2 files changed, 34 insertions, 24 deletions

diff --git a/src/main/java/com/it_jaros/jscanner/PollState.java b/src/main/java/com/it_jaros/jscanner/PollState.java
new file mode 100644
index 0000000..474c055
--- /dev/null
+++ b/src/main/java/com/it_jaros/jscanner/PollState.java
@@ -0,0 +1,13 @@
+package com.it_jaros.jscanner;
+
+/**
+ * A sealed interface representing the three possible outcomes of a CompletionService
+ * poll operation: successful result, task failure, or not ready yet.
+ * Makes error handling explicit — Failure and Unavailable are distinct and compile-time
+ * required to handle via exhaustiveness checking in switch statements.
+ */
+public sealed interface PollState<T> {
+    record Success<T>(T value) implements PollState<T> {}
+    record Failure<T>(Throwable error) implements PollState<T> {}
+    record Unavailable<T>() implements PollState<T> {}
+}
diff --git a/src/main/java/com/it_jaros/jscanner/Scanner.java b/src/main/java/com/it_jaros/jscanner/Scanner.java
index e3c886b..fd6c0e5 100644
--- a/src/main/java/com/it_jaros/jscanner/Scanner.java
+++ b/src/main/java/com/it_jaros/jscanner/Scanner.java
@@ -77,11 +77,12 @@ public class Scanner implements AutoCloseable {
         // with whatever is there already
         while (state.running().get() || state.inPipeline().get() > 0) {
             try {
-                ScanResult result = getHostResult(state);
-                if (result == null) {
-                    continue;
+                PollState<ScanResult> poll = getHostResult(state);
+                if (poll instanceof PollState.Success<ScanResult>(ScanResult value)) {
+                    consumer.accept(value);
+                } else if (poll instanceof PollState.Failure<ScanResult>(Throwable error)) {
+                    System.err.printf("Host task failed: %s%n", error.getMessage());
                 }
-                consumer.accept(result);
             } catch (InterruptedException e) {
                 Thread.currentThread().interrupt();
             }
@@ -90,25 +91,21 @@ public class Scanner implements AutoCloseable {
         scan.stop();
     }
 
-    private ScanResult getHostResult(ProducerState<ScanResult> state) throws InterruptedException {
-        // let's check for results and give add them to our scan data holder object
+    private PollState<ScanResult> getHostResult(ProducerState<ScanResult> state) throws InterruptedException {
         Future<ScanResult> finishedHost = state.completionService().poll(10, TimeUnit.MILLISECONDS);
         if (finishedHost == null) {
-            return null;
+            return new PollState.Unavailable<>();
         }
 
-        ScanResult result = null;
         try {
-            result = finishedHost.get();
+            ScanResult result = finishedHost.get();
+            return new PollState.Success<>(result);
         } catch (ExecutionException e) {
-            System.err.printf("%s -> %s%n", e.getClass().getSimpleName(), e.getMessage());
+            return new PollState.Failure<>(e.getCause());
         } finally {
-            // No matter what happens we have to free the resources after getting ScanResult
             state.activeWorkers().release();
             state.inPipeline().decrementAndGet();
         }
-
-        return result;
     }
 
     /**
@@ -251,12 +248,12 @@ public class Scanner implements AutoCloseable {
             final PortResultAccumulator accumulator = new PortResultAccumulator(host);
             while (!cancelled && (state.running().get() || state.inPipeline().get() > 0)) {
                 try {
-                    PortResult portResult = getPortResult(state);
-                    if (portResult == null) {
-                        continue;
+                    PollState<PortResult> poll = getPortResult(state);
+                    if (poll instanceof PollState.Success<PortResult>(PortResult value)) {
+                        accumulator.add(value);
+                    } else if (poll instanceof PollState.Failure(Throwable error)) {
+                        System.err.printf("Port task failed: %s -> %s%n", error.getClass().getSimpleName(), error.getMessage());
                     }
-
-                    accumulator.add(portResult);
                 } catch (InterruptedException ignored) {
                     Thread.currentThread().interrupt();
                 }
@@ -275,23 +272,23 @@ public class Scanner implements AutoCloseable {
             return false;
         }
 
-        private PortResult getPortResult(ProducerState<PortResult> state) throws InterruptedException {
+        private PollState<PortResult> getPortResult(ProducerState<PortResult> state) throws InterruptedException {
             Future<PortResult> portResultFuture = state.completionService().poll(10, TimeUnit.MILLISECONDS);
             if (portResultFuture == null) {
-                return null;
+                return new PollState.Unavailable<>();
             }
 
-            PortResult portResult = null;
+            PortResult portResult;
             try {
                 portResult = portResultFuture.get();
+                return new PollState.Success<>(portResult);
             } catch (ExecutionException e) {
-                // something more serious did not work
-                System.err.printf("%s -> %s%n", e.getClass().getSimpleName(), e.getMessage());
+                Throwable cause = e.getCause();
+                return new PollState.Failure<>(cause);
             } finally {
                 state.activeWorkers().release();
                 state.inPipeline().decrementAndGet();
             }
-            return portResult;
         }
     }