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For the purpose of the text that follows, O.O.P. stands for Object Oriented Programming.
Question 9.1: Study, compile and run the following code:
class PersonDriver1 begin classVar String homersName = "Homer Simpson"; classVar int homersAge = 40; // Homer's age in years classVar String fredsName = "Fred Flintstone"; classVar int fredsAge = 45; // Fred's age in years classVar String darthsName = "Darth Vader"; classVar int darthsAge = 55; // Darth's age in years function void growHomer() begin homersAge = homersAge + 1; end function void growFred() begin fredsAge = fredsAge + 1; end function void growDarth() begin darthsAge = darthsAge + 1; end function void knightHomer() begin homersName = "Sir " + homersName; end function void knightFred() begin fredsName = "Sir " + fredsName; end function void knightDarth() begin darthsName = "Sir " + darthsName; end function void printHomer() begin System.out.println("I am " + homersName + ", my age is " + homersAge); end function void printFred() begin System.out.println("I am " + fredsName + ", my age is " + fredsAge); end function void printDarth() begin System.out.println("I am " + darthsName + ", my age is " + darthsAge); end beginMain growHomer(); knightHomer(); printHomer(); printFred(); printDarth(); endMain end
Question 9.2: By copying the pattern established in the existing code write a some new class variables to represent a new person called Barack Obama. Note that he was born in 1945 so at the time of writing this manual he is 67 years old.
Question 9.3: Then write some functions to work with this new person.
Question 9.4: Finally call those functions from the main function.
As your program gets large (say over 1000 lines) then it becomes no longer practical to put all of your code in the same class. So it is natural to put each piece of related code in its own class.
Question 9.5: Study, compile and run the following code: Each of these classes can be put in their own file. For each class X, this class can be put into a file called X.jtw. However for the purposes of this tutorial you will probably find it easier to merge all of the classes into the same file into a file called PersonDriver2.jtw
class Homer begin classVar String name = "Homer Simpson"; classVar int age = 40; // Homer's age in years function void grow() begin age = age + 1; end function void knight() begin name = "Sir " + name; end function void print() begin System.out.println("I am " + name + ", my age is " + age); end end class Fred begin classVar String name = "Fred Flintstone"; classVar int age = 45; // Fred's age in years function void grow() begin age = age + 1; end function void knight() begin name = "Sir " + name; end function void print() begin System.out.println("I am " + name + ", my age is " + age); end end class Darth begin classVar String name = "Darth Vader"; classVar int age = 55; // Darth's age in years function void grow() begin age = age + 1; end function void knight() begin name = "Sir " + name; end function void print() begin System.out.println("I am " + name + ", my age is " + age); end end class PersonDriver2 begin beginMain Homer.grow(); Fred.knight(); Homer.print(); Fred.print(); Darth.print(); endMain end
Question 9.6: By copying the pattern established in the existing code write a new class to represent Barack Obama.
Question 9.7: Call the functions from the main function of the driver class.
To allow for more than one object per class, most if not all class variables needs to be made into what are called instance variables (or more simply and more commonly known as properties) and most if not all functions need to be made into what are called methods.
Question 9.8: Study, compile and run the following code:
class Person begin // // NOTE: the use of the "property" keyword here instead of the "classVar" keyword // property String name; // Person's full name property int age; // Person's age in years // // NOTE: the use of the "method" keyword here instead of the "function" keyword // method void grow() begin age = age + 1; end method void knight() begin name = "Sir " + name; end method void print() begin System.out.println("I am " + name + ", my age is " + age); end beginMain var Person h = new Person(); h.name = "Homer Simpson"; h.age = 40; var Person f = new Person(); f.name = "Fred Flintstone"; f.age = 45; var Person d = new Person(); d.name = "Darth Vader"; d.age = 55; h.grow(); h.knight(); h.print(); f.print(); d.print(); endMain end
In the above code, note the use of three references h, f and
Question 9.9: By copying the pattern established in the existing code add some code to the main function add some code to create a new person for Barack Obama.
A common design pattern in Java and one that I present for you in the following code is to make all of the properties of a class effectively read-only to all client classes by making all of the properties private and providing non-private getter methods for getting the values of the properties. It is possible for the original class to change the values of the properties but other classes (such as PersonTest below) are not capable of doing this, without calling a method of the original class such the grow and knight methods of the Person class. Finally an additional thing known as a constructor is used to ensure that objects are initialized with meaningful values for their properties.
Question 9.10: Study, compile and run the following code:
class Person begin private property String name; private property int age; // Age in years // // NOTE: Getter methods // public method String getName() begin return name; end public method int getAge() begin return age; end public constructor Person(String aName, int anAge) begin this.name = aName; this.age = anAge; end public method void grow() begin age = age + 1; end public method void knight() begin name = "Sir " + name; end public method void print() begin System.out.println("I am " + name + ", my age is " + age); end end class PersonDriver3 begin beginMain // // NOTE: In the following constructor calls the age and name are set by the constructor // var Person h = new Person("Homer Simpson",40); var Person f = new Person("Fred Flintstone",45); var Person d = new Person("Darth Vader",55); h.grow(); h.knight(); h.print(); f.print(); d.print(); h.name = "Luke Skywalker"; // ERROR: name is private h.age = h.age + 1; // ERROR: age is private System.out.println("name=" + h.name); // ERROR: name is private System.out.println("age=" + h.age); // ERROR: age is private System.out.println("name=" + h.getName()); // OK: getter is non-private System.out.println("age=" + h.getAge()); // OK: getter is non-private endMain end
Question 9.11: By copying the pattern established in the existing code add some code to the main function add some code to create a new person called Barack Obama.
Question 9.12: Add a method unknight() which removes the "Sir " title if he has one. One trap for young players in J.T.W. or Java is to use the operator == to compare strings like so:
function boolean myCompare(String a, String b) begin return a == b; // Works but not as expected! end
It compiles without error, but doesn't give you the result you were expecting. Instead you need to use the equals method of the String class like so:
function boolean myCompare(String a, String b) begin return a.equals(b); end
More generally, if x and y are a references to objects, then x == y returns whether or not x and y are pointing to the same object, whereas x.equals(y) returns whether or not the contents of the objects referred to by x and y are equal. The meaning of the word contents varies from class to class, but in the case of strings it means that the strings contain the same data.
You will also find the String class' substring and (toUpperCase or toLowerCase) methods useful here too. See the String class of the java.lang package in the following link:
for more details of these two methods.
As soon as you learn how to use references you need to know that all reference variables could conceivably hold the value null, meaning no value. In particular when properties are themselves references as you will discover in Tutorial 11, then those properties are initialized to null by default. Object arrays that you will learn about in Tutorial 10 using the second of two initialization syntaxes are also initialized to null by default.
If x is a reference to a class X (including this) and if m is a method of X and p is a property of X, and if x is currently null, then the following lines result in a NullPointerException being thrown when executed:
x.p; x.m();
whereas if x is null then
prints out, respectively:
If x is not null, it calls
so these expressions are safer to use than any other method or property in situations where x might be null. The syntax of the toString method is as follows:
public method String toString() begin // Code goes here... end
Importantly for reasons which will be explained later the toString method must be declared with public visibility. For other properties and methods to be used safely with null references you need to wrap a conditional if construct around the calling of the method or property like so for properties:
if (x != null) then begin System.out.println(x.p); end
or like so for methods:
if (x != null) then begin System.out.println(x.m()); end
Therefore the toString method is more convenient than any other method or property. Note that its use is without the explicit call to the toString method and only used with a variable name, including this for the current class. Most of the time the this keyword is optional which is why novices don't bother to learn it, but in the case of the toString method it is essential, as can be seen in the following example code:
System.out.println("x.toString()=" + x); System.out.println("this.toString()=" + this);
Question 9.13: Change the print method above from a method that prints out to the screen to a method called toString that returns a string.
Question 9.14: Call the toString method instead of the print methods in the main function.
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