Using composition over inheritance with Realm

Animal.java

interface Animal {
String getColor();
    void setColor(String color);

    int getLegCount();
    void setLegCount(int legCount);
}

AnimalEntity.java

@RealmClass
public class AnimalEntity implements RealmModel, Animal {

    private String color;
    private int legCount;

    @Override
    public String getColor() {
        return null;
    }

    @Override
    public void setColor(String color) {

    }

    @Override
    public int getLegCount() {
        return 0;
    }

    @Override
    public void setLegCount(int legCount) {

    }
}

Cat.java

interface Cat extends Animal {
    boolean isDeclawed();
    void setDeclawed(boolean declawed);
}

CatEntity.java

@RealmClass
public class CatEntity implements RealmModel, Cat {

    private boolean isDeclawed;
    private AnimalEntity commonAttributes;

    // In real app, need to check for null here when referencing commonAttributes.
    @Override
    public String getColor() { return commonAttributes.getColor(); }

    @Override
    public void setColor(String color) { commonAttributes.setColor(color); }

    @Override
    public int getLegCount() { return commonAttributes.getLegCount(); }

    @Override
    public void setLegCount(int legCount) { commonAttributes.setLegCount(legCount); }

    @Override
    public boolean isDeclawed() { return isDeclawed; }

    @Override
    public void setDeclawed(boolean declawed) { this.isDeclawed = declawed; }

}

Duck.java

interface Duck extends Animal {
    String getBillType();
    void setBillType(String billType);
}

DuckEntity.java

@RealmClass
public class DuckEntity implements RealmModel, Duck {
    private String billType;
    private AnimalEntity commonAttributes;

    // In real app, need to check for null here when referencing commonAttributes.
    @Override
    public String getColor() { return commonAttributes.getColor(); }

    @Override
    public void setColor(String color) { commonAttributes.setColor(color); }

    @Override
    public int getLegCount() { return commonAttributes.getLegCount(); }

    @Override
    public void setLegCount(int legCount) { commonAttributes.setLegCount(legCount); }

    @Override
    public String getBillType() { return billType;}

    @Override
    public void setBillType(String billType) { this.billType = billType; }
}

Usage.java

List<? extends Duck> ducks = db.where(DuckEntity.class).findAll();
for(Duck duck : ducks) {
    System.out.println("A duck has " + duck.getLegCount() + " legs, and this one has a " + duck.getBillType() + " type bill");
}