Update challenges.md

Author: bowbahdoe

src/generics/challenges.md

@@ -5,5 +5,85 @@ Remember the rules for this are
 - Try to use only the information given up to this point in this book.
 - Try not to give up until you've given it a solid attempt
 
+
+
 ## Challenge 1.
 
+
+Will the following code work? Why or why not?
+
+```java
+class Thing<T> {
+    T value;
+}
+
+class Cool {
+}
+class NotCool {
+}
+
+class Main {
+    void main() {
+        int n = Integer.parseInt(
+            IO.readln("Give a number: ")
+        );
+
+        Thing<Object> o = (n % 2 == 0) 
+            ? new Thing<Cool>() 
+            : new Thing<NotCool>();
+
+    }
+}
+```
+
+## Challenge 2.
+
+Make a class that holds a `NotNull` value. This class should be generic over the kind of
+data that it holds but it should throw an exception if the provided `value` is `null`.
+
+```java,editable
+// CODE HERE
+
+class Main {
+    void main() {
+        NotNull<String> s = new NotNull<>("abc");
+        IO.println(s.value);
+
+        NotNull<Integer> i = new NotNull<>(123);
+        IO.println(i.value);
+
+        // This should throw an exception
+        // NotNull<Double> d = new NotNull<>(null);
+    }
+}
+```
+
+
+## Challenge 3.
+
+The following class has 5 generic parameters.
+
+Correct the ones which do not follow expected naming conventions
+
+
+```java,editable
+class Organism<name, h, T, Cat, inch_worm> {
+    name name;
+    h h;
+    T t;
+    Cat cat;
+    inch_worm inchWorm;
+}
+
+class Main {
+    void main() {
+        var o = new Organism<String, Integer, Integer, String, String>();
+        o.name = "abc";
+        o.h = 123;
+        o.t = 5;
+        o.cat = "...";
+        o.inchWorm = "\\_/-\\--(*)";
+    }
+}
+```
+