fix validation errors & cleanup

* false-positive of the SpotBugs Plugin on Java 11
* better nullability specification + tests

Author: mtf90

build-tools/build-config/src/main/resources/learnlib-spotbugs-exclusions.xml

@@ -42,4 +42,11 @@ limitations under the License.
             <Class name="de.learnlib.datastructure.observationtable.writer.ObservationTableWriter"/>
         </Or>
     </Match>
+    <Match>
+        <!-- False-positive on Java 11. See https://github.com/spotbugs/spotbugs/issues/259 -->
+        <Bug pattern="RCN_REDUNDANT_NULLCHECK_OF_NONNULL_VALUE"/>
+        <Or>
+            <Class name="de.learnlib.examples.dfa.DFABenchmarks"/>
+        </Or>
+    </Match>
 </FindBugsFilter>

test-support/learning-examples/pom.xml

@@ -65,10 +65,15 @@ limitations under the License.
             <artifactId>checker-qual</artifactId>
         </dependency>
 
-        <!-- test -->
         <dependency>
             <groupId>org.slf4j</groupId>
             <artifactId>slf4j-api</artifactId>
         </dependency>
+
+        <!-- test -->
+        <dependency>
+            <groupId>org.testng</groupId>
+            <artifactId>testng</artifactId>
+        </dependency>
     </dependencies>
 </project>

test-support/learning-examples/src/main/java/de/learnlib/examples/dfa/DFABenchmarks.java

@@ -17,6 +17,7 @@
 
 import java.io.IOException;
 import java.io.InputStream;
+import java.util.Objects;
 
 import de.learnlib.examples.DefaultLearningExample.DefaultDFALearningExample;
 import de.learnlib.examples.LearningExample.DFALearningExample;
@@ -34,38 +35,41 @@ private DFABenchmarks() {
         // prevent instantiation
     }
 
-    public static @Nullable DFALearningExample<Integer> loadPots2() {
-        return loadLearnLibV2Benchmark("pots2");
-    }
-
     public static @Nullable DFALearningExample<Integer> loadLearnLibV2Benchmark(String name) {
-        String resourceName = "/automata/learnlibv2/" + name + ".dfa.gz";
-        if (DFABenchmarks.class.getResource(resourceName) == null) {
-            return null;
-        }
+        final String resourceName = "/automata/learnlibv2/" + name + ".dfa.gz";
+        final InputStream resourceStream = DFABenchmarks.class.getResourceAsStream(resourceName);
 
-        try (InputStream is = DFABenchmarks.class.getResourceAsStream(resourceName)) {
-            if (is == null) {
-                LOGGER.warn("Couldn't load resource '{}'", resourceName);
-                return null;
+        if (resourceStream == null) {
+            LOGGER.info("Couldn't find resource '{}'", resourceName);
+        } else {
+            try (InputStream is = resourceStream) {
+                CompactDFA<Integer> dfa = LearnLibV2Serialization.getInstance().readGenericDFA(is);
+                return new DefaultDFALearningExample<>(dfa);
+            } catch (IOException ex) {
+                LOGGER.error("Could not load benchmark", ex);
             }
-            CompactDFA<Integer> dfa = LearnLibV2Serialization.getInstance().readGenericDFA(is);
-            return new DefaultDFALearningExample<>(dfa);
-        } catch (IOException ex) {
-            LOGGER.error("Could not load benchmark", ex);
-            return null;
         }
+
+        return null;
+    }
+
+    public static DFALearningExample<Integer> loadPots2() {
+        return Objects.requireNonNull(loadLearnLibV2Benchmark("pots2"),
+                                      "Couldn't find 'pots2'. Are the correct JARs loaded?");
     }
 
-    public static @Nullable DFALearningExample<Integer> loadPots3() {
-        return loadLearnLibV2Benchmark("pots3");
+    public static DFALearningExample<Integer> loadPots3() {
+        return Objects.requireNonNull(loadLearnLibV2Benchmark("pots3"),
+                                      "Couldn't find 'pots3'. Are the correct JARs loaded?");
     }
 
-    public static @Nullable DFALearningExample<Integer> loadPeterson2() {
-        return loadLearnLibV2Benchmark("peterson2");
+    public static DFALearningExample<Integer> loadPeterson2() {
+        return Objects.requireNonNull(loadLearnLibV2Benchmark("peterson2"),
+                                      "Couldn't find 'peterson2'. Are the correct JARs loaded?");
     }
 
-    public static @Nullable DFALearningExample<Integer> loadPeterson3() {
-        return loadLearnLibV2Benchmark("peterson3");
+    public static DFALearningExample<Integer> loadPeterson3() {
+        return Objects.requireNonNull(loadLearnLibV2Benchmark("peterson3"),
+                                      "Couldn't find 'peterson3'. Are the correct JARs loaded?");
     }
 }

test-support/learning-examples/src/test/java/de/learnlib/examples/dfa/DFAExamplesTest.java

@@ -0,0 +1,42 @@
+/* Copyright (C) 2013-2019 TU Dortmund
+ * This file is part of LearnLib, http://www.learnlib.de/.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package de.learnlib.examples.dfa;
+
+import org.testng.Assert;
+import org.testng.annotations.Test;
+
+public class DFAExamplesTest {
+
+    @Test
+    public void testPots2() {
+        Assert.assertNotNull(DFABenchmarks.loadPots2());
+    }
+
+    @Test
+    public void testPots3() {
+        Assert.assertNotNull(DFABenchmarks.loadPots3());
+    }
+
+    @Test
+    public void testPeterson2() {
+        Assert.assertNotNull(DFABenchmarks.loadPeterson2());
+    }
+
+    @Test
+    public void testPeterson3() {
+        Assert.assertNotNull(DFABenchmarks.loadPeterson3());
+    }
+}