Week 4 [Sep 2] - Topics

The OOP concept Inheritance allows you to define a new class based on an existing class.

• Other names for Base class: Parent class, Super class
• Other names for Derived class: Child class, Sub class, Extended class

A superclass is said to be more general than the subclass. Conversely, a subclass is said to be more specialized than the superclass.

Applying inheritance on a group of similar classes can result in the common parts among classes being extracted into more general classes.

Inheritance implies the derived class can be considered as a sub-type of the base class (and the base class is a super-type of the derived class), resulting in an is a relationship.

Inheritance does not necessarily mean a sub-type relationship exists. However, the two often go hand-in-hand. For simplicity, at this point let us assume inheritance implies a sub-type relationship.

Inheritance relationships through a chain of classes can result in inheritance hierarchies (aka inheritance trees).

Two inheritance hierarchies/trees are given below. Note that the triangle points to the parent class. Observe how the Parrot is a Bird as well as it is an Animal.

Multiple Inheritance is when a class inherits directly from multiple classes. Multiple inheritance among classes is allowed in some languages (e.g., Python, C++) but not in other languages (e.g., Java, C#).

The Honey class inherits from the Food class and the Medicine class because honey can be consumed as a food as well as a medicine (in some oriental medicine practices). Similarly, a Car is an Vehicle, an Asset and a Liability.

Method overloading is when there are multiple methods with the same name but different type signatures. Overloading is used to indicate that multiple operations do similar things but take different parameters.

Method overriding is when a sub-class changes the behavior inherited from the parent class by re-implementing the method. Overridden methods have the same name, same type signature, and same return type.

Given below is an extract from the -- Java Tutorial, with slight adaptations.

public class Bicycle {

    public int gear;
    public int speed;

    public Bicycle(int startSpeed, int startGear) {
        gear = startGear;
        speed = startSpeed;
    }

    public void setGear(int newValue) {
        gear = newValue;
    }

    public void applyBrake(int decrement) {
        speed -= decrement;
    }

    public void speedUp(int increment) {
        speed += increment;
    }

}

public class MountainBike extends Bicycle {

    // the MountainBike subclass adds one field
    public int seatHeight;

    // the MountainBike subclass has one constructor
    public MountainBike(int startHeight, int startSpeed, int startGear) {
        super(startSpeed, startGear);
        seatHeight = startHeight;
    }

    // the MountainBike subclass adds one method
    public void setHeight(int newValue) {
        seatHeight = newValue;
    }
}

public class Superclass {

    public void printMethod() {
        System.out.println("Printed in Superclass.");
    }
}

public class Subclass extends Superclass {

    // overrides printMethod in Superclass
    public void printMethod() {
        super.printMethod();
        System.out.println("Printed in Subclass");
    }
    public static void main(String[] args) {
        Subclass s = new Subclass();
        s.printMethod();
    }
}

Printed in Superclass.
Printed in Subclass

public MountainBike(int startHeight, int startSpeed, int startGear) {
    super(startSpeed, startGear);
    seatHeight = startHeight;
}

As you know, all Java objects inherit from the Object class. Let us look at some of the useful methods in the Object class that can be used by other classes.

The toString method

class Time {
    int hours;
    int minutes;
    int seconds;

    Time(int hours, int minutes, int seconds) {
        this.hours = hours;
        this.minutes = minutes;
        this.seconds = seconds;
    }
}

 Time t = new Time(5, 20, 13);
 System.out.println(t);

class Time{

   //...

   @Override
   public String toString() {
       return String.format("%02d:%02d:%02d\n", this.hours, this.minutes, this.seconds);
   }
}

 Time t = new Time(5, 20, 13);
 System.out.println(t);

The equals method

Time time1 = new Time(9, 30, 0);
Time time2 = time1;
Time time3 = new Time(9, 30, 0);

public class Time {
    int hours;
    int minutes;
    int seconds;

    // ...

    @Override
    public boolean equals(Object o) {
        Time other = (Time) o;
        return this.hours == other.hours
                && this.minutes == other.minutes
                && this.seconds == other.seconds;
    }
}

Time t1 = new Time(5, 20, 13);
Time t2 = new Time(5, 20, 13);
System.out.println(t1 == t2);
System.out.println(t1.equals(t2));

false
true

Polymorphism allows you to write code targeting superclass objects, use that code on subclass objects, and achieve possibly different results based on the actual class of the object.

Polymorphism literally means "ability to take many forms".

Every instance of a subclass is an instance of the superclass, but not vice-versa. As a result, inheritance allows substitutability : the ability to substitute a child class object where a parent class object is expected.

an Academic is an instance of a Staff, but a Staff is not necessarily an instance of an Academic. i.e. wherever an object of the superclass is expected, it can be substituted by an object of any of its subclasses.

The following code is valid because an AcademicStaff object is substitutable as a Staff object.

Staff staff = new AcademicStaff (); // OK

But the following code is not valid because staff is declared as a Staff type and therefore its value may or may not be of type AcademicStaff, which is the type expected by variable academicStaff.

Staff staff;
...
AcademicStaff academicStaff = staff; // Not OK

Overridden methods are resolved using dynamic binding, and therefore resolves to the implementation in the actual type of the object.

Paradigms → OOP → Inheritance →

Overriding

Method overriding is when a sub-class changes the behavior inherited from the parent class by re-implementing the method. Overridden methods have the same name, same type signature, and same return type.

Consider the following case of EvaluationReport class inheriting the Report class:

Report methods	EvaluationReport methods	Overrides?
print()	print()	Yes
write(String)	write(String)	Yes
read():String	read(int):String	No. Reason: the two methods have different signatures; This is a case of overloading (rather than overriding).

Paradigms → OOP → Inheritance →

Overloading

Method overloading is when there are multiple methods with the same name but different type signatures. Overloading is used to indicate that multiple operations do similar things but take different parameters.

Type Signature: The type signature of an operation is the type sequence of the parameters. The return type and parameter names are not part of the type signature. However, the parameter order is significant.

Example:

Method	Type Signature
int add(int X, int Y)	(int, int)
void add(int A, int B)	(int, int)
void m(int X, double Y)	(int, double)
void m(double X, int Y)	(double, int)

In the case below, the calculate method is overloaded because the two methods have the same name but different type signatures (String) and (int)

• calculate(String): void
• calculate(int): void

Which of these methods override another method? A is the parent class. B inherits A.

• a
• b
• c
• d
• e

d

Explanation: Method overriding requires a method in a child class to use the same method name and same parameter sequence used by one of its ancestors

void adjustSalary(int byPercent) {
    for (Staff s: staff) {
        s.adjustSalary(byPercent);
    }
}

In contrast, overloaded methods are resolved using static binding.

class Account {

    Account () {
        // Signature: ()
        ...
    }

    Account (String name, String number, double balance) {
        // Signature: (String, String, double)
        ...
    }
}

void calculateGrade (int[] averages) { ... }
void calculateGrade (String matric) { ... }

Three concepts combine to achieve polymorphism: substitutability, operation overriding, and dynamic binding.

• Substitutability: Because of substitutability, you can write code that expects object of a parent class and yet use that code with objects of child classes. That is how polymorphism is able to treat objects of different types as one type.
• Overriding: To get polymorphic behavior from an operation, the operation in the superclass needs to be overridden in each of the subclasses. That is how overriding allows objects of different subclasses to display different behaviors in response to the same method call.
• Dynamic binding: Calls to overridden methods are bound to the implementation of the actual object's class dynamically during the runtime. That is how the polymorphic code can call the method of the parent class and yet execute the implementation of the child class.

Java is a strongly-typed language which means the code works with only the object types that it targets.

public class PetShelter {
    private static Cat[] cats = new Cat[]{
            new Cat("Mittens"),
            new Cat("Snowball")};

    public static void main(String[] args) {
        for (Cat c: cats){
            System.out.println(c.speak());
        }
    }
}

Mittens: Meow
Snowball: Meow

public class Cat {
    public Cat(String name) {
        super(name);
    }

    public String speak() {
        return name + ": Meow";
    }
}

This strong-typing can lead to unnecessary verbosity caused by repetitive similar code that do similar things with different object types.

public class PetShelter {
    private static Cat[] cats = new Cat[]{
            new Cat("Mittens"),
            new Cat("Snowball")};
    private static Dog[] dogs = new Dog[]{
            new Dog("Spot")};

    public static void main(String[] args) {
        for (Cat c: cats){
            System.out.println(c.speak());
        }
        for(Dog d: dogs){
            System.out.println(d.speak());
        }
    }
}

Mittens: Meow
Snowball: Meow
Spot: Woof

public class Dog {
    public Dog(String name) {
        super(name);
    }

    public String speak() {
        return name + ": Woof";
    }
}

A better way is to take advantage of polymorphism to write code that targets a superclass so that it works with any subclass objects.

public class PetShelter2 {
    private static Animal[] animals = new Animal[]{
            new Cat("Mittens"),
            new Cat("Snowball"),
            new Dog("Spot")};

    public static void main(String[] args) {
        for (Animal a: animals){
            System.out.println(a.speak());
        }
    }
}

Mittens: Meow
Snowball: Meow
Spot: Woof

public class Animal {

    protected String name;

    public Animal(String name){
        this.name = name;
    }
    public String speak(){
        return name;
    }
}

public class Cat extends Animal {
    public Cat(String name) {
        super(name);
    }

    @Override
    public String speak() {
        return name + ": Meow";
    }
}

public class Dog extends Animal {
    public Dog(String name) {
        super(name);
    }

    @Override
    public String speak() {
        return name + ": Woof";
    }
}

Polymorphic code is better in several ways:

• It is shorter.
• It is simpler.
• It is more flexible (in the above example, the main method will work even if we add more animal types).

When testing, we execute a set of test cases. A test case specifies how to perform a test. At a minimum, it specifies the input to the software under test (SUT) and the expected behavior.

Test cases can be determined based on the specification, reviewing similar existing systems, or comparing to the past behavior of the SUT.

For each test case we do the following:

1. Feed the input to the SUT
2. Observe the actual output
3. Compare actual output with the expected output

A test case failure is a mismatch between the expected behavior and the actual behavior. A failure indicates a potential defect (or a bug), unless the error is in the test case itself.

When we modify a system, the modification may result in some unintended and undesirable effects on the system. Such an effect is called a regression.

Regression testing is the re-testing of the software to detect regressions. Note that to detect regressions, we need to retest all related components, even if they had been tested before.

Regression testing is more effective when it is done frequently, after each small change. However, doing so can be prohibitively expensive if testing is done manually. Hence, regression testing is more practical when it is automated.

A simple way to semi-automate testing of a CLI(Command Line Interface) app is by using input/output re-direction.

• First, we feed the app with a sequence of test inputs that is stored in a file while redirecting the output to another file.
• Next, we compare the actual output file with another file containing the expected output.

Let us assume we are testing a CLI app called AddressBook. Here are the detailed steps:

1. Store the test input in the text file input.txt.

   add Valid Name p/12345 valid@email.butNoPrefix
   add Valid Name 12345 e/valid@email.butPhonePrefixMissing
   

2. Store the output we expect from the SUT in another text file expected.txt.

   Command: || [add Valid Name p/12345 valid@email.butNoPrefix]
   Invalid command format: add 
   
   Command: || [add Valid Name 12345 e/valid@email.butPhonePrefixMissing]
   Invalid command format: add 
   

3. Run the program as given below, which will redirect the text in input.txt as the input to AddressBook and similarly, will redirect the output of AddressBook to a text file output.txt. Note that this does not require any code changes to AddressBook.

   java AddressBook < input.txt > output.txt
   

   • The way to run a CLI program differs based on the language.
     e.g., In Python, assuming the code is in AddressBook.py file, use the command
     python AddressBook.py < input.txt > output.txt

   • If you are using Windows, use a normal command window to run the app, not a Power Shell window.

   More on the > operator and the < operator extra

   A CLI program takes input from the keyboard and outputs to the console. That is because those two are default input and output streams, respectively. But you can change that behavior using < and > operators. For example, if you run AddressBook in a command window, the output will be shown in the console, but if you run it like this,

   java AddressBook > output.txt 
   

   the Operating System then creates a file output.txt and stores the output in that file instead of displaying it in the console. No file I/O coding is required. Similarly, adding < input.txt (or any other filename) makes the OS redirect the contents of the file as input to the program, as if the user typed the content of the file one line at a time.

   Resources:

   • Using command redirection operators in Windows

4. Next, we compare output.txt with the expected.txt. This can be done using a utility such as Windows FC (i.e. File Compare) command, Unix diff command, or a GUI tool such as WinMerge.

   FC output.txt expected.txt
   

Note that the above technique is only suitable when testing CLI apps, and only if the exact output can be predetermined. If the output varies from one run to the other (e.g. it contains a time stamp), this technique will not work. In those cases we need more sophisticated ways of automating tests.

Debugging is the process of discovering defects in the program. Here are some approaches to debugging:

• Bad -- By inserting temporary print statements: This is an ad-hoc approach in which print statements are inserted in the program to print information relevant to debugging, such as variable values. e.g. Exiting process() method, x is 5.347. This approach is not recommended due to these reasons.
  • Incurs extra effort when inserting and removing the print statements.
  • These extraneous program modifications increases the risk of introducing errors into the program.
  • These print statements, if not removed promptly after the debugging, may even appear unexpectedly in the production version.
• Bad -- By manually tracing through the code: Otherwise known as ‘eye-balling’, this approach doesn't have the cons of the previous approach, but it too is not recommended (other than as a 'quick try') due to these reasons:
  • It is difficult, time consuming, and error-prone technique.
  • If you didn't spot the error while writing code, you might not spot the error when reading code too.
• Good -- Using a debugger: A debugger tool allows you to pause the execution, then step through one statement at a time while examining the internal state if necessary. Most IDEs come with an inbuilt debugger. This is the recommended approach for debugging.

• Intellij IDEA Documentation: Debugging Basics - Can be used as a reference document when you want to recall how to use a debugging feature.