Source(URL = "https://coders.com/",
+{% tabs annotations_7 %}
+{% tab 'Scala 2 and 3' for=annotations_7 %}
+```scala
+@Source(url = "https://coders.com/",
         mail = "[email protected]")
 class MyScalaClass ...
 ```
+{% endtab %}
+{% endtabs %}
 
 This syntax is quite tedious if the annotation contains only one element (without default value) so, by convention, if the name is specified as `value` it can be applied in Java using a constructor-like syntax:
 
-```
+{% tabs annotations_8 %}
+{% tab 'Java' for=annotations_8 %}
+```java
 @interface SourceURL {
     public String value();
     public String mail() default "";
 }
 ```
+{% endtab %}
+{% endtabs %}
 
-And then apply it as follows
+And then apply it as follows:
 
-```
+{% tabs annotations_9 %}
+{% tab 'Java' for=annotations_9 %}
+```java
 @SourceURL("https://coders.com/")
-public class MyClass extends TheirClass ...
+public class MyJavaClass extends TheirClass ...
 ```
+{% endtab %}
+{% endtabs %}
 
-In this case, Scala provides the same possibility
+In this case, Scala provides the same possibility:
 
-```
+{% tabs annotations_10 %}
+{% tab 'Scala 2 and 3' for=annotations_10 %}
+```scala
 @SourceURL("https://coders.com/")
 class MyScalaClass ...
 ```
+{% endtab %}
+{% endtabs %}
 
-The `mail` element was specified with a default value so we need not explicitly provide a value for it. However, if we need to do it we can not mix-and-match the two styles in Java:
+The `mail` element was specified with a default value so we need not explicitly provide a value for it.
+However, if we need to provide one then in Java we must also explicitly name the `value` parameter:
 
-```
+{% tabs annotations_11 %}
+{% tab 'Java' for=annotations_11 %}
+```java
 @SourceURL(value = "https://coders.com/",
            mail = "[email protected]")
-public class MyClass extends TheirClass ...
+public class MyJavaClass extends TheirClass ...
 ```
+{% endtab %}
+{% endtabs %}
 
-Scala provides more flexibility in this respect
+Scala provides more flexibility in this respect, so we can choose to only name the `mail` parameter:
 
-```
+{% tabs annotations_12 %}
+{% tab 'Scala 2 and 3' for=annotations_12 %}
+```scala
 @SourceURL("https://coders.com/",
            mail = "[email protected]")
-    class MyScalaClass ...
+class MyScalaClass ...
 ```
+{% endtab %}
+{% endtabs %}