StackOverflowError in hashCode due to recursion

[Dave Joyce]

Hi guys,

Well, I know what my problem is, but I don't know how to solve it. Here's the scenario:

I have an abstract superclass ContactsEntry, which has subclasses Group and Person. As a note, I'm using the HashCodeUtil and EqualsUtil classes based on Josh Bloch's discussions from Effective Java, which are found at Implementing hashCode and Implementing equals, respectively.

These domain model classes are structured to provide a nodetree-like structure, but I'm getting a StackOverflowError between Group and Person classes, due to the dreaded infinite recursion loop blowout in the hashCode() method.

import org.osframework.contacts.util.EqualsUtil;
import org.osframework.contacts.util.HashCodeUtil;
 
public abstract class ContactsEntry {
 
private Group parentGroup;
 
public ContactsEntry() {}
 
public ContactsEntry(Group parentGroup) {
this.parentGroup = parentGroup;
}
 
public Group getParentGroup() {
return parentGroup;
}
 
public void setParentGroup(Group parentGroup) {
this.parentGroup = parentGroup;
}
 
public abstract String getName();
 
public int hashCode() {
int result = HashCodeUtil.SEED;
result = HashCodeUtil.hash(result, parentGroup);
return result;
}
 
public boolean equals(Object obj) {
if (this == obj) return true;
if (!(obj instanceof ContactsEntry)) return false;
final ContactsEntry other = (ContactsEntry)obj;
return EqualsUtil.areEqual(parentGroup, other.parentGroup);
}
 
}

import java.util.HashSet;
import java.util.Set;
import org.osframework.contacts.util.EqualsUtil;
import org.osframework.contacts.util.HashCodeUtil;
 
public class Group extends ContactsEntry {
 
private String name;
private Set<ContactsEntry> children = new HashSet<ContactsEntry>(0);
 
public Group() {
super(null);
}
 
public Group(Group parentGroup, String name) {
super(parentGroup);
this.name = name;
}
 
public Group(Group parentGroup, String name, Set<ContactsEntry> children) {
this(parentGroup, name);
this.children = children;
}
 
@Override
public String getName() {
return name;
}
 
public void setName(String name) {
this.name = name;
}
 
public Set<ContactsEntry> getChildren() {
return children;
}
 
public void setChildren(Set<ContactsEntry> children) {
this.children = children;
}
 
public void addChild(ContactsEntry entry) {
children.add(entry);
}
 
public void removeChild(ContactsEntry entry) {
children.remove(entry);
}
 
@Override
public int hashCode() {
int result = HashCodeUtil.SEED;
result = super.hashCode();
result = HashCodeUtil.hash(result, name);
result = HashCodeUtil.hash(result, children);
return result;
}
 
@Override
public boolean equals(Object obj) {
if (this == obj) return true;
if (obj == null || getClass() != obj.getClass()) return false;
final Group other = (Group) obj;
return super.equals(obj) &&
       EqualsUtil.areEqual(name, other.name) &&
       EqualsUtil.areEqual(children, other.children);
}
 
}

import org.osframework.contacts.util.EqualsUtil;
import org.osframework.contacts.util.HashCodeUtil;
 
public class Person extends ContactsEntry {
 
private String firstName;
private String middleInitial;
private String lastName;
private String email;
 
public Person(Group parentGroup, String firstName, String lastName) {
super(parentGroup);
this.firstName = firstName;
this.lastName = lastName;
}
 
public Person(Group parentGroup, String firstName, String middleInitial, String lastName) {
super(parentGroup);
this.firstName = firstName;
this.middleInitial = middleInitial;
this.lastName = lastName;
}
 
public String getFirstName() {
return firstName;
}
 
public void setFirstName(String firstName) {
this.firstName = firstName;
}
 
public String getMiddleInitial() {
return middleInitial;
}
 
public void setMiddleInitial(String middleInitial) {
this.middleInitial = middleInitial;
}
 
public String getLastName() {
return lastName;
}
 
public void setLastName(String lastName) {
this.lastName = lastName;
}
 
public String getEmail() {
return email;
}
 
public void setEmail(String email) {
this.email = email;
}
 
@Override
public String getName() {
StringBuffer buf = new StringBuffer();
buf.append(firstName + " ");
buf.append(middleInitial == null ? "" : (middleInitial + ". "));
buf.append(lastName);
return buf.toString().trim();
}
 
@Override
public int hashCode() {
int result = HashCodeUtil.SEED;
result = super.hashCode();
result = HashCodeUtil.hash(result, firstName);
result = HashCodeUtil.hash(result, middleInitial);
result = HashCodeUtil.hash(result, lastName);
result = HashCodeUtil.hash(result, email);
return result;
}
 
@Override
public boolean equals(Object obj) {
if (this == obj) return true;
if (obj == null || getClass() != obj.getClass()) return false;
final Person other = (Person) obj;
return super.equals(obj) &&
       EqualsUtil.areEqual(firstName, other.firstName) &&
       EqualsUtil.areEqual(middleInitial, other.middleInitial) &&
       EqualsUtil.areEqual(lastName, other.lastName) &&
       EqualsUtil.areEqual(email, other.email);
}
 
}

Finally, here's the obvious stack trace:

java.lang.StackOverflowError
at java.util.HashMap$KeyIterator.<init>(HashMap.java:821)
at java.util.HashMap.newKeyIterator(HashMap.java:835)
at java.util.HashMap$KeySet.iterator(HashMap.java:867)
at java.util.HashSet.iterator(HashSet.java:154)
at java.util.AbstractSet.hashCode(AbstractSet.java:100)
at org.osframework.contacts.util.HashCodeUtil.hash(HashCodeUtil.java:88)
at org.osframework.contacts.model.Group.hashCode(Group.java:58)
at org.osframework.contacts.util.HashCodeUtil.hash(HashCodeUtil.java:88)
at org.osframework.contacts.model.Person.hashCode(Person.java:70)
at java.util.AbstractSet.hashCode(AbstractSet.java:104)
at org.osframework.contacts.util.HashCodeUtil.hash(HashCodeUtil.java:88)
at org.osframework.contacts.model.Group.hashCode(Group.java:58)
at org.osframework.contacts.util.HashCodeUtil.hash(HashCodeUtil.java:88)
at org.osframework.contacts.model.Person.hashCode(Person.java:70)
...

How do I break out of the recursive loop? Thanks!