Question

I have constructed a map and try to put subclasses of a given class into it. Problem is that map accepts some of them, but not all. Please let me know what is the problem, why some subclasses are not accepted and how to fix it. I also tried to make class Cat and Dog extend Rodent (since Hamster works here), but this doesn’t work. Thank you

Here is the code for each class:

package typeinfo.pets;

public class Pet extends Individual {
  public Pet(String name) { super(name); }
  public Pet() { super(); }
} ///:~

package typeinfo.pets;

public class Rodent extends Pet {
  public Rodent(String name) { super(name); }
  public Rodent() { super(); }
} ///:~

package typeinfo.pets;

public class Cat extends Pet {
  public Cat(String name) { super(name); }
  public Cat() { super(); }
} ///:~

package typeinfo.pets;

public class Hamster extends Rodent {
  public Hamster(String name) { super(name); }
  public Hamster() { super(); }
} ///:~

import typeinfo.pets.*;

import java.util.*;

import static net.mindview.util.Print.*;



public class PetMap {

  public static void main(String[] args) {

    Map<String,Pet> petMap = new HashMap<String,Pet>();

    petMap.put("My Hamster", new Hamster("Bosco"));

    //the two lines here cause problems "Map<String, Pet> is not 
    // applicable to <String, Cat>
    petMap.put("My Cat", new Cat("Molly"));

    petMap.put("My Dog", new Dog("Ginger"));

    print(petMap);

    Pet dog = petMap.get("My Dog");

    print(dog);

    print(petMap.containsKey("My Dog"));

    print(petMap.containsValue(dog));

  }

}

Answer 1

As most of the users suggested, your problem doesn’t lie in the inheritance tree, but somwhere along the lines of what classes you imported and from where.

The Cat and Dog you use in your main method may not be subclasses of Pet; probably that’s why Hamster is accepted and the others aren’t. I tried to make your code running and added what was missing. The following, for example works:

import java.util.*;

class Individual {
    public String name;
    public Individual(String name) {
        this.name = name;
    }
    public Individual () {
        this.name = new String();
    }
}
class Pet extends Individual {
  public Pet(String name) { super(name); }
  public Pet() { super(); }
} ///:~

class Rodent extends Pet {
  public Rodent(String name) { super(name); }
  public Rodent() { super(); }
} ///:~

class Cat extends Pet {
  public Cat(String name) { super(name); }
  public Cat() { super(); }
} ///:~

class Dog extends Pet {
      public Dog(String name) { super(name); }
      public Dog() { super(); }
    } ///:~

class Hamster extends Rodent {
  public Hamster(String name) { super(name); }
  public Hamster() { super(); }
} ///:~



public class PetMap {

  public static void main(String[] args) {

    Map<String,Pet> petMap = new HashMap<String,Pet>();

    petMap.put("My Hamster", new Hamster("Bosco"));

    // the two lines now work
    petMap.put("My Cat", new Cat("Molly"));

    petMap.put("My Dog", new Dog("Ginger"));

    System.out.println(petMap);

    Pet dog = petMap.get("My Dog");

    System.out.println(dog);

    System.out.println(petMap.containsKey("My Dog"));

    System.out.println(petMap.containsValue(dog));

  }

}

Please recheck your package definitions and imports.