interface WeatherFetcher {
String fetchWeather(String cityCode) throws IOException;
int fetchTemperature(String cityCode) throws IOException;
}Entries tagged [software-design]
The Singleton Pattern in Java
TweetPosted on Tuesday Sep 22, 2020 at 12:47PM in Technology
In this entry, I’ll introduce some common ways to apply the Singleton Pattern in Java and a problem that can harm maintainability of your code base and a solution for the problem.
What is it?
The Singleton Pattern is a commonly used idiom to create and maintain objects called singletons. A singleton is the only instance of a class.
Common use cases are an object which keeps the user’s preferences in a desktop application, cache or a resource pool. One example would be a TCP connection pool where there are multiple active connections to a backend service like a database server or a web service. Typically there should be only one pool object which maintains all of the connections. Having multiple separate pool objects in one application doesn’t make much sense in most cases because it will make the use of the pool less effective.
As an example, let’s imagine that there is the following interface which allows us to fetch the weather data from some external web service:
There are a couple of methods to fetch the weather (atmospheric conditions) and the temperature of a city. Since it involves network communcation, there can be an IOException.
Now imagine that you are working on implementing a feature which uses this interface. You are told that the performance requirement for this feature is strict, therefore you have decided to maintain a pool of active TCP connections to the external web service in order to keep the latency low. Applying the Singleton Pattern here sounds like a good idea because it will make sure that you will have only one pool object in your application.
Class with a private constructor
Let’s see how we can implement this with the Singleton Pattern. One simple implementation would be something like this:
public class SimpleSingleton implements WeatherFetcher {
private static final SimpleSingleton INSTANCE = new SimpleSingleton();
public static WeatherFetcher getInstance() { return INSTANCE; }
private SimpleSingleton() { System.out.println("Populating the connection pool..."); }
@Override
public int fetchTemperature(String cityCode) {
// gets an idle connection from the pool, sends a request
return 71; // please assume it's from a response
}
@Override
public String fetchWeather(String cityCode) {
// gets an idle connection from the pool, sends a request
return "Sunny"; // please assume it's from a response
}
// client code
public static void main(String[] args) throws IOException {
WeatherFetcher singleton = SimpleSingleton.getInstance();
System.out.printf("Weather in New York: %s, %dF\n",
singleton.fetchWeather("NewYork"), singleton.fetchTemperature("NewYork"));
}
}The class has a static final field called INSTANCE, where the only instance of this class is kept. There is a simple getter for that.
This class has a constructor populating the pool. Let’s imagine that it opens a lot of connections to an external web service and makes those ready to use. The constructor is private, which prevents creation of another instance of the class. Without this, any other class can create an instance of this class, which means that there is no guarantee that there is only one instance of this class. It also prevents inheritance because there is no constructor a subclass can call.
In order to get the singleton, the client needs to call the static method getInstance() first. After that, the client can fetch the weather data via the methods defined in the WeatherFetcher interface.
Class with a static holder class for lazy initialization
The SimpleSingleton class works fine, but there is one common requirement you might encounter. Populating the connection pool can be expensive and there might be a situation where the feature we just implemented will be used by only a very small number of your users, and you want to make sure that the connection pool gets populated only when it’s really needed.
We cannot guarantee that with the SimpleSingleton class. Let’s add a System.out.println() call to the main method and see what is happening there:
public static void main(String[] args) throws IOException {
System.out.println("Doing some other stuff - I don't need the singleton yet"); // added
WeatherFetcher singleton = SimpleSingleton.getInstance();
System.out.printf("Weather in New York: %s, %dF\n",
singleton.fetchWeather("NewYork"), singleton.fetchTemperature("NewYork"));
}The main method yields the following output:
Populating the connection pool...
Doing some other stuff - I don't need the singleton yet
Weather in New York: Sunny, 71FAs you can see, the pool has got initialized even though it’s not needed yet; the constructor gets executed before the getInstance() method is called because the constructor invocation is written in the static initializer of the class. It can lead to a situation where unnecessary resource consumption is imposed on the user even though the user might not need the feature. We can work around this with having a simple private static holder class like the following:
public class LazySingleton implements WeatherFetcher {
private static class SingletonHolder {
static final LazySingleton INSTANCE = new LazySingleton();
}
public static WeatherFetcher getInstance() { return SingletonHolder.INSTANCE; }
private LazySingleton() { System.out.println("Populating the connection pool..."); }
...
// client code
public static void main(String[] args) throws IOException {
System.out.println("Doing some other stuff - I don't need the singleton yet");
WeatherFetcher singleton = LazySingleton.getInstance();
System.out.printf("Weather in New York: %s, %dF\n",
singleton.fetchWeather("NewYork"), singleton.fetchTemperature("NewYork"));
}
}The LazySingleton singleton instance is kept in the static field in the static inner class called SingletonHolder and the getInstance() method returns that. Executing the main method will show the following output, as expected:
Doing some other stuff - I don't need the singleton yet
Populating the connection pool...
Weather in New York: Sunny, 71FA problem that can harm maintainability: getInstance() everywhere
Let’s imagine that now you need to implement some code for generating a report about the weather of some cities with the singleton we just wrote. It might be implemented like this:
public class WeatherReporter {
String generateWeatherReport(List<String> cityCodeList) {
StringBuilder sb = new StringBuilder("=== Weather Report ===\n");
for (String cityCode : cityCodeList) {
try {
String weather = LazySingleton.getInstance().fetchWeather(cityCode);
int temperature = LazySingleton.getInstance().fetchTemperature(cityCode);
sb.append(String.format("%s: %s, %dF\n", cityCode, weather, temperature));
} catch (IOException e) {
sb.append(String.format("%s: Failed to fetch data\n", cityCode));
}
}
return sb.toString();
}
public static void main(String[] args) {
WeatherReporter reporter = new WeatherReporter();
System.out.println(reporter.generateWeatherReport(
Arrays.asList("NewYork", "Montreal", "Tokyo")));
}
}It receives a List of city code which we need to create the report for. Then it constructs the header, iterates the list of city code and fetches the weather and the temperature for each city and builds the String which represents the report. If there is an I/O problem, we just mention that it failed to fetch the data for the city.
Executing the main method should produce the following output:
Populating the connection pool...
=== Weather Report ===
NewYork: Sunny, 71F
Montreal: Cloudy, 68F
Tokyo: Rainy, 69FThat’s a fairly complicated piece of code. Therefore, having some unit tests for the WeatherReporter class would be nice, but unfortunately it’s almost impossible because the current implementation is hard-coded to directly call the static getInstance() method to get the WeatherFetcher object, which means it always sends requests to the real external web service.
We cannot make a reliable assertion if it depends on the output of the real external service that way. One option would be running a fake of the external service, but it would require a lot of effort. And it would be nice if we could write a test case for IOException handling, but reproducing IOException in a real environment is also too much work. And the connection pool population might take a lot of time. The LazySingleton class might be hard-coded to create thousands of TCP connections and in that case it might take a few tens of seconds to populate. Waiting for such a long time with every unit test execution doesn’t sound reasonable.
So, the problem here is that we have business logic tightly coupled to an external data source. Having that kind of a method call in the middle of business logic makes writing unit tests very difficult or almost impossible because in order to make that kind of thing work, in many cases it requires some sort of specific setup or configuration and oftentimes that is too much work.
One thing we can try to work around this problem is replacing the static getInstance() method call by a functional interface. In our case, we can do something like this:
public class ImprovedWeatherReporter {
private final Supplier<? extends WeatherFetcher> weatherFetcherSupplier;
public ImprovedWeatherReporter(Supplier<? extends WeatherFetcher> weatherFetcherSupplier) {
this.weatherFetcherSupplier = weatherFetcherSupplier;
}
String generateWeatherReport(List<String> cityCodeList) {
StringBuilder sb = new StringBuilder("=== Weather Report ===\n");
for (String cityCode : cityCodeList) {
try {
String weather = weatherFetcherSupplier.get().fetchWeather(cityCode);
int temperature = weatherFetcherSupplier.get().fetchTemperature(cityCode);
sb.append(String.format("%s: %s, %dF\n", cityCode, weather, temperature));
} catch (IOException e) {
sb.append(String.format("%s: Failed to fetch data\n", cityCode));
}
}
return sb.toString();
}
public static void main(String[] args) {
ImprovedWeatherReporter reporter = new ImprovedWeatherReporter(LazySingleton::getInstance);
System.out.println(reporter.generateWeatherReport(
Arrays.asList("NewYork", "Montreal", "Tokyo")));
}
}The ImprovedWeatherReporter class is not tightly coupled to the getInstance() method anymore. Instead, now it has a field for a Supplier object which returns a WeatherFetcher object. The client of this class can inject the Supplier via its constructor. In our case, we can inject LazySingleton.getInstance() for the production use.
That seems to be not much change, but it is a great improvement from the perspective of unit testing. Now we can inject any Supplier returning a WeatherFeather object, and it enables us to write reliable, fast and stable unit tests like the following:
@ExtendWith(MockitoExtension.class)
class ImprovedWeatherReporterTest {
@Mock
WeatherFetcher weatherFetcher;
ImprovedWeatherReporter sut;
@BeforeEach
void setUp() {
sut = new ImprovedWeatherReporter(() -> weatherFetcher);
}
@Test
void producesReports() throws IOException {
when(weatherFetcher.fetchTemperature("NewYork")).thenReturn(71);
when(weatherFetcher.fetchWeather("NewYork")).thenReturn("Sunny");
when(weatherFetcher.fetchTemperature("Montreal")).thenReturn(68);
when(weatherFetcher.fetchWeather("Montreal")).thenReturn("Cloudy");
String actual = sut.generateWeatherReport(Arrays.asList("NewYork", "Montreal"));
assertThat(actual).isEqualTo("=== Weather Report ===\n" +
"NewYork: Sunny, 71F\n" +
"Montreal: Cloudy, 68F\n");
}
@Test
void exception() throws IOException {
when(weatherFetcher.fetchTemperature("Tokyo")).thenThrow(new IOException("catch this"));
String actual = sut.generateWeatherReport(Collections.singletonList("Tokyo"));
assertThat(actual).isEqualTo("=== Weather Report ===\n" +
"Tokyo: Failed to fetch data\n");
}
}First, we create a mock of WeatherFetcher and inject a Supplier which returns the mock. With that, we can have complete control of the WeatherFetcher object the ImprovedWeatherReporter class relies on. Now we can specify what the WeatherFetcher object returns for what parameter with the mock framework you use. It will even enable us to test a case where the IOException is thrown, which was impossible with the tightly-coupled version of the WeatherReporter class. Just having a simple abstraction layer between your business logic and an external service makes writing unit tests much easier.
Another solution you might want to check is using a dependency injection framework to maintain singletons. You can easily make an object singleton and let the framework inject the singleton to the objects the framework maintains. It will reduce a lot of boilerplates, and some of the code I introduced here will be unnecessary. Especially if your code base has a dependency injection framework already, I recommend checking the documentation of your framework. For Google Guice, check this out: https://github.com/google/guice/wiki/Scopes
Conclusion
We’ve seen a couple of common idioms that can be used to apply the Singleton Pattern in Java, and discussed a common maintainability issue that is sometimes caused by applying the pattern. When you see code where there are a lot of getInstance() calls in the middle of business logic, it might be good to take some time to see if you would be able to run your business logic in isolation for unit tests. If your code was unit test friendly, your code would be likely to have wider coverage of tests and it would make future maintenance easier and the code base more stable.
Favor composition over inheritance
TweetPosted on Friday Aug 14, 2020 at 04:52PM in Technology
In this entry, I’ll discuss problems of inheritance, which is often overused in software written in an object oriented programming language, and how we can do better with composition, which is usually a much better alternative to inheritance.
Inheritance can make your code fragile
Let’s think about some piece of software used by some cafe. It contains the following classes:
There is an interface called Beverage, which can return the price and description of a beverage. Most probably there are Coffee or Tea classes that implement the interface. And there is a class called Order, where you can add Beverage objects to it in order to calculate the grand total on the bill for a customer of the cafe. The code of these looks like this:
interface Beverage {
BigDecimal price();
String description();
}
class Order {
private static final BigDecimal TAX_RATE = new BigDecimal("0.1");
private BigDecimal subTotal = BigDecimal.ZERO;
void add(Beverage beverage) {
subTotal = subTotal.add(beverage.price());
}
void addAll(Collection<? extends Beverage> beverages) {
for (Beverage beverage : beverages)
subTotal = subTotal.add(beverage.price());
}
BigDecimal grandTotal() {
BigDecimal tax = subTotal.multiply(TAX_RATE);
return subTotal.add(tax);
}
}Now, let’s consider the following scenario: the owner of the cafe wants to start a campaign to boost their sales. The idea of the campaign is that if a customer orders more than 2 beverages at the same time, he/she will get 20% discount from the grand total. In order to implement this requirement, a programmer comes up with the CampaignOrder class which extends the Order class. It looks like the following:
class CampaignOrder extends Order {
private static final BigDecimal DISCOUNT_RATE = new BigDecimal("0.2");
private int numberOfBeverages;
@Override
void add(Beverage beverage) {
super.add(beverage);
numberOfBeverages++;
}
@Override
void addAll(Collection<? extends Beverage> beverages) {
super.addAll(beverages);
numberOfBeverages += beverages.size();
}
@Override
BigDecimal grandTotal() {
BigDecimal grandTotal = super.grandTotal();
if (numberOfBeverages > 2) {
BigDecimal discount = grandTotal.multiply(DISCOUNT_RATE);
grandTotal = grandTotal.subtract(discount);
}
return grandTotal;
}
}It captures add() and addAll() method calls, forwards them to its superclass and keeps track of the number of the beverages which are added in the numberOfBeverages variable. And it also captures grandTotal() method calls, forwards them to its superclass and applies the 20% discount to the grand total the superclass calculated depending on the numberOfBeverages variable.
It might look reasonable as it reuses the Order class effectively so that it won’t introduce any duplicate code. But this approach can lead to an unforseen issue due to the fact that the CampaignOrder class relies on a hidden behavior of the Order class, which is that the add() method and the addAll() method work independently. Consider that at some point some other programmer has done some quick refactoring in the addAll() method:
class Order {
...
void add(Beverage beverage) {
subTotal = subTotal.add(beverage.price());
}
void addAll(Collection<? extends Beverage> beverages) {
for (Beverage beverage : beverages)
// Someone has done refactoring. Original code was:
// subTotal = subTotal.add(beverage.price());
// Now:
add(beverage);
}
...
}It hasn’t broken anything in terms of the functionality of the Order class but unfortunately it has just broken the CampaignOrder class. Remember the implementation of the CampaignOrder class which captures both of the add() and the addAll() method calls and counts the number of the beverage objects it receives. Due to the fact that now the Order class calls the add() method from the addAll() method, whenever the addAll() method of the CampaignOrder class gets called, the number of the beverage objects gets counted twice in the CampaignOrder class. In other words, now the following test case fails:
@ExtendWith(MockitoExtension.class)
class CampaignOrderTest {
@Mock
Beverage coffee, tea;
CampaignOrder sut = new CampaignOrder();
@Test
void addAll() {
when(coffee.price()).thenReturn(new BigDecimal("2.0"));
when(tea.price()).thenReturn(new BigDecimal("3.0"));
sut.addAll(Arrays.asList(coffee, tea));
// It fails after the refactoring. Now grandTotal() returns 4.4
// The discount is applied unexpectedly since the logic which counts beverages is broken
// Now it's counted as 4, which is over the threshold of the discount
assertThat(sut.grandTotal()).isEqualByComparingTo("5.5");
}
}The person who has done this refactoring should not be blamed. In fact this person removed a duplicate piece of code, which is a good thing, and it’s not easy to catch such an error. The real problem here is the wrong use of inheritance, which is writing a subclass that relies on an implementation detail of its super class. That introduces fragility to the codebase. Hence using inheritance this way should be avoided.
And also there can be another problematic case where a new method has been added to the Order class. If the new method can be used for adding a beverage, we need to make sure that the CampaignOrder class captures method calls to the new method, but chances are we would not even notice that there was a subclass which we might have to change.
What could have been done instead of inheritance then? The most obvious approach is using composition instead. Let’s rework the class hierarchy and make it like this:
The implementation:
interface Order {
void add(Beverage beverage);
void addAll(Collection<? extends Beverage> beverages);
BigDecimal grandTotal();
}
class RegularOrder implements Order {
private static final BigDecimal TAX_RATE = new BigDecimal("0.1");
private BigDecimal subTotal = BigDecimal.ZERO;
@Override
public void add(Beverage beverage) {
subTotal = subTotal.add(beverage.price());
}
@Override
public void addAll(Collection<? extends Beverage> beverages) {
for (Beverage beverage : beverages)
subTotal = subTotal.add(beverage.price());
}
@Override
public BigDecimal grandTotal() {
BigDecimal tax = subTotal.multiply(TAX_RATE);
return subTotal.add(tax);
}
}
class CampaignOrder implements Order {
private static final BigDecimal DISCOUNT_RATE = new BigDecimal("0.2");
private int numberOfBeverages;
private final Order delegate;
CampaignOrder() {
this(new RegularOrder());
}
private CampaignOrder(Order delegate) {
this.delegate = delegate;
}
@Override
public void add(Beverage beverage) {
delegate.add(beverage);
numberOfBeverages++;
}
@Override
public void addAll(Collection<? extends Beverage> beverages) {
delegate.addAll(beverages);
numberOfBeverages += beverages.size();
}
@Override
public BigDecimal grandTotal() {
BigDecimal grandTotal = delegate.grandTotal();
if (numberOfBeverages > 2) {
BigDecimal discount = grandTotal.multiply(DISCOUNT_RATE);
grandTotal = grandTotal.subtract(discount);
}
return grandTotal;
}
}Now the Order class has become an interface which has 2 implementations. One is the RegularOrder class, which was formerly the Order class, and the other one is the CampaignOrder class. An instance of the CampaignOrder class has a reference to a RegularOrder instance in order to reuse its functionality, but the CampaignOrder class doesn’t have any superclass anymore. With the new design, no refactoring of the RegularCampaign class can break the CampaignOrder class as long as the RegularOrder class keeps the contract of the public methods.
One important difference from the inheritance approach is that now the CampaignOrder class no longer relies on any of the implementation details of the RegularOrder class. What it relies on now is the behavior of the public methods of the RegularOrder class, which are all defined in the Order interface.
The good thing about sticking with this approach is that as long as a class keeps its functionality on the public interface level the same, it can change its internal structure without you worrying about breaking something accidentally. It greatly reduces chances of unforseen breakage. Therefore, it will make your codebase more stable. Also, with having the interface, when a new method has been added to the interface, it will make the compilation of its implementors fail since the implementation is missing. With that, unlike with the inheritance solution, we can notice that we have to add the missing implementation to its implementors immediately.
Inheritance is inflexible
Now let’s think about some details of the Beverage interface in the codebase. It has an implemention class called Coffee. The customer can add a condiment such as milk, whip or sugar into it if they want to. For some reason, the original programmer implemented this requirement using inheritance:
interface Beverage {
BigDecimal price();
String description();
}
class Coffee implements Beverage {
@Override public BigDecimal price() { return new BigDecimal("1.99"); }
@Override public String description() { return "Coffee"; }
}
class CoffeeWithMilk extends Coffee {
@Override public BigDecimal price() { return super.price().add(new BigDecimal("0.10")); }
@Override public String description() { return super.description() + ", Milk"; }
}
class CoffeeWithWhip extends Coffee {
@Override public BigDecimal price() { return super.price().add(new BigDecimal("0.15")); }
@Override public String description() { return super.description() + ", Whip"; }
}
class CoffeeWithSugar extends Coffee {
@Override public BigDecimal price() { super.price().add(new BigDecimal("0.05")); }
@Override public String description() { return super.description() + ", Sugar"; }
}Now we’ve got a new requirement to implement, which is that a customer should be able to add multiple condiments into coffee. Let’s see if we can do that with inheritance. It would look like the following:
We ended up creating a lot of subclasses for all of the combinations. That will do for the time being but think about the future expansion. We will need to create many subclasses everytime we introduce a new condiment. And what if we want to reuse code which is responsible for a condiment for another beverage class, say, a Tea class? It’s not clear if we can do that in a reasonable way with this approach.
Let’s see if we could do better with composition. It would look like the following:
The implementation (the Beverage interface and the Coffee class are the same as the ones we have seen before):
class MilkWrapper implements Beverage {
private final Beverage delegate;
MilkWrapper(Beverage delegate) { this.delegate = delegate; }
@Override public BigDecimal price() { return delegate.price().add(new BigDecimal("0.10")); }
@Override public String description() { return delegate.description() + ", Milk"; }
}
class WhipWrapper implements Beverage {
private final Beverage delegate;
WhipWrapper(Beverage delegate) { this.delegate = delegate; }
@Override public BigDecimal price() { return delegate.price().add(new BigDecimal("0.15")); }
@Override public String description() { return delegate.description() + ", Whip"; }
}
class SugarWrapper implements Beverage {
private final Beverage delegate;
SugarWrapper(Beverage delegate) { this.delegate = delegate; }
@Override public BigDecimal price() { return delegate.price().add(new BigDecimal("0.05")); }
@Override public String description() { return delegate.description() + ", Sugar"; }
}In the new implementation, the classes responsible for condiments hold a reference to a Beverage object instead of extending the Coffee class. They can still reuse the code of the Coffee class but through the Beverage interface. Instead of creating an instance of a subclass of the Coffee class, you need to provide a Coffee instance to the constructor of one of the wrapper classes. Let’s do it like this:
@Test
void coffeeWithMilk() {
Beverage coffeeWithMilk = new MilkWrapper(new Coffee());
assertThat(coffeeWithMilk.description()).isEqualTo("Coffee, Milk");
assertThat(coffeeWithMilk.price()).isEqualByComparingTo("2.09");
}
@Test
void coffeeWithWhip() {
Beverage coffeeWithWhip = new WhipWrapper(new Coffee());
assertThat(coffeeWithWhip.description()).isEqualTo("Coffee, Whip");
assertThat(coffeeWithWhip.price()).isEqualByComparingTo("2.14");
}
@Test
void coffeeWithSugar() {
Beverage coffeeWithSugar = new SugarWrapper(new Coffee());
assertThat(coffeeWithSugar.description()).isEqualTo("Coffee, Sugar");
assertThat(coffeeWithSugar.price()).isEqualByComparingTo("2.04");
}Now, let’s see how we can implement the new requirement about multiple condiments with the new design. In fact, no change is needed in the Coffee class and the other classes in the diagram. We can do that like this:
@Test
void coffeeWithMilkAndWhip() {
Beverage coffee = new Coffee();
coffee = new MilkWrapper(coffee);
coffee = new WhipWrapper(coffee);
assertThat(coffee.description()).isEqualTo("Coffee, Milk, Whip");
assertThat(coffee.price()).isEqualByComparingTo("2.24");
}
@Test
void coffeeWithMilkAndSugar() {
Beverage coffee = new Coffee();
coffee = new MilkWrapper(coffee);
coffee = new SugarWrapper(coffee);
assertThat(coffee.description()).isEqualTo("Coffee, Milk, Sugar");
assertThat(coffee.price()).isEqualByComparingTo("2.14");
}
@Test
void coffeeWithMilkAndWhipAndSugar() {
Beverage coffee = new Coffee();
coffee = new MilkWrapper(coffee);
coffee = new WhipWrapper(coffee);
coffee = new SugarWrapper(coffee);
assertThat(coffee.description()).isEqualTo("Coffee, Milk, Whip, Sugar");
assertThat(coffee.price()).isEqualByComparingTo("2.29");
}First, we create an instance of the Coffee class and then wrap it with the wrapper classes as needed. This is much more flexible than the old approach which required having subclasses for all of the combinations. We can even combine one particular condiment multiple times, which would have been much harder with the inheritance approach:
@Test
void coffeeWithTripleWhip() {
Beverage coffee = new Coffee();
coffee = new WhipWrapper(coffee);
coffee = new WhipWrapper(coffee);
coffee = new WhipWrapper(coffee);
assertThat(coffee.description()).isEqualTo("Coffee, Whip, Whip, Whip");
assertThat(coffee.price()).isEqualByComparingTo("2.44");
}This design is also better in terms of maintainability. Introducing a new condiment doesn’t impact any other class in the diagram (i.e. there will be no class explosion). Those wrapper classes are highly reusable because they can be reused for anything which implements the Beverage interface. Also, they don’t depend on anything but the Beverage interface. They solely rely on the public methods in the Beverage interface. There will be no breakage as long as the contract of the Beverage interface stays the same.
Conclusion
We’ve seen how improper use of inheritance can make your code fragile and inflexible. Using inheritance just for code reuse can lead to an unforseen problem in the future. And inheritance is not flexible as you might expect. When you are tempted to use inheritance, I recommend considering if you can do it using composition instead. In my experience, inheritance is not the best option for most of such cases.
I would also like to mention that composition opens up a whole new world of clever design ideas in an object oriented programming language. If you want to learn more about it, I recommend checking out the following books:
Tags: oop software-design