Quick Recap: Chapters 6 – 14

Chapter 6: Encapsulation

• Encapsulation hides implementation details behind an Interface or an API
• Encapsulated code has two features:
     o Instance variables are hidden (Usually with private modifier)
     o Getter and Setter methods are provided to give access to instance variables

Chapter 7: Inheritance

• Inheritance allows a class to be a subclass of a superclass and thereby inherit code and functionality of the superclass
• All classes are subclasses of Object and therefore they inherit Objects methods
• IS-A relationship is expressed with the keyword extends
• HAS-A means an instance of one class “has a” reference to an instance of another class
• Single, Multilevel, Multiple (Partial of course) and Hybrid are the types of inheritance

Chapter 8: Polymorphism

• Polymorphism means many forms
• A reference variable is always of a single, unchangeable type, but it can refer to a subtype object
• A single object can be referred to by reference variables of many different types, as long as they are the same type or a supertype of the object
• The reference variables type (not the objects type), determines which methods are called
• Polymorphic method invocations apply only to overridden instance methods

Overriding & Overloading:

• Methods can be both overridden and overloaded
• Constructors can only be overloaded
• Abstract methods must be overridden by the first concrete class (non abstract class)
• Overridden methods:
     o Must have the same argument list
     o Must have the same return type or a covariant return
     o Must not have a more restrictive access modifier
     o Must not throw new or broader checked exceptions
     o May throw fewer or narrower checked exceptions or any unchecked exception
• Final methods cannot be overridden
• Only inherited methods may be overridden (private methods are not inherited)
• You can use super.OverriddenMethodName() to call the super class’s version of the method
• Overloading means reusing a method name
• Overloaded methods
     o Must have different arguments list
     o May have different return types (But you cannot overload a method by just changing the return type)
     o May have different access modifiers
     o May throw different exceptions
     o Overloading is not Polymorphism
• Reference type determines which overloaded method will be used at compile time (In case you have overloaded a method that is available in the parent class)

Chapter 9: Reference Variable Casting

• There are two types of reference variable casting: downcasting and upcasting
• Downcasting: If you have a reference variable that refers to a subtype object, you can assign it to a reference variable of the subtype. You must make an explicit cast to do this, and the result is that you can access the subtype’s members with this new reference variable.
• Upcasting: You can assign a reference variable to a supertype reference variable explicitly or implicitly. This is an inherently safe operation because the assignment restricts the access capabilities of the new variable.


Chapter 10: Implementing an Interface

• When you implement an Interface, you are fulfilling its contract
• If you implement an interface you must provide concrete overriding for all methods defined in the interface
• If you don’t provide implementation for all methods, you must mark your class Abstract
• A single class can implement many interfaces


Chapter 11: Return Types

• Overloaded methods can change return types
• Overridden methods cannot change return types (Except in case of covariant returns)
• Object reference return types can accept null as a return value
• An array is a perfectly legal return type, both to declare and to return as a value
• For methods with primitive return types, any value that can be implicitly converted to the return type can be returned
• Nothing can be returned from a method that has the void modifier
• You can use the return keyword to get out of a method early but you cannot return anything from a void method and you cannot use an empty return in a non-void method
• Methods with an object reference return type, can return a subtype
• Methods with an interface return type, can return any implementer

Chapter 12: Constructors and Instantiation

• A Constructor is always invoked when a new object of a class is created
• Each superclass in an objects inheritance tree will have a constructor called
• Every class (Even an abstract class) has atleast one constructor
• Even if you don’t write a constructor explicitly, the compiler will add a default no-arg constructor to your class
• If you write a constructor that takes arguments, the compiler will not place a no-arg constructor in your class
• Constructors must have the same name as the class (Case Sensitive)
• Constructors don’t have a return type (If you see a return type, then it’s a method and not a constructor)
• Constructors can use any access modifier including private
• The default constructor is always the no-arg constructor
• The first statement of every constructor must be a call to either this() or super()
• You cannot have both this() and super() in the same constructor
• Constructors are never inherited, so they cannot be overridden
• A constructor can be directly invoked only by another constructor
• You cannot call a constructor explicitly

Chapter 13: Statics

• Use static methods to implement behavior that will not be affected by the state of any instances
• Use static variables to hold data that is class specific. There will be only one copy of a static variable irrespective of how many instances you make of a class
• All static members of a class belong to the class and not any instance
• A static method cannot access instance variables
• Static methods cannot be overridden

Chapter 14: Coupling and Cohesion

• Coupling refers to the degree to which one class knows about another class
• Loose coupling is a desirable state of having classes that are well encapsulated, minimize references to one another
• Tight coupling is undesirable
• Cohesion refers to the degree in which a class has a single, well defined role or responsibility
• High cohesion is a desirable state and low cohesion is undesirable


Previous Chapter: Chapter 14: Coupling and Cohesion

Next Chapter: Self Test Chapters 6 to 14

Chapter 14: Coupling and Cohesion

You would have heard or learnt a lot about coupling and cohesion when you learnt object oriented concepts or while learning other programming languages like c++. Let me tell you up front that, this chapter is going to be from the SCJP exam perspective and is going to cover concepts related to these two topics only from the exam point of you and not the overall dig deep into the topics. Frankly speaking, you’ll have very few questions about coupling and cohesion on the real exam.

Lets get started.

These two topics, coupling and cohesion, have to do with the quality of an OO design. In general, good OO design calls for loose coupling and shuns tight coupling, and good OO design calls for high cohesion, and shuns low cohesion. As with most OO design discussions, the goals for an application are

• Ease of creation
• Ease of maintenance
• Ease of enhancement

Coupling

Coupling is the degree to which one class knows about another class. If the only knowledge that class A has about class B, is what class B has exposed through its interface, then class A and class B are said to be loosely coupled. If, on the other hand, class A relies on parts of class B that are not part of class B’s interface, then the coupling between the classes is tighter. In other words, if A knows more than it should about the way in which B was implemented, then A and B are tightly coupled.

Using this second scenario, imagine what happens when class B is enhanced. It’s quite possible that the developer enhancing class B has no knowledge of class A, (why would he/she?) Class B’s developer ought to feel that any enhancements that don’t break the class’s interface should be safe, so she might change some non-interface parts of the class, which then causes class A to break.

At the far end of the coupling spectrum is the horrible situation in which class A knows non-API stuff about class B, and class B knows non-API stuff about class A. (This is REALLY BAD CODING). If either class is ever changed, there’s a chance that the other class will break. Let’s look at an obvious example of tight coupling, which has been enabled by poor encapsulation:

class CalculateTaxes {
float rate;
float doIndia() {
TaxRatesInIndia str = new TaxRatesInIndia();
rate = str.salesRate; // ouch
// this should be a method call:
// rate = str.getSalesTaxRates("CO");
// do stuff with rate
}
}

class TaxRatesInIndia {
public float salesRate; // should be private
public float adjustedSalesRate; // should be private

public float getSalesTaxRates(String region) {
salesRate = new CalculateTaxes().doIndia(); // ouch again!
// do country-based calculations
return adjustedSalesRate;
}
}

All large OO applications are a mix of many classes and interfaces working together. Ideally, all interactions between objects in an OO system should use the APIs, in other words, the contracts, of the objects’ respective classes. Theoretically, if all of the classes in an application have well-designed APIs, then it should be possible for all interclass interactions to use those APIs exclusively. If you make changes to the way one class behaves, in a loosely coupled environment, you shouldn't be getting surprise errors in other classes. As we discussed earlier in this chapter, an aspect of good class and API design is that classes should be well encapsulated.

The point here is that coupling is a somewhat subjective concept. Because of this, the SCJP exam will test you on really obvious examples of tight coupling only. So don't worry much about having to make design decisions about code.

Cohesion

While coupling has to do with how classes interact with each other, cohesion is all about how a single class is designed. The term cohesion is used to indicate the degree to which a class has a single, well-focused purpose. Keep in mind that cohesion too is a subjective concept. The more focused a class is, the higher its cohesiveness. The key benefit of high cohesion is that such classes are typically much easier to maintain (and less frequently changed) than classes with low cohesion. Another benefit of high cohesion is that classes with a well-focused purpose tend to be more reusable than other classes. Let’s take a look at a pseudocode example:

class SalesReport {
void connectToDb(){ }
void generateSalesReport() { }
void saveAsFile() { }
void print() { }
}

Now imagine your manager comes along and says, “Hey you know that accounting application we’re working on? The clients just decided that they’re also going to want to generate a revenue projection report, oh and they want to do some inventory reporting also. They do like our reporting features however, so make sure that all of these reports will let them choose a database, choose a printer, and save generated reports to data files...”

Rather than putting all the printing code into one report class, we probably would have been better off with the following design right from the start:

class SalesReport {
Options getReportingOptions() { }
void generateSalesReport(Options o) { }
}

class ConnectToDb {
DBconnection getDb() { }
}

class PrintStuff {
PrintOptions getPrintOptions() { }
}

class FileSaver {
SaveOptions getFileSaveOptions() { }
}

This design is much more cohesive. Instead of one class that does everything, we’ve broken the system into four main classes, each with a very specific, or cohesive, role. Because we’ve built these specialized, reusable classes, it’ll be much easier to write a new report, since we’ve already got the database connection class, the printing class, and the file saver class, and that means they can be reused by other classes that might want to print a report. Again, as in Coupling, you may not get too many questions about cohesion but if you are (un)lucky you may get one or two…

Previous Chapter: Chapter 13: Statics

Next Chapter: Quick Review: Chapters 6 to 14