Persistent data structures in Java

Question

Does anyone know a library or some at least some research on creating and using persistent data structures in Java? I don\'t refer to persistence as long term storage but persis

Answer 1

Do you want immutability :

1. so external code cannot change the data?
2. so once set a value cannot be changed?

In both cases there are easier ways to accomplish the desired result.

Stopping external code from changing the data is easy with interfaces:

public interface Person {
   String getName();
   Address getAddress();
}
public interface PersonImplementor extends Person {
   void setName(String name);
   void setAddress(Address address);
}

public interface Address {
    String getCity();
}


public interface AddressImplementor {
    void setCity(String city);
}

Then to stop changes to a value once set is also "easy" using java.util.concurrent.atomic.AtomicReference (although hibernate or some other persistence layer usage may need to be modified):

class PersonImpl implements PersonImplementor {
    private AtomicReference<String> name;
    private AtomicReference<Address> address;

    public void setName(String name) {
        if ( !this.name.compareAndSet(name, name) 
           && !this.name.compareAndSet(null, name)) {
            throw new IllegalStateException("name already set to "+this.name.get()+" cannot set to "+name);
        }
    }
    // .. similar code follows....
}

But why do you need anything more than just interfaces to accomplish the task?

Answer 2

It is very difficult, if not impossible, to make things immutable that ain't designed so.

If you can design from ground up:

• use only final fields
• do not reference non immutable objects