I’m learning about Java. I would like to override the return type of ArrayList to subclass. Is it possible?
The compiler said that can override only the same type…

unit classes

import java.util.ArrayList;

abstract public class BaseUnit {
  public BaseUnit() {
  }

  public abstract ArrayList<BaseElement> getList();
}

public class UnitA extends BaseUnit {
  private ArrayList<ElementA> listA;

  UnitA() {
    super();
    listA = new ArrayList<ElementA>();
    listA.add(new ElementA());
  }

  public ArrayList<ElementA> getList() {
    return listA;
  }
}

element classes

public class BaseElement {
  protected String name = "";
  public BaseElement() {

  }
  
  public void sayName() {
    System.out.println(name);
  }
}

public class ElementA extends BaseElement {
  public ElementA() {
    name = "A";
  }
}

main

public static void main() {
  UnitA unitA = new UnitA();
  unitA.getList().get(0).sayName();
}

>Solution :

The generics of the list in both types have to be the same. For what you want to do you basically have two options: Use the extends keyword, or make the type in the base class totally generic.

The first solution would look like this:

import java.util.ArrayList;

abstract public class BaseUnit {
  public BaseUnit() {
  }

  public abstract ArrayList<? extends BaseElement> getList();
}

The second like this:

import java.util.ArrayList;

abstract public class BaseUnit<K> {
    public BaseUnit() {
    }

     public abstract ArrayList<K> getList();
}

public class UnitA extends BaseUnit<ElementA> {
    private ArrayList<ElementA> listA;

    UnitA() {
        super();
        listA = new ArrayList<ElementA>();
        listA.add(new ElementA());
     }

    public ArrayList<ElementA> getList() {
        return listA;
    }
}