Update challenges.md

Author: bowbahdoe

src/streams/challenges.md

@@ -1 +1,65 @@
 # Challenges
+
+
+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.
+
+Translate the following code using a for-loop to code using streams.
+
+```java,editable
+import module java.base;
+
+class Main {
+    void main() {
+        for (int i = 0; i < 10; i++) {
+            IO.println(i);
+        }
+    }
+}
+```
+
+## Challenge 2.
+
+Translate the following code using a for-loop to code using streams.
+
+```java
+import module java.base;
+
+class Main {
+    void main() {
+        List<Integer> timestamps = List.of(1, 1756137441);
+
+        Set<Instant> instants = new HashSet<>();
+        for (int timestamp : timestamps) {
+            instants.add(Instant.ofEpochSecond(timestamp));
+        }
+
+        IO.println(instants);
+    }
+}
+```
+
+## Challenge 3.
+
+Read the documentation on [`Collector`](https://docs.oracle.com/en/java/javase/24/docs/api/java.base/java/util/stream/Collector.html) and [`Collectors`](https://docs.oracle.com/en/java/javase/24/docs/api/java.base/java/util/stream/Collector.html).
+
+Make an implementation of `Collector` that can collect elements into `MySpecialList`.
+
+```java
+import module java.base;
+
+class MySpecialList<T> extends ArrayList<T> {}
+
+class Main {
+    // CODE HERE
+
+    void main() {
+        MySpecialList<Integer> l = Stream.of(1, 2, 3)
+            .collect(/* CODE HERE */);
+    }
+}
+```