Thread: Splitting up main method / class

Originally Posted by aussiemcgr

Tell me which option you would like. It is really just a matter of opinion

I think the version where the Scanner is integrated into the class and rather than the main method seem's more 'proper' for some reason? I really don't know though.

Originally Posted by aussiemcgr

The createAnimal() method should accept the animal the user wants as the parameter in the method. When this method is called, we are assuming that the desired animal has already been read by the user

OK, so I've put that into my createAnimal(String Animal) method.... And I've put the scanner before that method, though I'm not sure if that's where the scanner should be or not.

here's the code that I've tried for the FarmClass :

import java.util.Scanner;
 
public class FarmClass {
 
	int pigCount;
	int cowCount;
	int sheepCount;
 
 
 
 
	//Prompt user method
 
	public String promptUser() {
		System.out.println("what kind of animal would you like to create?");
		Scanner scan = new Scanner(System.in);
 
		String animal = scan.nextLine();
 
		return animal;
	}
 
 
	// Not sure if this scanner's in the right place... It should be in this
	// class though?
 
 
 
	// create animal method
 
	public void createAnimal(String animal) {
 
		if (animal.equalsIgnoreCase("cow")) {
 
			cowCount++;
 
			// Not sure what I'm doing here... I'm trying to think of a way to
			// add the cow count to the newly created cow, but I can't really
			// work it. Also, I'm not sure If I'm meant to create the cow in
			// this section, or how I'm meant to go about creating it.
			Animal newCow = new cow[cowCount];
			String sound = "moo";
 
		}
 
		else if (animal.equalsIgnoreCase("pig")) {
			pigCount++;
 
 
			String sound = "oink";
 
 
		}
 
		else if (animal.equalsIgnoreCase("sheep")) {
 
			sheepCount++;
 
			String sound = "baaa";
		}
 
 
	}
 
}

Not sure if any of that's in the right ball park.... There should be some fairly obvious mistakes in there though. I'm unsure about creating a new animal from the createAnimal method of the FarmClass... I'm not really sure about the scanner either and how that's going to work...

cheers

I'll have a bash at creating the other classes now.... I'll post back when I'm done!

--- Update ---

Ok I've made those...

I'm still kind of waiting for the dot's to join up around the scanner usage, the counting and the calling between classes, but I think it's there! Onto the main method?

Farm Class :

import java.util.Scanner;
 
public class FarmClass {
 
	int pigCount;
	int cowCount;
	int sheepCount;
 
	// Prompt user method
 
	public String promptUser() {
		System.out.println("what kind of animal would you like to create?");
		Scanner scan = new Scanner(System.in);
 
		String animal = scan.nextLine();
 
		return animal;
	}
 
	public void createAnimal(String animal) {
 
		if (animal.equalsIgnoreCase("cow")) {
 
			cowCount++;
			Cow newCow = new Cow();
			System.out.println("The cow say's " + newCow.getSound());
		}
 
		else if (animal.equalsIgnoreCase("pig")) {
			pigCount++;
			Pig newPig = new Pig();
			System.out.println("The pig goes " + newPig.getSound());
		}
 
		else if (animal.equalsIgnoreCase("sheep")) {
 
			sheepCount++;
			Sheep newSheep = new Sheep();
			System.out.println("The sheep says " + newSheep.getSound());
 
		}
 
	}
 
}

Cow:

public class Cow {
 
	public String sound = "Moo";
 
	public String getSound() {
		return sound;
	}
 
}

Pig:

public class Pig {
 
	public String sound = "Oink";
 
	public String getSound() {
		return sound;
	}
 
}

Sheep:

public class Sheep {
 
	public String sound = "Baaa";
 
	public String getSound() {
		return sound;
	}
 
}

Will try this now - I'm creating a main method separate to the FarmClass. I thought that the scanner in the promptUser() method of the FarmClass was capturing the users input..?

Does this mean that there should be a getAnimal(); method created in the FarmClass? I've tried to make a main method but it doesn't seem to be incrementing the count.

I guess I'm stuck on 3 - I'm getting confused because I thought that the promptUser method was meant to do all that...

Here's the main method that I've written so far :

 
import java.util.Scanner;
 
 
public class FarmMain {
 
	public static void main(String[] args) {
 
		FarmClass myFarm = new FarmClass();
 
 
 
		Scanner scan = new Scanner(System.in);
 
 
		for(int i = 0 ; i < 4; i++){
 
			myFarm.promptUser();
 
			/*String userAnimal = scan.nextLine();
*/			
			System.out.println(myFarm.pigCount);
 
		}
 
 
 
 
	}
 
}

here's the return from the console after running that method;

what kind of animal would you like to create?
pig
0
what kind of animal would you like to create?

Not really - where has the userAnimal variable come from? or is that created by invoking that method?

Ahhhh I see you've created a String variable then passed that into myFarm.promptUser();

OK that's cool ! What's that type of thing called? I guess you can do that with any relating method's return type? So if it were numbers you'd have int userAnimalNum = myFarm.promptUser(); ?


now the output of the program is :

 
what kind of animal would you like to create?
pig
The pig goes Oink
1
what kind of animal would you like to create?
cow
The cow say's Moo
1
what kind of animal would you like to create?
sheep
The sheep says Baaa
1
what kind of animal would you like to create?
pig
The pig goes Oink
2

Not sure what the next stage is... creating an array update from the counts, or the user exit?

nice one

Just looking at this... going to have a bash. I think that I'm removing all the count int's and replacing them with this <list> interface. Going to try that now.

--- Update ---

Cool that's worked nicely, I'm trying to do a get out loop at the moment but I'm not sure how's best, I've been trying a while with a scanner input, but I don't really get where to put it... I've I nest the for loop into the while loop it doesn't seem to work, and If I have the while loop in the for loop then it's not really doing anything....

It would be ideal If I could access the promptUser scanner, but because that's made within the other class I can't get to it....

I've been trying if statement but I'm having the same problem, it either skips the for loop or it doesn't do anything.

here's the (not working) code at the moment though :

import java.util.Scanner;
 
public class FarmMain {
 
	public static void main(String[] args) {
 
		FarmClass myFarm = new FarmClass();
 
		Scanner scan = new Scanner(System.in);
 
		String makeMore = null;
		String end = "Y";
 
		if (makeMore.equalsIgnoreCase(end)){
 
			for (int i = 0; i < 4; i++) {
 
				String userAnimal = myFarm.promptUser();
				myFarm.createAnimal(userAnimal);
 
				System.out.println("Would you like to make more animals? Y/N");
 
				String makeMoreAnswer = scan.nextLine();
				makeMore = makeMoreAnswer;
				}
 
			break;
 
		}
 
		System.out.println("\nAnimals created!");
 
		myFarm.getAnimalTotals();
 
	}
 
}

and here's the FarmClass now :

import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;
 
public class FarmClass {
 
	List pigList = new ArrayList<Pig>();
	List cowList = new ArrayList<Cow>();
	List sheepList = new ArrayList<Sheep>();
 
	public String promptUser() {
		System.out.println("what kind of animal would you like to create?");
		Scanner scan = new Scanner(System.in);
 
		String animal = scan.nextLine();
 
		return animal;
	}
 
	public void createAnimal(String animal) {
 
		if (animal.equalsIgnoreCase("cow")) {
 
			Cow cow = new Cow();
			cowList.add(cow);
			System.out.println("Cow count : " + cowList.size());
			System.out.println("The cow say's " + cow.getSound());
		}
 
		else if (animal.equalsIgnoreCase("pig")) {
			Pig pig = new Pig();
			pigList.add(pig);
			System.out.println("Pig count : " + pigList.size());
			System.out.println("The pig goes " + pig.getSound());
		}
 
		else if (animal.equalsIgnoreCase("sheep")) {
 
			Sheep sheep = new Sheep();
			sheepList.add(sheep);
			System.out.println("Sheep count : " + sheepList.size());
			System.out.println("The sheep says " + sheep.getSound());
 
		}
 
	}
 
	public void getAnimalTotals() {
		System.out.println("\nCow count : " + cowList.size()
				+ "\nSheep count : " + sheepList.size() + "\nPig count : "
				+ pigList.size());
	}
 
}

--- Update ---

I'm knackered again, I'll have time tomorrow for this though. cheers!

I don't really see how the nested if statement can break out of the for loop? Because it's nested won't it just break out of the if statement back into the for loop etc...?

OK wow I just tried it and it worked I didn't think that would work (for reasons above!)

Here's the amended code :

Farm Main

• Added the if statement with the break

• changed the for loop to an infinite one

• Added some basic validation, It's kind of do-able for a list this size but if there were 20 animal's in the farm I imagine the the method I've used here would be a bit of a chore, or just not an option

import java.util.Scanner;
 
public class FarmMain {
 
	public static void main(String[] args) {
 
		FarmClass myFarm = new FarmClass();
 
		Scanner scan = new Scanner(System.in);
 
		String makeMore = null;
		String end = "Y";
 
			for (;;) {
 
				String userAnimal = myFarm.promptUser();
				myFarm.createAnimal(userAnimal);
 
 
				if(!userAnimal.equalsIgnoreCase("cow") && (!userAnimal.equalsIgnoreCase("pig") && (!userAnimal.equalsIgnoreCase("Sheep")))){
					System.out.println("Sorry! we don't have any of those\n");
 
				}
 
 
				if(userAnimal.equalsIgnoreCase("exit")){
					break;
				}
 
				}
 
 
		System.out.println("\nAnimals created!");
 
		myFarm.getAnimalTotals();
 
	}
 
}

here's the FarmClass just in case some of those references aren't expected ;

FarmClass

import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;
 
public class FarmClass {
 
	List<Pig> pigList = new ArrayList<Pig>();
	List<Cow> cowList = new ArrayList<Cow>();
	List<Sheep> sheepList = new ArrayList<Sheep>();
 
	public String promptUser() {
		System.out.println("what kind of animal would you like to create? Or enter 'exit' to terminate the program");
		Scanner scan = new Scanner(System.in);
 
		String animal = scan.nextLine();
 
		return animal;
	}
 
	public void createAnimal(String animal) {
 
		if (animal.equalsIgnoreCase("cow")) {
 
			Cow cow = new Cow();
			cowList.add(cow);
			System.out.println("Cow count : " + cowList.size());
			System.out.println("The cow say's " + cow.getSound());
		}
 
		else if (animal.equalsIgnoreCase("pig")) {
			Pig pig = new Pig();
			pigList.add(pig);
			System.out.println("Pig count : " + pigList.size());
			System.out.println("The pig goes " + pig.getSound());
		}
 
		else if (animal.equalsIgnoreCase("sheep")) {
 
			Sheep sheep = new Sheep();
			sheepList.add(sheep);
			System.out.println("Sheep count : " + sheepList.size());
			System.out.println("The sheep says " + sheep.getSound());
 
		}
 
	}
 
	public void getAnimalTotals() {
		System.out.println("\nCow count : " + cowList.size()
				+ "\nSheep count : " + sheepList.size() + "\nPig count : "
				+ pigList.size());
	}
 
}

It can be just an else statement at the end. The reason for this is to reduce computations. If the user input is neither of those things

Ah of course, that'll be the easier way to do that! Although typing exit

In the above code, I labeled the outer loop: OUTER, and specified the loop I wanted to break out of after the break statement. That is just a handy thing to keep in your head

Yes - I've seen this before with a continue statement, completely forgot about it... handy

for (;;)

This should be changed to this:

while(true)

Ok cool - it looks neater like that... makes sense!


I've amended the classes now, here's what the classes looks like ;

FarmMain

import java.util.Scanner;
 
public class FarmMain {
 
	public static void main(String[] args) {
 
		FarmClass myFarm = new FarmClass();
 
		Scanner scan = new Scanner(System.in);
 
		String makeMore;
		String end = "Y";
 
		for (;;) {
 
			String userAnimal = myFarm.promptUser();
 
 
 
			if (userAnimal.equalsIgnoreCase("exit")) {
				break;
			}
 
			myFarm.createAnimal(userAnimal);
		}
 
		System.out.println("\nAnimals created!");
 
		myFarm.getAnimalTotals();
 
	}
 
}

FarmClass

import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;
 
public class FarmClass {
 
	List<Pig> pigList = new ArrayList<Pig>();
	List<Cow> cowList = new ArrayList<Cow>();
	List<Sheep> sheepList = new ArrayList<Sheep>();
 
	public String promptUser() {
		System.out
				.println("what kind of animal would you like to create? Or enter 'exit' to terminate the program");
		Scanner scan = new Scanner(System.in);
 
		String animal = scan.nextLine();
 
		return animal;
	}
 
	public void createAnimal(String animal) {
 
		if (animal.equalsIgnoreCase("cow")) {
 
			Cow cow = new Cow();
			cowList.add(cow);
			System.out.println("The " + cow.getName() + " count : " + cowList.size());
			System.out.println("The cow say's " + cow.getSound());
		}
 
		else if (animal.equalsIgnoreCase("pig")) {
			Pig pig = new Pig();
			pigList.add(pig);
			System.out.println("The " + pig.getName() +" count : " + pigList.size());
			System.out.println("The pig goes " + pig.getSound());
		}
 
		else if (animal.equalsIgnoreCase("sheep")) {
 
			Sheep sheep = new Sheep();
			sheepList.add(sheep);
			System.out.println("The " + sheep.getName() +" count : " + sheepList.size());
			System.out.println("The sheep says " + sheep.getSound());
		} 
 
		else {
			System.out.println("Sorry! we don't have any of those\n");
		}
 
	}
 
	public void getAnimalTotals() {
		System.out.println("\nCow count : " + cowList.size()
				+ "\nSheep count : " + sheepList.size() + "\nPig count : "
				+ pigList.size());
	}
 
}

And one of the animal classes ;

PigClass

public class Pig {
 
	public String sound = "Oink\n";
 
	public String name = "Pig";
 
	public String getName() {
		return name;
	}
 
	public String getSound() {
		return sound;
	}
 
}

We will create the abstract Animal class, which all the animal types will extend from (we can create an abstract class or an interface, it is up to you)

I've never really used either so I'm not sure - I guess which ever you feel would either be best to learn, most applicable to this example or the one that you would use personally!

Thanks

Ace cheers mcgr - have read through the post a couple of times...

So an interface - contain's all of the instance variables that are going to be used in further classes, If someone was making an interface for dogs they might have

public interface dogInterface {
            public void barkAtStrangers();
            public void weight();
            public boolean pedigree();
 
}

Or something of that nature.... Or maybe they would have the dog as a subclass of animal, unless their program was just about dogs....

Then abstract class, out of the two abstract class seem's like a bit of an obvious preferences, if it's more flexible?

But then the abstract class is (this is only my 'understanding' here....) about something abstract. So 'Animal' would be an abstract class as you can't have an animal without it being a subset of. Although perhaps someone would argue that with dog? You can't just have a 'dog', you have a border collie, or a Labrador, or a mongrel etc. So I guess in that sense 'dog' could be used as an abstract class or a subclass... maybe!

I can't really see why an interface would be beneficial at the moment, but seeing as I've never put either into practice that's not much of a surprise. You can implement many interfaces but only one abstract class I guess that's the main reason....

In the example of the interface that you used :

public void methodOne() {
		System.out.println("Method One");
	}

this method could be called one by using

someObject.methodOne();

and would return the following to the console :

Method one

And in interfaces you can only instantiate the methods / variables - you can't actually provide the code that they would execute.

Is that correct?

Tell me if you understand all that. It is very conceptual

Well, I think that I'm happy to learn it, though I get that will come from actually using them. Already going through this example with you there have been thing's that I've read but actually using them and seeing how they work is the only way to comprehend them (so thanks again!)

for your purposes, it doesn't really matter either way. Let's give it a try with an abstract class

Yeah that's cool....

Let me know if there's anything it sound's like I'm missing the point on, and if there's any reading I should do before the next part..

Ace, well here's the Animal class (I used Eclipse to generate the getters.... not sure if this is a not-to-do...):

public abstract class Animal {
 
	public String sound;
	public String name;
 
	public String getSound() {
		return sound;
	}
 
	public String getName() {
		return name;
	}
 
}

Pig

public class Pig extends Animal {
 
	public String sound = "Oink\n";
 
	public String name = "Pig";
 
	public String getName() {
		return name;
	}
 
	public String getSound() {
		return sound;
	}
 
}

Sheep

public class Sheep extends Animal {
 
	public String sound = "Baaa\n";
 
	public String name = "Sheep";
 
 
	public String getName(){
		return name;
	}
 
	public String getSound() {
		return sound;
	}
 
}

Cow

public class Cow extends Animal {
 
	public String sound = "Moo\n";
 
	public String name = "Cow";
 
 
	public String getName(){
		return name;
	}
 
	public String getSound() {
		return sound;
	}
 
}

Haven't made any changes to the main method or FarmClass...

Dog extends/implements Animal, and Border Collie, Labrador, ect. extend/implement Dog.
If you want to make it so that you MUST have a type of Dog (meaning: just a Dog object would not be enough for your desired implementation, and you MUST have the type of Dog), you would probably make Dog abstract. This means you would not be able to create just a generic Dog object, but you would still be able to use all of the Dog's "traits" to create a Border Collie, Labrador, ect

I'm not really fully following along with this, it kind of makes sense but it's not pictured in my head. But as this is the design element I guess it's a matter of doing a bit of wax on wax off for it to sink in.

With an abstract class, you at least have some implementation here and there

So you can implement an abstract class say, if there was an abstract class called someAbstract. You could have ?:

someAbstract myAbstract = new someAbstract();

And that would work, but if it was an interface it wouldn't. Interfaces don't have any parenthesis so they can't receive any input (Not sure if that's correct or not) thy can only declare methods not variables.

Ace... Times vanished in the UK, I should be able to read up on stuff and have a stab at this before work tomorrow though...

1. Get rid of your Pig, Cow, and Sheep list and create a single Animal list

FarmClass

public class FarmClass {
 
	List<Animal> animalList = new ArrayList<Animal>();

I've made that, but I'm struggling with everything else...

2. Create 3 methods: getPigCount(), getCowCount(), and getSheepCount(). These methods will return an int, representing the number of each animal. These methods should loop through the Animal list and check each item. When you pull an item out of the Animal list (with the List.get(int index) method), you will have a generic Animal object. Now, we know that the object is not really a generic Animal object. In reality, it is either a Pig, a Cow, or a Sheep. But we don't know which one. There are two ways we can check

I've tried to create these method's in the farm class, but I'm not sure how I'm meant to go through the arrayList with these methods. I've read bit's about list.add method's and stuff, it that something that should be in the main method or the prompt user section? I'm guessing that there need's to be some kind of :

 
for (int i = 0 ; i ; i++) {
       System.out.println(animalList.get[i]);
}

But that's printing out, I'm not sure how to 'check' each item, or what exactly that means... Check that they're there? So, like the validation stage where it ensures that those animal's are actually available?

When you pull an item out of the Animal list (with the List.get(int index) method), you will have a generic Animal object

so I would use the code animalList.get(3) to return the third animal in the list. And the list would store cow's, pigs, sheep all together... so as an array it might look like

animalArray[0] = "Sheep";
animalArray[1] = "Pig";
animalArray[2] = "Cow";
animalArray[3] = "Pig";

Obviously it's not an array in that sense... but there are stored like that?

3. Change your createAnimal() method. To do this, we will follow these steps:
a) create a variable before the if statements:

 
 
public void createAnimal(String animal) {
 
	Animal tempAnimal = null;
 
	if (animal.equalsIgnoreCase("cow")) {
...

I've tried to do that, although I'm not really joining the dots... Here's a part of my ammended create animal method though;

public void createAnimal(String animal) {
 
		if (animal.equalsIgnoreCase("cow")) {
 
			Cow cow = new Cow();
			cowList.add(cow);
			System.out.println("The " + cow.getName() + " count : " + cowList.size());
			System.out.println("The cow say's " + cow.getSound());
		}

Question about the promptUser method : In the method the variable animal is instantiated with the line

String animal = scan.nextLine();

This variable animal is declared within the braces of the prompt user method. We can access this variable with other methods because at the end of the promptUser() method the variable is returned with

return animal;

I just wanted to check my understanding on that one...

I'm not sure how to tie in the counts with this, here're the classes in case I've done anything daft... As it stands in the FarmClass the get*Count() methods are just kind of hanging...



FarmClass

import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;
 
public class FarmClass {
 
	List<Animal> animalList = new ArrayList<Animal>();
 
 
	public int getPigCount();
	public int getSheepCount();
	public int getCowCount();
 
 
 
 
	public String promptUser() {
		System.out
				.println("what kind of animal would you like to create? Or enter 'exit' to terminate the program");
		Scanner scan = new Scanner(System.in);
 
		String animal = scan.nextLine();
 
		return animal;
	}
 
	public void createAnimal(String animal) {
 
		Animal tempAnimal = null;
 
		if (animal.equalsIgnoreCase("cow")) {
 
			tempAnimal = new Cow();
 
		}
 
		else if (animal.equalsIgnoreCase("pig")) {
			tempAnimal = new Pig();
 
					}
 
		else if (animal.equalsIgnoreCase("sheep")) {
 
			tempAnimal = new Sheep();
					} 
 
		else {
			System.out.println("Sorry! we don't have any of those\n");
		}
 
		animalList.add(tempAnimal);
		System.out.println("The " + tempAnimal.getName() + " says " + tempAnimal.getSound());
	}
 
	/*public void getAnimalTotals() {
		System.out.println("\nCow count : " + cowList.size()
				+ "\nSheep count : " + sheepList.size() + "\nPig count : "
				+ pigList.size());
	}
*/
}

AnimalAbstractClass

public abstract class Animal {
 
	public abstract String getSound();
	public abstract String getName();
 
}

FarmMain

import java.util.Scanner;
 
public class FarmMain {
 
	public static void main(String[] args) {
 
		FarmClass myFarm = new FarmClass();
 
		Scanner scan = new Scanner(System.in);
		while (true) {
 
			String userAnimal = myFarm.promptUser();
 
 
 
			if (userAnimal.equalsIgnoreCase("exit")) {
				break;
			}
 
			myFarm.createAnimal(userAnimal);
		}
 
		System.out.println("\nAnimals created!");
 
		//myFarm.getAnimalTotals();
 
	}
 
}

Actually that would get the 4th item

Ah yeah of course...count from zero

your for loop would be incorrect because you haven't specified the limit

Yeah I left that blank for some reason...

your for loop would be incorrect because you haven't specified the limit. It should read:

for (int i = 0 ; i<animalList.size(); i++)

Cool - I've seen these method's with Strings as well I think.

When one class extends another, you say the subclass "is-a" super class. So when Pig extends Animal, Pig "is-a" Animal

Yeah this makes sense It can be confusing sometimes when I read super class because I see the Animal class as the super class, but of course by extending the Parent class the PigClass becomes the animal...

every item we pull out of the animalList is not "really" an Animal object, but rather an object for one of Animal's subclasses. But when you pull an item out of the animalList, you pull it out as an Animal object

This is slightly confusing - so the program pulls out an object from the animal list as an Animal - as in the parent class. So say In my animal list I had :

Pig, Sheep, Cow, Pig, Sheep

And pulled the 3rd element : Cow. Java / my program would register this as an Animal rather than a Pig object / extension of Animal... Or would it see Pig, register Pig, but consider this as an animal because it extends animal...?

At runtime, the Animal object we pull out will already know what type of Animal it is.

Or is this (the above comment) what happens at runtime? Sorry, just got a bit confused around that one... maybe I need to try it a few times.

you can set an instance of an object to a variable with type of a superclass without losing any inherited behaviour

A obj0 = new A(); // This is allowed for obvious reasons
A obj1 = new B(); // This is allowed because B is-a A
A obj2 = new C(); // This is allowed because C is-a B, and B is-a A, so C is-a A
A obj3 = new D(); // This is allowed because D is-a A
B obj4 = new C(): // This is allowed because C is-a B
B obj5 = new A(); // This is NOT allowed, because A is not a B. Inheritence is one way, B is-a A, but A is not a B

I think I understand the logic with this, but what I'm not sure about is the actual.... So the First letter is the Class that is being assigned to the variable obj * , and then a new Class is created. You can create a new class from an object that's further up the parent - child class, not not further down (parent being high, child low). But you can't assign a variable to an object that a lower class than the one that you're instantiating with new....

edit like a dog is an animal, and a labrador is an animal but a dog isn't a Labrador, because it could be a Scottish terrier.... And an animal isn't a dog, because it could be a cat...

Hope that reads / show's whether it's ticking over

Thanks

the compiler doesn't know or care if the object returned by the get() method will be an instance of an Animal, or an instance of some subclass of Animal when the program is actually ran. All it cares about is that it is at least an Animal

Right, is this in a way analogous to a folder system in a GUI? (I should probably stay away from using GUI as examples with this but it seemed to fit....) The highest most 'folder' (or parent class) that could work for the example is used by the compiler

Animal animal = animalList.get(i);

Within this analogy sub-folders (Sub-Classes / Child-Classes) are irrelevant to the compiler, It can sort these out after. Initially it just checks the highest level is correct, that animalList.get(i); is an Animal. An example for a higher level class might be life form, Animal might be a lower class than life form, although I'm not sure that if LifeForm was being used and Animal was an extension of LifeForm, would Pig then be an extension of Animal? (Can you have 'nested' extensions?) Perhaps I'm wobbling a bit off topic there...

So, at "runtime", Java now knows that the object
retrieved by the get() method is not just an Animal, but a Pig. So when it
performs method calls on the animal variable, it knows to call the methods
associated to the Pig class, not the Animal class

We / The Java program will know this because we're going to use the

if(animal instanceof Pig)

Statement.... without this statement would it know? Or rather, this statement is just to enable us to return certain thing's based on what class / object is returned by the list....

So at RunTime the animal is what ever sub-class was entered by the user, cow / sheep / pig... So for example say the PigClass had a method called makeBacon(), at what point could we use

animal.makeBacon();

I'm assuming that this is somehow wrong, because it would be a bit odd to have all the pig's, cow's and sheep to be called 'animal'.

At compile-time, they are all variables of type: A. But at runtime, their real types are known, and the methods for their real types are used, instead of the methods for type: A.

Tell me if that makes sense.

It really does make sense, I think that I understand how the logic work's within the examples given and on a more general level. There are still some uncertainties about details though, such as the makeBacon() example above.

I'm looking at the code at the moment (for the FarmClass) and at the bottom of the createAnimal method there is the :

animalList.add(tempAnimal);
		System.out.println("The " + tempAnimal.getName() + " says "
				+ tempAnimal.getSound());

section. Now, we've created a new Animal with the promptUser() method.

String animal = scan.nextLine();

So that's created a new String called Animal, so it's not actually created an Animal, just a String called Animal that represents the type of animal that they would like to create. Then the promptUser() method returns this String.

So referencing the main method FarmMain we have the creation of the FarmClass with

FarmClass myFarm = new FarmClass();

Then we move into the while loop - which creates a String called userAnimal from the promptUser method. this means that the animal String from the promptUser method is now userAnimal.

Then we use the userAnimal String variable in the if statement to validate and if user has entered break clause the program will end.

However if they have entered a correct Animal (one that we have) into the userAnimal String the program will move onto the createAnimal method. I'm a little unsure here actually with how the variable userAnimal and promptUser animal is being passed around.

In the main method the userAnimal is created from the promptUser method - so the animal String variable from promptUser is replaced by the user defined userAnimal.... But in the FarmClass our createAnimal method has an input of String Animal - this input is the animal variable from the promptUser class, that is now the userAnimal.

So - the animal variable still exists within the FarmClass - but it's value has been replaced by the value input by the user. This happens before the code from the createAnimal method is processed, so by the time the process get's to createAnimal the variable animal is the animal that was input by the user. We can use the animal variable in the createAnimal class because it's returned by it's method.

So now we're in the createAnimal class, the user having specified which they'd like to create. We create a tempAnimal and assign it's value to null. This is because we've already used the variable animal in the class (as input) so need to have another variable that can be used within the createAnimal class. Although tempAnimal is of type Animal where as animal is a String - so hmm. So what is tempAnimal at the stage of

Animal tempAnimal = null;

It's value is null - but is it an 'animal' ? Has this in some way declared an abstract class? I know it's not with the 'new' operator, but still. So at this point is tempAnimal an Animal, and because it's abstract its value is null?

Anyway, tempAnimal is then assigned to the relevant object depending on the String input by the user in the promptUser method.


After this has been assigned (or custom error returned) we use

animalList.add(tempAnimal);

to add this newly created animal to our list....

So tempAnimal is added to the list, but because it's either a sheep, cow or pig, these are what value the list will return if say

animalList.get(0);

is used... So does Java convert them as it's putting them into the list or as it's pulling them out? (Does that even matter?) I'm just curious as to whether the list is

animalList[0] = tempAnimal
animalList[1] = tempAnimal
animalList[2] = tempAnimal
animalList[3] = tempAnimal

and then when we call animalList.get(3) java pulls tempAnimal, knows that it's a sheep and presents it to us as a sheep. Or if it converts them on the way in and the list would look like

animalList(0) = pig
animalList(1) = sheep
animalList(2) = cow
animalList(3) = pig

I'm pretty sure that the nomenclatures wrong there - but I hope the point translates... If it's irrelevent then fair enough!

So that's where I think I'm at, I think that the concept of not being able to create an animal from a dog, but dog from an animal makes sense. Hopefully I've explained myself well enough to highlight any confusions though. Sorry it's a bit of an essay