added ex R

src/exercises/ex_r/PostOffice.java

@@ -0,0 +1,19 @@
+package exercises.ex_r;
+
+import java.util.concurrent.BlockingQueue;
+import java.util.concurrent.LinkedBlockingQueue;
+
+public class PostOffice {
+    private BlockingQueue<Package> packages = new LinkedBlockingQueue<>();
+
+    public void registerPackage(Package pkg) {
+        packages.add(pkg);
+        System.out.println("Registered " + pkg + " (now " + packages.size() + ")");
+    }
+
+    public Package takePackage() throws InterruptedException {
+        Package pkg = packages.take();
+        System.out.println("Withdrawn " + pkg + " (now " + packages.size() + ")");
+        return pkg;
+    }
+}