Simplify concept about if-statements (#2508)

* Deprecate concept exercise `blackjack`

* Rename concept to `if-else-statements`

* Remove `if` statements from `numbers` concept

* Remove introduction.md.tpl from `blackjack`

* Add if-else-statements concept to cars-assemble

Author: sanderploegsma

concepts/conditionals-if/.meta/config.json

@@ -0,0 +1,5 @@
+{
+  "blurb": "If-else statements can be used to perform conditional logic.",
+  "authors": ["TalesDias", "sanderploegsma"],
+  "contributors": []
+}

concepts/conditionals-if/about.md

@@ -0,0 +1,62 @@
+# About
+
+## The _if-then_ statement
+
+The most basic control flow statement in Java is the [_if-then_ statement][if-statement].
+This statement is used to only execute a section of code if a particular condition is `true`.
+An _if-then_ statement is defined using the `if` clause:
+
+```java
+class Car {
+    void drive() {
+        // the "if" clause: the car needs to have fuel left to drive
+        if (fuel > 0) {
+            // the "then" clause: the car drives, consuming fuel
+            fuel--;
+        }
+    }
+}
+```
+
+In the above example, if the car is out of fuel, calling the `Car.drive` method will do nothing.
+
+## The _if-then-else_ statement
+
+The _if-then-else_ statement provides an alternative path of execution for when the condition in the `if` clause evaluates to `false`.
+This alternative path of execution follows an `if` clause and is defined using the `else` clause:
+
+```java
+class Car {
+    void drive() {
+        if (fuel > 0) {
+            fuel--;
+        } else {
+            stop();
+        }
+    }
+}
+```
+
+In the above example, if the car is out of fuel, calling the `Car.drive` method will call another method to stop the car.
+
+The _if-then-else_ statement also supports multiple conditions by using the `else if` clause:
+
+```java
+class Car {
+    void drive() {
+        if (fuel > 5) {
+            fuel--;
+        } else if (fuel > 0) {
+            turnOnFuelLight();
+            fuel--;
+        } else {
+            stop();
+        }
+    }
+}
+```
+
+In the above example, driving the car when the fuel is less then or equal to `5` will drive the car, but it will turn on the fuel light.
+When the fuel reaches `0`, the car will stop driving.
+
+[if-statement]: https://docs.oracle.com/javase/tutorial/java/nutsandbolts/if.html

concepts/conditionals-if/introduction.md

@@ -0,0 +1,60 @@
+# Introduction
+
+## The _if-then_ statement
+
+The most basic control flow statement in Java is the _if-then_ statement.
+This statement is used to only execute a section of code if a particular condition is `true`.
+An _if-then_ statement is defined using the `if` clause:
+
+```java
+class Car {
+    void drive() {
+        // the "if" clause: the car needs to have fuel left to drive
+        if (fuel > 0) {
+            // the "then" clause: the car drives, consuming fuel
+            fuel--;
+        }
+    }
+}
+```
+
+In the above example, if the car is out of fuel, calling the `Car.drive` method will do nothing.
+
+## The _if-then-else_ statement
+
+The _if-then-else_ statement provides an alternative path of execution for when the condition in the `if` clause evaluates to `false`.
+This alternative path of execution follows an `if` clause and is defined using the `else` clause:
+
+```java
+class Car {
+    void drive() {
+        if (fuel > 0) {
+            fuel--;
+        } else {
+            stop();
+        }
+    }
+}
+```
+
+In the above example, if the car is out of fuel, calling the `Car.drive` method will call another method to stop the car.
+
+The _if-then-else_ statement also supports multiple conditions by using the `else if` clause:
+
+```java
+class Car {
+    void drive() {
+        if (fuel > 5) {
+            fuel--;
+        } else if (fuel > 0) {
+            turnOnFuelLight();
+            fuel--;
+        } else {
+            stop();
+        }
+    }
+}
+```
+
+In the above example, driving the car when the fuel is less then or equal to `5` will drive the car, but it will turn on the fuel light.
+When the fuel reaches `0`, the car will stop driving.

concepts/conditionals-if/links.json

@@ -0,0 +1,10 @@
+[
+  {
+    "url": "https://docs.oracle.com/javase/tutorial/java/nutsandbolts/if.html",
+    "description": "Java tutorial on if-then-else statements"
+  },
+  {
+    "url": "https://www.javatpoint.com/java-if-else",
+    "description": "Example if-then-else statements with flow charts"
+  }
+]

concepts/if-else-statements/.meta/config.json

@@ -1,7 +1,5 @@
 {
   "blurb": "Java includes various numeric types including integer and floating-point numbers.",
-  "authors": [
-    "TalesDias"
-  ],
-  "contributors": []
+  "authors": ["TalesDias"],
+  "contributors": ["sanderploegsma"]
 }

concepts/if-else-statements/about.md

@@ -62,20 +62,6 @@ double fromInt = i;
 int fromDouble = (int)d;
 ```
 
-An `if` statement can be used to conditionally execute code. The condition of an `if` statement must be of type `boolean`. Java has no concept of _truthy_ values.
-
-```java
-int x = 6;
-
-if (x == 5){
-    // Execute logic if x equals 5
-} else if (x > 7){
-    // Execute logic if x greater than 7
-} else{
-    // Execute logic in all other cases
-}
-```
-
 [arithmetic-operators]: https://docs.oracle.com/javase/tutorial/java/nutsandbolts/op1.html
 [comparison-operators]: https://docs.oracle.com/javase/tutorial/java/nutsandbolts/op2.html
 [type-casting]: https://www.programiz.com/java-programming/typecasting