XMLEncoder - Stackoverflowerror - overriding equals

[Jonathan Janisch]

Hi, I'm trying to write out a class to XML using XMLEncoder.

There's a few unique things about this class.

1) It has no default constructor
2) I'm overriding the equals method.
3) The class has a static int count which defaults to 0. Each object is assigned an ID based on the current count. The ID is used to compare two objects.

Since it has no default constructor, I'm using the defaultpersistencedelegate. Here's the code:

public class Person {
    
    private String name;
    
    private int id;
    private static int count = 0;
    
    /**
     * Creates a new instance of Person
     */
    public Person(String name) {
        this.name = name;
        this.id = count;
        count++;
        System.out.println("Person constructor called.  Count = " + count);
    }
 
    public boolean equals(Object obj) {
        System.out.println("Equals? This: " + this + " Obj: " + obj);
        if (obj instanceof Person) {
            System.out.println("Person: " + obj);
            return id == ((Person)obj).id;
        } else {
            return false;
        }
    }
 
    public int hashCode() {
        return id;
    }
    
    public String getName() {
        return name;
    }
    
    public int getId() {
        return id;
    }
 
    public void setId(int id) {
        this.id = id;
        System.out.println("setId called");
    }
    
    public static void main(String[] args) {
        Person p1 = new Person("Java Duke");
        Person p2 = new Person("John Smith");
        Person p3 = new Person("Bob Jones");
        
        java.beans.XMLEncoder enc = new java.beans.XMLEncoder(System.out);
        enc.setPersistenceDelegate(Person.class,
                new java.beans.DefaultPersistenceDelegate(
                new String[]{ "name"}));
        enc.writeObject(p1);
        enc.writeObject(p2);
        enc.writeObject(p3);
        enc.close();
    }
}

If you run this, you will get a stackoverflowerror (atleast under jdk1.6.0_01 on Windows). I'm not an XMLEncoder expert, I've narrowed the problem down and I came up with this simple Person.java example.

The combination of the static int ID and override equals seems to be the problem. If you change the equals to:

    public boolean equals(Object obj) {
        return super.equals(obj);
    }

It will still give the same error. However, if you remove the equals method completely, it will work fine and the xml output is as I expected. But unfortunately, I need to override the equals method.

1) Does anyone know how I can fix this?
2) Why would "return super.equals(obj)" give different behavior than having no equals method at all?

Note: I realize if my class were really a Person class, the equals method should compare SSN, first and last name, etc. and not some made up int ID - this is just a simple example to demonstrate the problem.

Thank you!

[Jonathan Janisch]

I solved it using my own persistence delegate to create a new Person. I don't understand why I had to do this, since I thought that was the purpose of the defaultpersistencedelegate. I did see some stuff in the source for defaultpersistencedelegate that involved checks for overriding equals - but I won't pretend like I understand how it works.

public class PersonPersistenceDelegate extends DefaultPersistenceDelegate {
    
    @Override
    protected Expression instantiate(Object oldInstance, Encoder out) {
        System.out.println("Instantiate called");
        Person old = (Person)oldInstance;
        return new Expression(oldInstance, oldInstance.getClass(), "new", new Object[] { old.getName() });
    }
}