The Demons Command Each Other
From Huben's Wiki
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| + | This needs to be extended to explain the idea of execution flow during a call, especially the idea of returning to where you were called. Then the test can also be extended. | ||
| + | Have some code they can run, explaining how to step through a program in Dr. Java. | ||
| + | --> | ||
| + | [[Image:Gort.jpeg]] | ||
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We have penetrated the mysteries of finding demon commands. Now we must seek to use those commands, and thus control our demons. | We have penetrated the mysteries of finding demon commands. Now we must seek to use those commands, and thus control our demons. | ||
| − | ==First Call== | + | ==(A) First Call== |
Place this class into Dr.Java, save, compile and run it. | Place this class into Dr.Java, save, compile and run it. | ||
<pre> | <pre> | ||
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* Create another method, named speakThrice(), that calls speak() three times. Change main() to call speakThrice() two times. How many times is the world saved? Why? | * Create another method, named speakThrice(), that calls speak() three times. Change main() to call speakThrice() two times. How many times is the world saved? Why? | ||
| − | ==Calling Between Classes== | + | ==(B) Calling Between Classes== |
Click the New button to start a new Java class, place this code in it, save it and compile it. | Click the New button to start a new Java class, place this code in it, save it and compile it. | ||
<pre> | <pre> | ||
public class Nietzsche | public class Nietzsche | ||
{ | { | ||
| − | public void speak() | + | static public void speak() |
{ | { | ||
System.out.println("Thus spake Zarathustra!"); | System.out.println("Thus spake Zarathustra!"); | ||
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* The third line of main() shows another way to invoke the speak() of DayTheEarthStoodStill. Why does this work? | * The third line of main() shows another way to invoke the speak() of DayTheEarthStoodStill. Why does this work? | ||
* What rule would you make to explain why sometimes you can call with just the name of a method and other times you need something more complex? | * What rule would you make to explain why sometimes you can call with just the name of a method and other times you need something more complex? | ||
| + | * If you are looking at Nietzsche and hit run, for once the error message makes sense. Why does this give an error? | ||
| + | |||
| + | ==(C) Method Calls In Real Programs== | ||
| + | It is easy to identify method calls in the toy programs on this page. It is also easy to identify them in real programs. | ||
Class names start with upper case letters and method names start with lower case letters. Statements such as | Class names start with upper case letters and method names start with lower case letters. Statements such as | ||
| Line 70: | Line 80: | ||
* x is a '''variable''' that is the '''argument''' we are giving to Math.cos(). | * x is a '''variable''' that is the '''argument''' we are giving to Math.cos(). | ||
* y is a '''variable''' where we are storing the '''return value''' from calling Math.cos(). | * y is a '''variable''' where we are storing the '''return value''' from calling Math.cos(). | ||
| − | So in this example, we are giving the Math demon x, a value, and will receive in trade the value cosine of x, which we store in y. If the variable x has a 0 in it, the demon will return a 1 to place into the variable y. More on variables soon. | + | * Variable names also start with lower case letters. |
| + | So in this example, we are giving the Math demon x, a value, and will receive in trade the value cosine of x, which we store in y. If the variable x has a 0 in it, the demon will return a 1 to place into the variable y. Variables are places we can store information, such as numbers. More on variables soon. | ||
Slightly more complicated, | Slightly more complicated, | ||
<pre> | <pre> | ||
| − | sound = animal.makeSound(aTypeOfSound); | + | sound = animal.makeSound(aTypeOfSound, volume); |
</pre> | </pre> | ||
| + | in this example "animal" is not a class. It is a kind of variable called an '''object'''. Objects belong to classes, and can call the methods of their class. Thus: | ||
| + | * makeSound() is a method in some class. (We can figure out which class later.) | ||
| + | * animal is a variable that is an object. | ||
| + | * animal's class has the method makeSound(). | ||
| + | * aTypeOfSound and volume are variables that are arguments for makeSound(). | ||
| + | * sound is a variable where we store the result of animal.makeSound(aTypeOfSound, volume). | ||
| + | * We can be calling methods of different classes by using objects of different classes to make the calls. Notice the dot! Just like the dot when we call with a Class name. | ||
| + | |||
| + | Let's look at some real code now. Here is the main from a class Cryptogram: | ||
| + | <pre> | ||
| + | public static void main(String[] args) | ||
| + | { | ||
| + | Cryptogram window = new Cryptogram(); | ||
| + | window.setDefaultCloseOperation(EXIT_ON_CLOSE); | ||
| + | window.setBounds(30, 30, 800, 600); | ||
| + | window.setVisible(true); | ||
| + | } | ||
| + | </pre> | ||
| + | * The first statement, "Cryptogram window = new Cryptogram();" creates an object named "window" that has class Cryptogram. | ||
| + | * The first statement uses the method Cryptogram() to create the object. As we mentioned before, methods with the same name as their class start with capital letters and are called '''constructors'''. They make new objects, in this case window. | ||
| + | * The remaining statements are all method calls. | ||
| + | * What class is window? | ||
| + | * What class are these methods for? | ||
| + | * How many arguments does setBounds() have? | ||
| + | * Do these 3 last method calls return any values? | ||
| + | |||
| + | ==(D) Do you get it?== | ||
| + | |||
| + | Look at studentdisk/Ch07/Craps/RollingDie.java. | ||
| + | * List the methods of class RollingDie. | ||
| + | * How many arguments does each of those methods need? | ||
| + | * Where are each of those methods called in class RollingDie (0 to many places.) | ||
| + | * What is the name of the constructor method? Is there a coincidence involved? | ||
| + | * How many different methods of class Math are called? | ||
| + | |||
| + | All you need now are variables, objects, arguments and conditionals: then you shall understand most of the wiles of these demons in Java code. | ||
