Entity Inheritance

ObjectBox allows entity inheritance to share persisted properties in super classes. While ObjectBox always allowed entities to extend a non-entity base class, ObjectBox 1.4+ also allows extending entities. In addition to the @Entity annotation, we introduced a @BaseEntity annotation for base classes, which can be used instead of @Entity .

There three types of base classes, which are defined via annotations:

No annotation: The base class and its properties are not considered for persistence.
@BaseEntity: Properties are considered for persistence in sub classes, but the base class itself cannot be persisted.
@Entity: Properties are considered for persistence in sub classes, and the base class itself is a normally persisted entity.

For example:

// base class:
@BaseEntity
public abstract class Base {
    
    @Id long id;
    String baseString;

    public Base() {
    }

    public Base(long id, String baseString) {
        this.id = id;
        this.baseString = baseString;
    }
}

// sub class:
@Entity
public class Sub extends Base {
    
    String subString;

    public Sub() {
    }

    public Sub(long id, String baseString, String subString) {
        super(id, baseString);
        this.subString = subString;
    }
}

// base class:

@BaseEntity

public abstract class Base {

@Id long id;

String baseString;

public Base() {

}

public Base(long id, String baseString) {

this.id = id;

this.baseString = baseString;

}

// sub class:

@Entity

public class Sub extends Base {

String subString;

public Sub() {

}

public Sub(long id, String baseString, String subString) {

super(id, baseString);

this.subString = subString;

}

The model for Sub, Sub_, will now include all properties: id , baseString and subString .

It is also possible to inherit properties from another entity:

// entities inherit properties from entities
@Entity
public class SubSub extends Sub {
    
    String subSubString;

    public SubSub() {
    }

    public SubSub(long id, String baseString, String subString, String subSubString) {
        super(id, baseString, subString);
        this.subSubString = subSubString;
    }
}

// entities inherit properties from entities

@Entity

public class SubSub extends Sub {

String subSubString;

public SubSub() {

}

public SubSub(long id, String baseString, String subString, String subSubString) {

super(id, baseString, subString);

this.subSubString = subSubString;

}

Notes on usage

It is possible to have classes in the inheritance chain that are not annotated with @BaseEntity. Their properties will be ignored and will not become part of the entity model.
It is not generally recommend to have a base entity class consisting of an ID property only. E.g. Java imposes an additional overhead to construct objects with a sub class.
Depending on your use case using interfaces may be more straightforward.

Restrictions

Superclasses annotated with @BaseEntity can not be part of a library.
There are no polymorphic queries (e.g. you cannot query for a base class and expect results from sub classes).
Currently any superclass, whether it is an @Entity or @BaseEntity, can not have any relations (like a ToOne or ToMany property).

// THIS WILL NOT WORK
@BaseEntity
public abstract class Base {
    @Id long id;
    ToOne<OtherEntity> other; 
    ToMany<OtherEntity> others; 
}

// THIS WILL NOT WORK

@BaseEntity

public abstract class Base {

@Id long id;

ToOne<OtherEntity> other;

ToMany<OtherEntity> others;

}