Change interfaces to make methods return an Optional instead of null

jcel-core/src/main/java/de/tudresden/inf/lat/jcel/core/algorithm/cel/CelProcessor.java

@@ -163,7 +163,7 @@ private Map<Integer, Set<Integer>> computeDirectTypes(IntegerHierarchicalGraph h
 		Map<Integer, Set<Integer>> ret = new HashMap<>();
 		Set<Integer> individuals = getEntityManager().getEntities(IntegerEntityType.INDIVIDUAL, false);
 		individuals.forEach(indiv -> {
-			Set<Integer> subsumers = hierarchicalGraph.getParents(getEntityManager().getAuxiliaryNominal(indiv));
+			Set<Integer> subsumers = hierarchicalGraph.getParents(getEntityManager().getAuxiliaryNominal(indiv).get());
 			subsumers.forEach(elem -> {
 				if (getEntityManager().getAuxiliaryNominals().contains(elem)) {
 					throw new IllegalStateException("An individual has another individual as direct subsumer.");
@@ -179,11 +179,11 @@ private Map<Integer, Set<Integer>> computeSameIndividualMap(IntegerHierarchicalG
 		Set<Integer> individuals = getEntityManager().getEntities(IntegerEntityType.INDIVIDUAL, false);
 		individuals.forEach(indiv -> {
 			Set<Integer> equivalentClasses = hierarchicalGraph
-					.getEquivalents(getEntityManager().getAuxiliaryNominal(indiv));
+					.getEquivalents(getEntityManager().getAuxiliaryNominal(indiv).get());
 			Set<Integer> equivalents = new HashSet<>();
 			equivalentClasses.forEach(elem -> {
 				if (getEntityManager().getAuxiliaryNominals().contains(elem)) {
-					equivalents.add(getEntityManager().getIndividual(elem));
+					equivalents.add(getEntityManager().getIndividual(elem).get());
 				}
 			});
 			ret.put(indiv, Collections.unmodifiableSet(equivalents));
@@ -560,9 +560,9 @@ public boolean process() {
 
 	private void process(Integer cA, ExtensionEntry eX) {
 		if (eX.isImplication()) {
-			processImplication(cA, eX.asImplication());
+			processImplication(cA, eX.asImplication().get());
 		} else if (eX.isExistential()) {
-			processExistential(cA, eX.asExistential());
+			processExistential(cA, eX.asExistential().get());
 		} else {
 			throw new RuntimeException("Internal error: entry was not recognized " + eX);
 		}

jcel-core/src/main/java/de/tudresden/inf/lat/jcel/core/algorithm/cel/ExistentialEntry.java

@@ -47,6 +47,7 @@
 package de.tudresden.inf.lat.jcel.core.algorithm.cel;
 
 import java.util.Objects;
+import java.util.Optional;
 
 /**
  * This is an auxiliary class which corresponds to an existential entry in the
@@ -77,13 +78,13 @@ public ExistentialEntry(Integer property, Integer cls) {
 	}
 
 	@Override
-	public ExistentialEntry asExistential() {
-		return this;
+	public Optional<ExistentialEntry> asExistential() {
+		return Optional.of(this);
 	}
 
 	@Override
-	public ImplicationEntry asImplication() {
-		return null;
+	public Optional<ImplicationEntry> asImplication() {
+		return Optional.empty();
 	}
 
 	@Override

jcel-core/src/main/java/de/tudresden/inf/lat/jcel/core/algorithm/cel/ExtensionEntry.java

@@ -46,6 +46,8 @@
 
 package de.tudresden.inf.lat.jcel.core.algorithm.cel;
 
+import java.util.Optional;
+
 /**
  * This class models an entry for the queues. There are only two kinds of
  * entries : the existential and the implication.
@@ -55,20 +57,22 @@
 public interface ExtensionEntry {
 
 	/**
-	 * Returns this entry as an existential entry.
+	 * Returns an optional containing this entry if this is as an existential
+	 * entry, or an empty optional otherwise.
 	 * 
-	 * @return <code>this</code> if this is an existential entry, or
-	 *         <code>null</code> if not
+	 * @return an optional containing this entry if this is as an existential
+	 *         entry, or an empty optional otherwise
 	 */
-	ExistentialEntry asExistential();
+	Optional<ExistentialEntry> asExistential();
 
 	/**
-	 * Returns this entry as an implication entry.
+	 * Returns an optional containing this entry if this is as an implication
+	 * entry, or an empty optional otherwise.
 	 * 
-	 * @return <code>this</code> if this is an implication entry, or
-	 *         <code>null</code> if not
+	 * @return an optional containing this entry if this is as an implication
+	 *         entry, or an empty optional otherwise
 	 */
-	ImplicationEntry asImplication();
+	Optional<ImplicationEntry> asImplication();
 
 	/**
 	 * Tell whether or not this is an existential entry.

jcel-core/src/main/java/de/tudresden/inf/lat/jcel/core/algorithm/cel/ImplicationEntry.java

@@ -48,6 +48,7 @@
 
 import java.util.Collections;
 import java.util.Objects;
+import java.util.Optional;
 import java.util.Set;
 
 /**
@@ -79,13 +80,13 @@ public ImplicationEntry(Set<Integer> left, Integer right) {
 	}
 
 	@Override
-	public ExistentialEntry asExistential() {
-		return null;
+	public Optional<ExistentialEntry> asExistential() {
+		return Optional.empty();
 	}
 
 	@Override
-	public ImplicationEntry asImplication() {
-		return this;
+	public Optional<ImplicationEntry> asImplication() {
+		return Optional.of(this);
 	}
 
 	@Override

jcel-core/src/main/java/de/tudresden/inf/lat/jcel/core/algorithm/rulebased/RuleBasedProcessor.java

@@ -196,7 +196,7 @@ private Map<Integer, Set<Integer>> computeDirectTypes(IntegerHierarchicalGraph h
 		Map<Integer, Set<Integer>> ret = new HashMap<>();
 		Set<Integer> individuals = getEntityManager().getIndividuals();
 		individuals.forEach(indiv -> {
-			Set<Integer> subsumers = hierarchicalGraph.getParents(getEntityManager().getAuxiliaryNominal(indiv));
+			Set<Integer> subsumers = hierarchicalGraph.getParents(getEntityManager().getAuxiliaryNominal(indiv).get());
 			subsumers.forEach(elem -> {
 				if (getEntityManager().getAuxiliaryNominals().contains(elem)) {
 					throw new IllegalStateException("An individual has another individual as direct subsumer.");
@@ -249,11 +249,11 @@ private Map<Integer, Set<Integer>> computeSameIndividualMap(IntegerHierarchicalG
 		Set<Integer> individuals = getEntityManager().getIndividuals();
 		individuals.forEach(indiv -> {
 			Set<Integer> equivalentClasses = hierarchicalGraph
-					.getEquivalents(getEntityManager().getAuxiliaryNominal(indiv));
+					.getEquivalents(getEntityManager().getAuxiliaryNominal(indiv).get());
 			Set<Integer> equivalents = new HashSet<>();
 			equivalentClasses.forEach(elem -> {
 				if (getEntityManager().getAuxiliaryNominals().contains(elem)) {
-					equivalents.add(getEntityManager().getIndividual(elem));
+					equivalents.add(getEntityManager().getIndividual(elem).get());
 				}
 			});
 			ret.put(indiv, Collections.unmodifiableSet(equivalents));

jcel-coreontology/src/main/java/de/tudresden/inf/lat/jcel/coreontology/datatype/IntegerEntityManager.java

@@ -46,6 +46,7 @@
 
 package de.tudresden.inf.lat.jcel.coreontology.datatype;
 
+import java.util.Optional;
 import java.util.Set;
 
 /**
@@ -121,14 +122,17 @@ public interface IntegerEntityManager {
 	Set<Integer> getAuxiliaryInverseObjectProperties();
 
 	/**
-	 * This method gives the auxiliary nominal related to a specific individual.
+	 * Returns an optional containing the auxiliary nominal related to a
+	 * specific individual, or an empty optional if the individual does not have
+	 * any related auxiliary nominal.
 	 *
 	 * @param individual
 	 *            the individual
-	 * @return the requested class or <code>null</code> if the individual does
-	 *         not have any related auxiliary nominal.
+	 * @return an optional containing the auxiliary nominal related to a
+	 *         specific individual, or an empty optional if the individual does
+	 *         not have any related auxiliary nominal
 	 */
-	Integer getAuxiliaryNominal(Integer individual);
+	Optional<Integer> getAuxiliaryNominal(Integer individual);
 
 	/**
 	 * Returns the set of auxiliary nominals.
@@ -162,14 +166,17 @@ public interface IntegerEntityManager {
 	Set<Integer> getEntities(IntegerEntityType type, boolean auxiliary);
 
 	/**
-	 * This method gives the individual related to a specific auxiliary nominal.
+	 * Returns an optional containing the individual related to a specific
+	 * auxiliary nominal, or an empty optional if the auxiliary nominal does not
+	 * have any related individual.
 	 *
 	 * @param auxNominal
 	 *            the auxiliary nominal
-	 * @return the requested individual or <code>null</code> if the auxiliary
-	 *         nominal does not have any related individual.
+	 * @return an optional containing the individual related to a specific
+	 *         auxiliary nominal, or an empty optional if the auxiliary nominal
+	 *         does not have any related individual.
 	 */
-	Integer getIndividual(Integer auxNominal);
+	Optional<Integer> getIndividual(Integer auxNominal);
 
 	/**
 	 * Returns the set of individuals.

jcel-coreontology/src/main/java/de/tudresden/inf/lat/jcel/coreontology/datatype/IntegerEntityManagerImpl.java

@@ -51,6 +51,7 @@
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Objects;
+import java.util.Optional;
 import java.util.Set;
 import java.util.TreeSet;
 
@@ -155,9 +156,9 @@ public Set<Integer> getAuxiliaryInverseObjectProperties() {
 	}
 
 	@Override
-	public Integer getAuxiliaryNominal(Integer individual) {
+	public Optional<Integer> getAuxiliaryNominal(Integer individual) {
 		Objects.requireNonNull(individual);
-		return this.auxNominalMap.get(individual);
+		return Optional.ofNullable(this.auxNominalMap.get(individual));
 	}
 
 	@Override
@@ -197,9 +198,9 @@ public Set<Integer> getEntities(IntegerEntityType type, boolean auxiliary) {
 	}
 
 	@Override
-	public Integer getIndividual(Integer auxNominal) {
+	public Optional<Integer> getIndividual(Integer auxNominal) {
 		Objects.requireNonNull(auxNominal);
-		return this.auxNominalInvMap.get(auxNominal);
+		return Optional.ofNullable(this.auxNominalInvMap.get(auxNominal));
 	}
 
 	@Override