Java：如何创建从继承的类中获取对象的getter方法？

Question

I'm trying to form a getter method of an object Mammal within a class Catalog. The get method has no parameters, and has a return type Mammal (?)

public class Mammal extends Animal {
    private double intellect;
    private double hitpoints;

    public Mammal(String firstName, String lastName, String nickname) {
        super(firstName, lastName, nickname);
    }

    public void setIntellect(double intellect) {
        this.intellect = intellect;
    }

    public void setHitpoints(double hitpoints) {
        this.hitpoints = hitpoints;
    }
    public double getIntellect() {
        return intellect;
    }
    public double getHitpoints() {
        return hitpoints;
    }

I know it's a pretty simple question and you didn't have to see that, just giving context. I coded several classes like Mammal, such as Feline, Birds, etc. and am the last part of the assignment, Catalog, a series of get methods. I just need to getMammal, getBirds, getFeline, and the return type is either Mammal, or Birds[], etc. So the object itself or an array of objects. My attempt:

public class Catalogue{
    private Mammal mam;

    public getMammal() {
       Mammal mam = new Mammal();
       return mam;
    }
}

so I'm clearly not understanding what to do at this step. Any help would be appreciated! The compile error I get is "invalid method declaration; return type required". I'm trying it out in a few different ways, but can't seem to get past this error.

Answer 1

I think what you need looks something like this:

public class Catalogue 
{
    private Mammal mammal;
    private Bird[] birds;
    public Catalogue(Mammal mammal, Bird[] birds)
    {
       this.mammal = mammal;
       this.birds = birds;
    }
    public Mammal getMammal()
    {
     return mammal;
    }
    public Bird[] getBirds()
    {
      return birds;
    }
}

This creates a Catalogue with instance variables mammals and birds , and allows you to get those values. And then to construct a Catalogue , you call:

Catalogue c = new Catalogue(mammal, birds);

Where mammal is an instance of Mammal and birds is an instance of Bird[] .

Answer 2

Replace

public getMammal() {
   Mammal mam = new Mammal();
   return mam;
}

by

public Mammal getMammal() {
   Mammal mam = new Mammal();
   return mam;
}

You need to be explicit and define the return type of getter methods, as well as any non-constructor methods.

You might think it is obvious since you are clearly returning a Mammal , but Java is strongly and typed.

There are programming languages which can infer the return type of a method simply by examining the code, but Java is not one of them.

Scala, for instance, is strongly typed but can usually infer what the return type of methods is.

Answer 3

public Mammal getMammal() {
    Mammal mam = new Mammal();
    return mam;
}

you have to write the type of object, that you will return. and to write in brackets "new Mammal(firstName,lastName,nickname)"

Answer 4

If i understand you correctly this Catalog class should be something to create instances of All the Animal type that you created.. one of the solution is simple and will be like:

public class Catalogue{

    public Mammal getMammal() {
       return new Mammal();
    }
}

You don't need Catalog to hold on instance.. Another generic solution will be:

public class Catalogue{

    public static Animal getAnimalByType(Class type) {
       return type.newInstance();
    }
}

And when you call this you will know the type.. So this will be generic.

Mammal m = (Mammal) Catalogue.getAnimalByType(Mammal.class);

The casting is required because in you generic type you will return Animal, so this will be generic and work for all animal types.. And when you call the method you know what kind of animal you want.. Because you passing this as a parameter in constructor

Hope that helps.. Of course this can be more sophisticated but this will do the work for now .

EDIT:

In order to create new instance of Mammal you also need to add construtor:

public class Mammal extends Animal {
    private double intellect;
    private double hitpoints;

    public Mammal() {
        super();
    }

    public Mammal(String firstName, String lastName, String nickname) {
        super(firstName, lastName, nickname);
    }

    public void setIntellect(double intellect) {
        this.intellect = intellect;
    }

    public void setHitpoints(double hitpoints) {
        this.hitpoints = hitpoints;
    }
    public double getIntellect() {
        return intellect;
    }
    public double getHitpoints() {
        return hitpoints;
    }