Review changes to deal with cycles

src/main/java/de/danielbechler/diff/BeanDiffer.java

@@ -16,6 +16,9 @@
 
 package de.danielbechler.diff;
 
+import java.util.HashSet;
+import java.util.Set;
+
 import de.danielbechler.diff.accessor.*;
 import de.danielbechler.diff.introspect.*;
 import de.danielbechler.diff.node.*;
@@ -29,6 +32,7 @@
 final class BeanDiffer extends AbstractDiffer
 {
 	private Introspector introspector = new StandardIntrospector();
+	private Set<InstanceOutline> visitedInstances = new HashSet<InstanceOutline>();
 
 	BeanDiffer()
 	{
@@ -64,6 +68,8 @@ else if (instances.getType() == null)
 		}
 		else
 		{
+			if(checkAlreadyVisited(instances))
+				return node;
 			return compareBean(parentNode, instances);
 		}
 		return node;
@@ -143,4 +149,18 @@ void setIntrospector(final Introspector introspector)
 		Assert.notNull(introspector, "introspector");
 		this.introspector = introspector;
 	}
+
+	private boolean checkAlreadyVisited(Instances instances) 
+	{
+		InstanceOutline outline = InstanceOutline.from(instances);
+		if(outline != null) {
+			if(visitedInstances.contains(outline)) {
+				// if line below is commented in test case bidirectionalGraphStackOverflow creates a stack overflow
+				// visitedInstances.remove(outline);
+				return true;
+			}
+			visitedInstances.add(outline);
+		}	
+		return false;
+	}
 }

src/main/java/de/danielbechler/diff/InstanceOutline.java

@@ -0,0 +1,69 @@
+package de.danielbechler.diff;
+
+public class InstanceOutline {
+
+	private Object comparisonObject;
+	private Object working;
+	private Object base;
+
+	public InstanceOutline() {
+		super();
+	}
+
+	public InstanceOutline(Object comparisonObject, Object working, Object base) {
+		super();
+		this.comparisonObject = comparisonObject;
+		this.working = working;
+		this.base = base;
+	}
+
+
+	public static InstanceOutline from(Instances instances) {
+		if(instances.getWorking() == null)
+			return null;
+		if(instances.getBase() == null)
+			return null;
+		Object comparisonObject = instances.getComparisonObject();
+		if(comparisonObject == null)
+			return null;
+		return new InstanceOutline(comparisonObject, instances.getWorking(), instances.getBase());
+	}
+
+	@Override
+	public int hashCode() {
+		final int prime = 31;
+		int result = 1;
+		result = prime * result + ((base == null) ? 0 : base.hashCode());
+		result = prime * result + ((comparisonObject == null) ? 0 : comparisonObject.hashCode());
+		result = prime * result + ((working == null) ? 0 : working.hashCode());
+		return result;
+	}
+
+	@Override
+	public boolean equals(Object obj) {
+		if (this == obj)
+			return true;
+		if (obj == null)
+			return false;
+		if (getClass() != obj.getClass())
+			return false;
+		InstanceOutline other = (InstanceOutline) obj;
+		if (base == null) {
+			if (other.base != null)
+				return false;
+		} else if (!base.equals(other.base))
+			return false;
+		if (comparisonObject == null) {
+			if (other.comparisonObject != null)
+				return false;
+		} else if (!comparisonObject.equals(other.comparisonObject))
+			return false;
+		if (working == null) {
+			if (other.working != null)
+				return false;
+		} else if (!working.equals(other.working))
+			return false;
+		return true;
+	}
+
+}

src/main/java/de/danielbechler/diff/Instances.java

@@ -169,4 +169,11 @@ public PropertyPath getPropertyPath(final Node parentNode)
 				.withElement(sourceAccessor.getPathElement())
 				.build();
 	}
+
+	public Object getComparisonObject() 
+	{
+		if(sourceAccessor == null)
+			return null;		
+		return sourceAccessor.getComparisonObject();		
+	}
 }

src/main/java/de/danielbechler/diff/accessor/AbstractAccessor.java

@@ -54,4 +54,9 @@ public void setIgnored(final boolean ignored)
 	{
 		this.ignored = ignored;
 	}
+
+	public Object getComparisonObject() {
+		// answer the default, redefine in subclass as appropriate
+		return null;
+	}
 }

src/main/java/de/danielbechler/diff/accessor/Accessor.java

@@ -24,4 +24,6 @@ public interface Accessor extends PropertyDescriptor
 	void set(Object target, Object value);
 
 	void unset(Object target);
+
+	Object getComparisonObject();
 }

src/main/java/de/danielbechler/diff/accessor/CollectionItemAccessor.java

@@ -95,4 +95,9 @@ public void unset(final Object target)
 			targetCollection.remove(referenceItem);
 		}
 	}
+
+	@Override
+	public Object getComparisonObject() {
+		return referenceItem;
+	}
 }

src/main/java/de/danielbechler/diff/accessor/MapEntryAccessor.java

@@ -89,4 +89,9 @@ public void unset(final Object target)
 			targetMap.remove(getReferenceKey());
 		}
 	}
+
+	@Override
+	public Object getComparisonObject() {
+		return getReferenceKey();
+	}
 }

src/main/java/de/danielbechler/diff/accessor/PropertyAccessor.java

@@ -19,6 +19,7 @@
 import de.danielbechler.diff.accessor.exception.*;
 import de.danielbechler.diff.path.*;
 import de.danielbechler.util.*;
+
 import org.slf4j.*;
 
 import java.lang.reflect.*;
@@ -189,4 +190,13 @@ public Element getPathElement()
 	{
 		return new NamedPropertyElement(this.propertyName);
 	}
+
+	@Override
+	public Object getComparisonObject() {
+		if(Classes.isSimpleType(getType())) {
+			// ignore, because no comparison on the level of user-defined objects
+			return null;
+		}
+		return getPropertyName();
+	}
 }

src/main/java/de/danielbechler/diff/node/DefaultNode.java

@@ -387,4 +387,10 @@ else if (getChildren().size() > 1)
 		sb.append(" }");
 		return sb.toString();
 	}
+
+	@Override
+	public Object getComparisonObject() {
+		// not needed here, but has to be implemented, because of the Accessor interface
+		throw new UnsupportedOperationException();
+	}
 }