Teaching Java to kids -- sample section from free ebook
This user comes from kidslike.info
Find other posts in the same category: | | |

More details

This is from a free 200 page ebook by a respected Java author
http://www.kidslike.info/free_ebook_java_for_kids


Long description

Java programs consist of classes that represent objects from the real world. Even though people may have different preferences as to how to write programs, most of them agree that it's better to do it in a so-called object-oriented style. This means that good programmers start with deciding which objects have to be included in the program and which Java classes will represent them. Only after this part is done, they start writing Java code.
Classes and Objects
Classes in Java may have methods and attributes.
Methods define actions that a class can perform.
Attributes describe the class.
Let's create and discuss a class named VideoGame. This class may have several methods, which can tell what objects of this class can do: start the game, stop it, save the score, and so on. This class also may have some attributes or properties: price, screen color, number of remote controls and others.
In Java language this class may look like this:
class VideoGame {
String color;
int price;

void start () {
}
void stop () {
}
void saveScore(String playerName, int score) {
}
}
Our class VideoGame should be similar to other classes that represent video games - all of them have screens of different size and color, all of them perform similar actions, and all of them cost money.
We can be more specific and create another Java class called GameBoyAdvance. It also belongs to the family of video games, but has some properties that are specific to the model GameBoy Advance, for example a cartridge type.
class GameBoyAdvance {
String cartridgeType;
int screenWidth;

void startGame() {
}
void stopGame() {
}
}
In this example the class GameBoyAdvance defines two attributes - cartridgeType and screenWidth and two methods - startGame() and stopGame(). But these methods can't perform any actions just yet, because they have no Java code between the curly braces.
In addition to the word class, you'll have to get used to the new meaning of the word object.

The phrase "to create an instance of an object" means to create a copy of this object in the computer's memory according to the definition of its class.
A factory description of the GameBoy Advance relates to an actual game the same way as a Java class relates to its instance in memory. The process of building actual games based on this description in the game factory is similar to the process of creating instances of GameBoy objects in Java.
In many cases, a program can use a Java class only after its instance has been created. Vendors also create thousands of game copies based on the same description. Even though these copies represent the same class, they may have different values in their attributes - some of them are blue, while others are silver, and so on. In other words, a program may create multiple instances of the GameBoyAdvance objects.
Creation of a Pet
Let's design and create a class Pet. First we need to decide what actions our pet will be able to do. How about eat, sleep, and say? We'll program these actions in the methods of the class Pet. We'll also give our pet the following attributes: age, height, weight, and color.
Start with creating a new Java class called Pet in My First Project. Now we are ready to declare attributes and methods in the class Pet. Java classes and methods enclose their bodies in curly braces. Every open curly brace must have a matching closing brace:
class Pet{
}
To declare variables for class attributes we should pick data types for them. I suggest an int type for the age, float for weight and height, and String for a pet's color.
class Pet{
int age;
float weight;
float height;
String color;
}
The next step is to add some methods to this class. Before declaring a method you should decide if it should take any arguments and return a value:
The method sleep() will just print a message Good night, see you tomorrow - it does not need any arguments and will not return any value.
The same is true for the method eat().It will print the message I'm so hungry…let me have a snack, like nachos!.
The method say() will also print a message, but the pet will "say" the word or a phrase that we give to it. We'll pass this word to the method say() as a method argument. The method will build a phrase using this argument and will return it back to the calling program.
The new version of the class Pet will look like this:
public class Pet {
int age;
float weight;
float height;
String color;

public void sleep(){
System.out.println("Good night, see you tomorrow");
}

public void eat(){
System.out.println(
"I'm so hungry…let me have a snack, like nachos!");
}

public String say(String aWord){
String petResponse = "OK!! OK!! " +aWord;
return petResponse;
}
}
This class represents a friendly creature from the real world:
Let's talk now about the signature of the method sleep():
public void sleep()
It tells us that this method can be called from any other Java class (public), it does not return any data (void). The empty parentheses mean that this method does not have any arguments, because it does not need any data from the outside world - it always prints the same text.
The signature of the method say() looks like this:
public String say(String aWord)
This method can also be called from any other Java class, but has to return some text, and this is the meaning of the keyword String in front of the method name. Besides, it expects some text data from outside, hence the argument String aWord.
How do you decide if a method should or should not return a value? If a method performs some data manipulations and has to give the result of these manipulations back to a calling class, it has to return a value. You may say, that the class Pet does not have any calling class! That's correct, so let's create one called PetMaster. This class will have a method main()containing the code to communicate with the class Pet. Just create yet another class PetMaster, and this time select the option in Eclipse that creates the method main(). Remember, without this method you can not run this class as a program. Modify the code generated by Eclipse to look like this:
public class PetMaster {

public static void main(String[] args) {

String petReaction;

Pet myPet = new Pet();

myPet.eat();
petReaction = myPet.say("Tweet!! Tweet!!");
System.out.println(petReaction);

myPet.sleep();

}
}
Do not forget to press Ctrl-S to save and compile this class! To run the class PetMaster click on the Eclipse menus Run, Run..., New and type the name of the main class: PetMaster. Push the button Run and the program will print the following text:
I'm so hungry…let me have a snack like nachos!
OK!! OK!! Tweet!! Tweet!!
Good night, see you tomorrow
The PetMaster is the calling class, and it starts with creating an instance of the object Pet. It declares a variable myPet and uses the Java operator new:
Pet myPet = new Pet();
This line declares a variable of the type Pet (that's right, you can treat any classes created by you as new Java data types). Now the variable myPet knows where the Pet instance was created in the computer's memory, and you can use this variable to call any methods from the class Pet, for example:
myPet.eat();
If a method returns a value, you should call this method in a different way. Declare a variable that has the same type as the return value of the method, and assign it to this variable. Now you can call this method:
String petReaction;
petReaction = myPet.say("Tweet!! Tweet!!");
At this point the returned value is stored in the variable petReaction and if you want to see what's in there, be my guest:
System.out.println(petReaction);
Inheritance - a Fish is Also a Pet
Our class Pet will help us learn yet another important feature of Java called inheritance. In the real life, every person inherits some features from his or her parents. Similarly, in the Java world you can also create a new class, based on the existing one.
The class Pet has behavior and attributes that are shared by many pets - they eat, sleep, some of them make sounds, their skins have different colors, and so on. On the other hand, pets are different - dogs bark, fish swim and do not make sounds, parakeets talk better than dogs. But all of them eat, sleep, have weight and height. That's why it's easier to create a class Fish that will inherit some common behaviors and attributes from the class Pet, rather than creating the classes Dog, Parrot or Fish from scratch every time.
A special keyword extends that will do the trick:
class Fish extends Pet{
}
You can say that our Fish is a subclass of the class Pet, and the class Pet is a superclass of the class Fish. In other words, you use the class Pet as a template for creating a class Fish.
Even if you will leave the class Fish as it is now, you can still use every method and attribute inherited from the class Pet. Take a look:
Fish myLittleFish = new Fish();
myLittleFish.sleep();
Even though we have not declared any methods in the class Fish yet, we are allowed to call the method sleep() from its superclass!
Not all pets can dive, but fish certainly can. Let's add a new method dive() to the class Fish now.

public class Fish extends Pet {

int currentDepth=0;

public int dive(int howDeep){
currentDepth=currentDepth + howDeep;
System.out.println("Diving for " + howDeep +" feet");

System.out.println("I'm at " + currentDepth +
" feet below sea level");
return currentDepth;
}
}
The method dive() has an argument howDeep that tells the fish how deep it should go. We've also declared a class variable currentDepth that will store and update the current depth every time you call the method dive(). This method returns the current value of the variable currenDepth to the calling class.
Please create another class FishMaster that will look like this:
public class FishMaster {

public static void main(String[] args) {

Fish myFish = new Fish();

myFish.dive(2);
myFish.dive(3);

myFish.sleep();
}
}
The method main() instantiates the object Fish and calls its method dive() twice with different arguments. After that, it calls the method sleep(). When you run the program FishMaster, it will print the following messages:
Diving for 2 feet
I'm at 2 feet below sea level
Diving for 3 feet
I'm at 5 feet below sea level
Good night, see you tomorrow

Have you noticed that beside methods defined in the class Fish, the FishMaster also calls methods from its superclass Pet? That's the whole point of inheritance - you do not have to copy and paste code from the class Pet - just use the word extends, and the class Fish can use Pet's methods!
One more thing, even though the method dive() returns the value of currentDepth, our FishMaster does not use it. That's fine, our FishMaster does not need this value, but there may be some other classes that will also use Fish, and they may find it useful. For example, think of a class FishTrafficDispatcher that has to know positions of other fish under the sea before allowing diving to avoid traffic accidents :) X World RIA Conference - Saving Yo


To see articles on the same topic, click the links below the name of the author at the top of this page.