Re: classes and inheritance revisited

Hendrik Maryns <>
Thu, 15 Feb 2007 13:10:42 +0100
Hash: SHA1

John T schreef:

After all the discussion about the last set of questions I posted, I
decided to proceed with a simpler version of the Animal class hierarchy.
Everything is in one package (main..bad name but it works).
I have an Animal (hey RedGritty...I made those changes...still not quite
what I want)

Indeed. I gave some comments:

package main;

How about package animals?

public class Animal {

It would help very much if you included some comments about what all
these variables represent. The names are good, but not enough.

    private String type;

This is superfluous: the class defines the type of animal. A Cat does
not have to store a string saying it is a cat, since it is, well, a Cat.

    private char gender;

Why not boolean? (Ok, snails and stuff, then use an enum, see below)

    private int numberOfLegs;
    private char warmOrCold;

But this really should be a boolean. Don???t try to encode stuff in
strange letters. People (maybe only you, but still) have to remember
what letter stands for what, and it gives room for inconsistency (what
if someone set it to 'z'?

    private char birthType;

Same comment. But here, a boolean is not appropriate. Instead, have a
look at enums (since Java 5): enum BirthType { EGG, LIVE, CELL_SPLIT }

    private char eatType;

Idem, use an enum.

    public Animal(String type, char gender, int numberOfLegs, char
warmOrCold, char birthType, char eatType) {
    this.type = type;
    this.gender = gender;
    this.numberOfLegs = numberOfLegs;
    this.warmOrCold = warmOrCold;
    this.birthType = birthType;
    this.eatType = eatType;

    void move() {}
    void sleep(){}
    void giveBirth() {}
    void eat() {}

Those 4 should be abstract (see below).

    public char getBirthType() {
    return birthType;
    public void setBirthType(char birthType) {
    this.birthType = birthType;

Are you sure you want people to be able to set the birth type for any
animal? I suppose a cat gives live birth, so you do not want to let
people make it lay eggs. Since birth type is predestined, this variable
should be set only once. I.e. you want it to be a final variable, and
remove the setter. Same for all four below.

    public char getEatType() {
    return eatType;
    public void setEatType(char eatType) {
    this.eatType = eatType;
    public char getGender() {
    return gender;
    public void setGender(char gender) {
    this.gender = gender;
    public int getNumberOfLegs() {
    return numberOfLegs;
    public void setNumberOfLegs(int numberOfLegs) {
    this.numberOfLegs = numberOfLegs;
    public String getType() {
    return type;
    public void setType(String type) {
    this.type = type;
    public char getWarmOrCold() {
    return warmOrCold;
    public void setWarmOrCold(char warmOrCold) {
    this.warmOrCold = warmOrCold;

    void showAll() {
    System.out.println("type: " + type );
    System.out.println("gender: " + gender);
    System.out.println("numberOfLegs: " + numberOfLegs);
    System.out.println("warmOrCold: " + warmOrCold );
    System.out.println("birthType: " + birthType );
    System.out.println("eatType: " + eatType );

Here is a Cat

package main;

public class Cat extends Animal {

    public Cat(String type, char gender, int numberOfLegs, char warmOrCold,
        char birthType, char eatType) {
    super(type, gender, numberOfLegs, warmOrCold, birthType, eatType);

I don???t think you need numberOfLegs, warmOrCold, birthType and eatType
in the constructor. They are predestined for cats. So, supposing you
have a public enum Gender{MALE, FEMALE, ANDROGYNOUS} (remember I threw
out type):

  public Cat(Gender gender) {
    super(gender, 4, WARM, LIVE, CARNIVOROUS);


Here is a HouseCat

package main;

public class HouseCat extends Cat {
    String catName;
    public HouseCat(String houseCatName, String type, char gender, int
numberOfLegs, char warmOrCold, char birthType, char eatType) {
        super(type, gender, numberOfLegs, warmOrCold, birthType, eatType);
        // TODO Auto-generated constructor stub
        this.catName = houseCatName;

Since Cat only asks for a gender, you???ll have to adapt this constructor too:

  public HouseCat(String name, Gender gender) {
    this.catName = name;

    void move() {
    void eat() {
    void showAll() {
        System.out.println("Name: " + catName);

And here is my HouseCatTest

package main;

public class HouseCatTest {
    public static void main(String[] args) {
    HouseCat hc = new HouseCat("Sparkles", "tabby", 'm', 4, 'w', 'l', 'c');

  HouseCat hc = new HouseCat("Sparkles", MALE);



type: tabby
gender: m
numberOfLegs: 4
warmOrCold: w
birthType: l
eatType: c
Name: Sparkles

Which is exactly what I wanted.

See the thing is: I see Animal as more of an Interface that is
implemented by Cat than as a Class. But an Interface has no instance
variables so how can I make Animal an interface? That would mean that
the instance variable of Animal would have to be contained in the
classes that implement the interface. Can you say, "code duplication"?
Is there an alternative or a better way to work this together.

Indeed, you need an abstract class and interfaces. As Patricia said,
interfaces are nice, but you have to put the methods somewhere. And
indeed, you don???t want code duplication, so you try to pull them up in
you hierarchy as high as possible. Animal is appropriate for (most of)
what you put in there, but you do not want people to instantiate some
Animal. So you make it abstract, that way only subtypes of animal can
be instantiated.

Here's my image

An animal knows about:
 type : What kind of animal is it
 gender : mALE or fEMALE
 numberOfLegs : 0, 2, 4 whatever
 birthType : lIVE or eGGS

and probably a bunch more

An animal knows how to:

Not all animals sleep or move. So define interfaces Moving and
Sleeping, and have moving animals implement them. You might want a
(still abstract) class Mammal between Animal and Cat, and have it
implement Moving and Sleeping, since all Mammals sleep and move, and
also make it warmblooded by default. In Mammal, sleep() and move() will
also be abstract, such that they have to be implemented by all mammals.
 But concrete mammals such as Cat will not have to think about whether
thay are warmblooded, since Mammal knows that.

and probably a bunch more

A cat knows about everything that an Animal knows about (a cat IS-A animal)

But a Cat also knows about:

itsTail : some Animals don't have one so this instance variable needs to
be in cat

As above: this might be appropriate in Mammal (but there are always
exceptions, such as Humans, arguably), or even some other class
TailedAnimal in between. It all depends on how detailed you want to
make your application.

breed: breaking down Animal further

and it knows how to:
makeNoise (not all Animals make audible noises)

This is also a candidate interface, which, I think, Mammal should
implement, but Fish not.

A HouseCat knows about everything that a Cat knows about (a housecat
IS-A cat)

But a HouseCat also knows about:
itsName: you give your pets names don't you?

and knows how to

I think all Cats know that.

So to me, Animal in an interface.
Cat is an interface that implements Animal (but an interface can't and
implement an interface, at least that's what Eclipse is saying)

An interface can extend another interface.

And HouseCat should both extend and implement Cat, but I can't do that
either. This is getting frustrating...what am I missing.

That you need some place to put the code, so it is impossible to have
only interfaces. Interfaces are just a way to keep multiple inheritance
out of Java anyway. Maybe you could forget about interfaces for a while
and think in classes which you keep as simple as possible, then look
where you have multiple inheritance, and make the less essential
ancestors interfaces.

- --
Hendrik Maryns
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