How can I use same methods in different classes type ArrayList?

Question

I'm doing a project that should be stored in two differents text files. Let say I have 2 classes Person and Activity, each with only these attributes in common: id and isActive. But there are also many that are not common.

Also I have 2 classes ArrayList type:

public class RegistryPerson extends ArrayList<Person> {
     public void add(Person obj){
              ....
     }
     public boolean isDuplicate(Person obj){
         for(Person p: this){
             if(obj.equals(p)){ 
                 return true;
             }
         }
         return false;
     } 
     public Person search(int id){
              ....
     }
     public void readFile(){
         otherClass.readFile(String txtfilePerson);
     } 
     public void activate(Person obj){
              obj.setActivate;
     }
     //more methods
}

.

public class RegistryActivity extends ArrayList<Activity> {
     public void add(Activity obj){
              ....
     }
     public boolean isDuplicate(Activity obj){
         for(Activity p: this){
             if(obj.equals(p)){ 
                 return true;
             }
         }
         return false;
     } 
     public Activity search(int id){
              ....
     }
     public void readFile(){
         otherClass.readFile(String txtfileActivity);
     } 
     public void activate(Activity obj){
              obj.setActivate;
     }
     //more methods
}

Both classes have the same methods

As you see both classes type ArrayList RegitryPerson and RegistryActivigy have same methods, but some used different kind of object.

I just don't wanna have almost same code in differents classes. Can I use an interface or abstract class? and most important, How can implement that?. Or I am complicating everything?

Thanks.

Answer 1

You can follow Program to an interface, not implementations design principle here.

Create an interface say Entity that would be implemented by both Person and Activity

Entity.java

public interface Entity {
    public Boolean equals(Entity e);
    //other common methods
}

This interface would be implemented by both Person and Activity

Person.java

public class Person implements Entity {
    ...
    @Override
    public boolean equals(Entity e) {
        ...
    }
    ...
}

Activity.java

public class Activity implements Entity {
    ...
    @Override
    public boolean equals(Entity e) {
        ...
    }
    ...
}

Now create a parent Class Registry

Registry.java

public class Registry extends ArrayList<Entity> {
     public void add(Entity obj){
              ....
     }
     public boolean isDuplicate(Entity obj){
         for(Entity p: this){
             if(obj.equals(p)){ 
                 return true;
             }
         }
         return false;
     } 
     public Entity search(int id){
              ....
     }
     public void readFile(){
         otherClass.readFile(String txtfilePerson);
     } 
     public void activate(Entity obj){
              obj.setActivate;
     }
     //more methods
}

Now you can extend this Registry class to both of your implementations, ie, RegistryPerson and RegistryActivity

RegistryPerson.java

public class RegistryPerson extends Registry {
    ..
}

RegistryActivity.java

public class RegistryActivity extends Registry {
    ..
}

PS: All of the classes listed above many contain more common methods. This is just to give you basic introduction to this design principle.

Answer 2

I had the same idea as rD but used generics to create the registry. I also moved the List inside the class. Methods to read from file and add the registry could be handled by other classes. I try to avoid inheritance and abstract classes at all costs. Interfaces are generics are quite powerful.

interface IdObject {
    int getId();
}

class Registry<T extends IdObject> {
    private List<T> list = new ArrayList<T>();

    public void add(T obj){
        list.add(obj);
    }

    public boolean isDuplicate(T obj){
        for(T t: list){
            if(obj.equals(t)){
                return true;
            }
        }
        return false;
    }
    public T search(int id){
        for(T t: list){
            if(t.getId() == id)){
                return t;
            }
        }
        return null;
    }
}

class Example {
    Registery<Person> personRegistery = new Registry<>();
    Registery<Activity> activityRegistery = new Registry<>();
}