Make getRuleListeners and getRulesEngineListeners return unmodifiable lists

Resolves #157

easy-rules-core/src/main/java/org/jeasy/rules/core/AbstractRuleEngine.java

@@ -28,6 +28,7 @@ import org.jeasy.rules.api.RulesEngine;
 import org.jeasy.rules.api.RulesEngineListener;
 
 import java.util.ArrayList;
+import java.util.Collections;
 import java.util.List;
 
 /**
@@ -60,12 +61,12 @@ abstract class AbstractRuleEngine implements RulesEngine {
 
     @Override
     public List<RuleListener> getRuleListeners() {
-        return ruleListeners;
+        return Collections.unmodifiableList(ruleListeners);
     }
 
     @Override
     public List<RulesEngineListener> getRulesEngineListeners() {
-        return rulesEngineListeners;
+        return Collections.unmodifiableList(rulesEngineListeners);
     }
 
     public void registerRuleListener(RuleListener ruleListener) {

easy-rules-core/src/test/java/org/jeasy/rules/core/DefaultRulesEngineTest.java

@@ -40,12 +40,17 @@ import org.jeasy.rules.api.RuleListener;
 import org.jeasy.rules.api.RulesEngineListener;
 import org.junit.After;
 import org.junit.Before;
+import org.junit.Rule;
 import org.junit.Test;
+import org.junit.rules.ExpectedException;
 import org.mockito.InOrder;
 import org.mockito.Mock;
 
 public class DefaultRulesEngineTest extends AbstractTest {
 
+    @Rule
+    public ExpectedException expectedException = ExpectedException.none();
+
     @Mock
     private RuleListener ruleListener;
 
@@ -225,6 +230,21 @@ public class DefaultRulesEngineTest extends AbstractTest {
         assertThat(ruleListeners).contains(ruleListener);
     }
 
+    @Test
+    public void getRuleListenersShouldReturnAnUnmodifiableList() {
+        // Given
+        expectedException.expect(UnsupportedOperationException.class);
+        DefaultRulesEngine rulesEngine = new DefaultRulesEngine();
+        rulesEngine.registerRuleListener(ruleListener);
+
+        // When
+        List<RuleListener> ruleListeners = rulesEngine.getRuleListeners();
+        ruleListeners.clear();
+
+        // Then
+        // expected exception
+    }
+
     @Test
     public void testGetRulesEngineListeners() throws Exception {
         // Given
@@ -238,6 +258,21 @@ public class DefaultRulesEngineTest extends AbstractTest {
         assertThat(rulesEngineListeners).contains(rulesEngineListener);
     }
 
+    @Test
+    public void getRulesEngineListenersShouldReturnAnUnmodifiableList() {
+        // Given
+        expectedException.expect(UnsupportedOperationException.class);
+        DefaultRulesEngine rulesEngine = new DefaultRulesEngine();
+        rulesEngine.registerRulesEngineListener(rulesEngineListener);
+
+        // When
+        List<RulesEngineListener> rulesEngineListeners = rulesEngine.getRulesEngineListeners();
+        rulesEngineListeners.clear();
+
+        // Then
+        // excepted exception
+    }
+
     @After
     public void clearRules() {
         rules.clear();