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Design Patterns

Interview Questions

General Questions

What are Design Patterns?

Each design pattern offers a reusable solution for a for common (repeatable) design problem.

What is Gang of Four (GOF)?

The authors of the book “Design Patterns: Elements of Reusable Object-Oriented Software (1994)”

Name the types of the Design Patterns

There are many types, but if the question is about the fundamental patterns like GoF Design Patterns, there are 3 types: creational, structural and behavioral. If the question is about all the patterns, some of they are: architectural, integration etc.

Explain the idea behind Creational Patterns

To deal with object creation/ instantiation.

Explain the idea behind Structural Patterns

To deal with the relations between objects

Explain the idea behind Behavioral Patterns

To deal with communication between objects

Name some of the design patterns, used in JAVA API.

Factory Method: 

Calendar.getInstance()

Builder: 

Calendar cal = new Calendar.Builder()
    .setCalendarType("iso8601").setWeekDate(2013, 1, MONDAY).build();

Locale aLocale = new Builder().setLanguage("sr").setScript("Latn").setRegion("RS").build(); (since java 7)

Adapter: 

java.util.Arrays#asList()

Decorator: 

All subclasses of java.io.InputStream, OutputStream, Reader and Writer have a constructor taking an instance of same type.

Proxy: 

java.lang.reflect.Proxy

Chain of Responsibility: 

javax.servlet.Filter#doFilter() - Servlet Filters in Java that allow multiple filters to process an HTTP request

Command: 

java.lang.Runnable

Iterator: 

All implementations of java.util.Iterator (thus among others also java.util.Scanner!).
All implementations of java.util.Enumeration

Mediator: 

java.util.Timer (all scheduleXXX() methods)

Strategy: 

java.util.Comparator#compare(), executed by among others Collections#sort()

Singleton Questions

Can we create a clone of a singleton object?

Yes, we can.

How to prevent cloning of a singleton object?

Implement the Cloneable interface and make clone() method to do nothing or to throw an Exception

Singleton with double checked locking - what is that and how is implemented?

Does the check twice. You can also mark the instance as volatile (to indicate that a variable’s value will be modified by different threads)

    public static Singleton getInstance() {
        if (instance == null) { // First Check
            synchronized(Singleton.class) {
                if (instance == null) { // Second check
                    instance = new Singleton();
                }
            }
        }
        return instance;
    }

How to make Singleton serializable?

Implement the Serializable interface. The problem is that when we deserialize it, it will create a new instance of the class. This will be solved by implementing readResolve or if we use enum.

protected Object readResolve() {
    return getInstance();
}

What are the differences between a static class and a singleton class?

  • The Singleton is object and can be passed as parameter, while the static class can not.
  • The Singleton can implement interfaces, extend other classes, while the static class can not.
  • The Singleton is stored in the heap (as the other objects), while statics are stored in the stack.

Comparison Questions

Adapter vs Decorator vs Facade vs Proxy - Explain the differences

  • Adapter: Makes two incompatible interfaces compatible
  • Decorator: Adds functionality based on some input or configuration
  • Facade: The facade provides a different front to the client and hides complexity
  • Proxy: May provide some functionality like logging etc.

Strategy vs Decorator

The key difference is in the change vs augment

  • As an example, imagine you are writing something to sort a collection of elements. So you write an interface ISortingStrategy you can then implement several different sorting strategies BubbleSortStrategy, QuickSortStrategy, RadixSortStrategy, then your application, based on some criteria of the existing list chooses the most appropriate strategy to use to sort the list. So for example if the list has fewer then 10 items we will use RadixSortStrategy, if fewer than 10 items have been added to the list since the last sort we will use BubbleSortStrategy otherwise we will use QuickSortStrategy.We are changing the type of sort at runtime (to be more efficient based on some extra information.) this is the strategy pattern.

  • Now imagine someone asks us to provide a log of how often each sorting algorithm is used to do an actual sort and to restrict sorting to admin users. We can add both of these pieces of functionality by creating a decorator which enhances any ISortingStrategy. We could create a decorator which logs that it was used to sort something and the type of the decorated sorting strategy. And we could add another decorator that checked if the current user was an administrator before it called the decorated sorting strategy.

Strategy vs Visitor vs Decorator

  • One Type that may apply many algorithms of a single family of algorithms (i.e. different sorting algorithms) = Strategy*

  • Many Types that can apply same or multiple algorithms = Visitor*

  • If you want to add some functionality, not related to the main algorithm i.e. to log when an algorithm is used = Decorator*