Common Knowledge Questions
Last updated
Last updated
Interpreted languages don’t run at the machine code level. They are interpreted by another program that executes the instructions. Sometimes interpreted programs are compiled to bytecode and the bytecode is what is interpreted, which allows the programs to be smaller and run more quickly.
Compiled languages are converted to machine code to execute directly on the CPU. This makes them typically much faster than interpreted programs.
[Technically any language could be compiled or interpreted, but there are some languages that typically get compiled (Go, C, C++, etc.) and some that typically get interpreted (Java, Python, JavaScript, etc.)]
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[This specific question, “stack vs. heap” is about where a language allocates memory for different types of objects. It’s not directly about the stack data structure or heap data structure.]
When a program executes, the stack is used to hold local variables and function arguments. When a function gets called, a new stack frame is allocated on top of the stack with just enough room to hold all the local variables and arguments to the function. When the function returns, its stack frame is popped from the stack and the variables are forgotten. (Which is OK, because they were only needed for as long as the function was active.)
The heap is used for longer-term storage. Objects that were allocated with new or dynamically allocated another way go on the heap. Constant strings and global variables go there, as well.
Recursion is when a function calls itself.
It typically keeps calling itself until a base case is met that causes it to no longer call itself. (i.e. when the work is done.)
Problems that are composed of identical independent subproblems are good candidates for recursive solutions. (e.g a binary search tree is made up of smaller binary search trees, so traversing them has a nice recursive solution.)
In software engineering, the singleton pattern is a software design pattern that restricts the instantiation of a class to one "single" instance. This is useful when exactly one object is needed to coordinate actions across the system.
The term comes from the mathematical concept of a singleton.
The singleton design pattern is one of the twenty-three well-known "Gang of Four" design patterns that describe how to solve recurring design problems to design flexible and reusable object-oriented software with the aim of making it easier to implement, change, test, and reuse objects.[1]
The singleton design pattern solves problems by allowing it to:[2]
Ensure that a class only has one instance
Easily access the sole instance of a class
Control its instantiation
Restrict the number of instances
Access a global variable
The singleton design pattern describes how to solve such problems:
Hide the constructors of the class.
Define a public static operation (getInstance()) that returns the sole instance of the class.
In essence, the singleton pattern forces it to be responsible for ensuring that it is only instantiated once. A hidden constructor—declared private or protected—ensures that the class can never be instantiated from outside the class. The public static operation can be accessed by using the class name and operation name, e.g., Singleton.getInstance().
The abstract factory, factory method, builder, and prototype patterns can use singletons.
Facade objects are often singletons because only one facade object is required.
State objects are often singletons.
Singletons are often preferred to global variables because:[3]
They permit lazy allocation and initialization, whereas global variables in many languages will always consume resources.
Logging is a classic example of a singleton.[4]
In object-oriented programming (OOP), a factory is an object for creating other objects – formally a factory is a function or method that returns objects of a varying prototype or class[1] from some method call, which is assumed to be "new".[a] More broadly, a subroutine that returns a "new" object may be referred to as a "factory", as in factory method or factory function. This is a basic concept in OOP, and forms the basis for a number of related software design patterns.
In class-based programming, a factory is an abstraction of a constructor of a class, while in prototype-based programming a factory is an abstraction of a prototype object. A constructor is concrete in that it creates objects as instances of a single class, and by a specified process (class instantiation), while a factory can create objects by instantiating various classes, or by using other allocation schemes such as an object pool. A prototype object is concrete in that it is used to create objects by being cloned, while a factory can create objects by cloning various prototypes, or by other allocation schemes.
A factory may be implemented in various ways. Most often it is implemented as a method, in which case it is called a factory method. Sometimes it is implemented as a function, in which case it is called a factory function. In some languages, constructors are themselves factories. However, in most languages they are not, and constructors are invoked in a way that is idiomatic to the language, such as by using the keyword new, while a factory has no special status and is invoked via an ordinary method call or function call. In these languages, a factory is an abstraction of a constructor, but not strictly a generalization, as constructors are not themselves factories.
Terminology differs as to whether the concept of a factory is itself a design pattern – in Design Patterns there is no "factory pattern", but instead two patterns (factory method pattern and abstract factory pattern) that use factories. Some sources refer to the concept as the factory pattern,[2][3] while others consider the concept itself a programming idiom,[4] reserving the term "factory pattern" or "factory patterns" to more complicated patterns that use factories, most often the factory method pattern; in this context, the concept of a factory itself may be referred to as a simple factory.[4] In other contexts, particularly the Python language, "factory" itself is used, as in this article.[5] More broadly, "factory" may be applied not just to an object that returns objects from some method call, but to a subroutine that returns objects, as in a factory function (even if functions are not objects) or factory method.[6] Because in many languages factories are invoked by calling a method, the general concept of a factory is often confused with the specific factory method pattern design pattern.
OOP provides polymorphism on object use by method dispatch, formally subtype polymorphism via single dispatch determined by the type of the object on which the method is called. However, this does not work for constructors, as constructors create an object of some type, rather than use an existing object. More concretely, when a constructor is called, there is no object yet on which to dispatch.[b]
Using factories instead of constructors or prototypes allows one to use polymorphism for object creation, not only object use. Specifically, using factories provides encapsulation, and means the code is not tied to specific classes or objects, and thus the class hierarchy or prototypes can be changed or refactored without needing to change code that uses them – they abstract from the class hierarchy or prototypes.
More technically, in languages where factories generalize constructors, factories can usually be used anywhere constructors can be,[c] meaning that interfaces that accept a constructor can also in general accept a factory – usually one only need something that creates an object, rather than needing to specify a class and instantiation.
For example, in Python, the collections.defaultdict class[7] has a constructor which creates an object of type defaultdict[d] whose default values are produced by invoking a factory. The factory is passed as an argument to the constructor, and can itself be a constructor, or any thing that behaves like a constructor – a callable object that returns an object, i.e., a factory. For example, using the list constructor for lists:
Factory objects are used in situations where getting hold of an object of a particular kind is a more complex process than simply creating a new object, notably if complex allocation or initialization is desired. Some of the processes required in the creation of an object include determining which object to create, managing the lifetime of the object, and managing specialized build-up and tear-down concerns of the object. The factory object might decide to create the object's class (if applicable) dynamically, return it from an object pool, do complex configuration on the object, or other things. Similarly, using this definition, a singleton implemented by the singleton pattern is a formal factory – it returns an object, but does not create new objects beyond the single instance.
The simplest example of a factory is a simple factory function, which just invokes a constructor and returns the result. In Python, a factory function f that instantiates a class A can be implemented as:
A simple factory function implementing the singleton pattern is:
This will create an object when first called, and always return the same object thereafter.
Factories may be invoked in various ways, most often a method call (a factory method), sometimes by being called as a function if the factory is a callable object (a factory function). In some languages constructors and factories have identical syntax, while in others constructors have special syntax. In languages where constructors and factories have identical syntax, like Python, Perl, Ruby, Object Pascal, and F#,[e] constructors can be transparently replaced by factories. In languages where they differ, one must distinguish them in interfaces, and switching between constructors and factories requires changing the calls.
In languages where objects are dynamically allocated, as in Java or Python, factories are semantically equivalent to constructors. However, in languages such as C++ that allow some objects to be statically allocated, factories are different from constructors for statically allocated classes, as the latter can have memory allocation determined at compile time, while allocation of the return values of factories must be determined at run time. If a constructor can be passed as an argument to a function, then invocation of the constructor and allocation of the return value must be done dynamically at run time, and thus have similar or identical semantics to invoking a factory.
Main article: Creational pattern
Factories are used in various design patterns, specifically in creational patterns such as the Design pattern object library. Specific recipes have been developed to implement them in many languages. For example, several "GoF patterns", like the "Factory method pattern", the "Builder" or even the "Singleton" are implementations of this concept. The "Abstract factory pattern" instead is a method to build collections of factories.
In some design patterns, a factory object has a method for every kind of object it is capable of creating. These methods optionally accept parameters defining how the object is created, and then return the created object.
Factory objects are common in toolkits and frameworks where library code needs to create objects of types which may be subclassed by applications using the framework. They are also used in test-driven development to allow classes to be put under test.[8]
Factories determine the actual concrete type of object to be created, and it is here that the object is actually created. As the factory only returns an abstract interface to the object, the client code does not know – and is not burdened by – the actual concrete type of the object which was just created. However, the type of a concrete object is known by the abstract factory. In particular, this means:
The client code has no knowledge whatsoever of the concrete type, not needing to include any header files or class declarations relating to the concrete type. The client code deals only with the abstract type. Objects of a concrete type are indeed created by the factory, but the client code accesses such objects only through their abstract interface.
Adding new concrete types is done by modifying the client code to use a different factory, a modification which is typically one line in one file. This is significantly easier than modifying the client code to instantiate a new type, which would require changing every location in the code where a new object is created.
Factories can be used when:
The creation of an object makes reuse impossible without significant duplication of code.
The creation of an object requires access to information or resources that should not be contained within the composing class.
The lifetime management of the generated objects must be centralized to ensure a consistent behavior within the application.
Factories, specifically factory methods, are common in toolkits and frameworks, where library code needs to create objects of types that may be subclassed by applications using the framework.
Parallel class hierarchies often require objects from one hierarchy to be able to create appropriate objects from another.
Factory methods are used in test-driven development to allow classes to be put under test.[9] If such a class Foo creates another object Dangerous that can't be put under automated unit tests (perhaps it communicates with a production database that isn't always available), then the creation of Dangerous objects is placed in the virtual factory method createDangerous in class Foo. For testing, TestFoo (a subclass of Foo) is then created, with the virtual factory method createDangerous overridden to create and return FakeDangerous, a fake object. Unit tests then use TestFoo to test the functionality of Foo without incurring the side effect of using a real Dangerous object.
Tip: Be transparent about what a real web development project looks like for you. Highlight your wins, of course, but don’t shy away from being real about the challenges. Interviewers aren’t looking to hear that you never have setbacks (that’s not realistic). They want to hear how you get past setbacks and ultimately succeed.
Tip: Similar to the last question, interviewers are looking for honesty here. Sometimes projects go badly, and that’s OK. What’s important is how you respond to failures and learn from them so they don’t happen next time.
Tip: This question is pretty straightforward—let the interviewer know which languages you’re familiar with and how you use them. Ideally, you’ve scoped out the job beforehand and know that your experience syncs with what the employer needs. At the same time, have some new languages in mind that you’d like to learn. This highlights your willingness to keep growing professionally.
Tip: It’s a common pitfall to interview for a job and never explicitly say WHY you want to work in this specific field or for this particular employer/company. Even if the question doesn’t get asked, find a way to touch on it during the interview.
Tip: You may be tempted to say whatever you think the interviewer is looking for, but it’s way better to be honest. If the team you’ll be working with has a work style that’s completely outside of your comfort zone, then this particular job might not be a good fit for you. That being said, most development teams are dynamic and flexible, and if your employer knows what kind of environment suits you best, they can help find a spot on the team that WILL work for you.
Tip: You DO keep up with industry news, don’t you? If so, simply rattle off your list of favorite news sources and why they’re effective for keeping you in the know. And if tech news is something you’ve overlooked while being in the weeds of learning tech skills, take a few minutes to find a few suitable news blogs and tech Twitter accounts to put in your hip pocket (and be ready to bust them out at your next interview).
Tip: The gist here is to demonstrate that you understand the importance of keeping clients and stakeholders up to date, and that you have ideas for establishing systems of communication (or that you’re willing to work with systems like Agile or Scrum if they’re used by your employer).
Tip: Having an effective communication strategy with stakeholders doesn’t mean you won’t sometimes receive negative feedback. So how do you respond? Do you get defensive? Shut down? Give up? Or do you find creative ways to accept that feedback and address client or shareholder concerns? Interviewers are looking for candidates who can adapt to and recover from hard times, so either think of a real example that you can share, or develop a client appeasement gameplan that you’ll use when the time comes.
Tip: As a web developer you’ll find yourself in situations where you need to talk “tech” with non-techies. Making your work make sense to people who have no idea what it is you actually do is a valuable skill. Take off your developer hat for a day and think of some ways to describe web development to someone who doesn’t know Java from JavaScript.
Tip: Interviewers may ask you to provide an example of a website you think is less than stellar, then ask you to describe what you think is lacking and what you’d do to improve it. It’s a good idea to have examples and explanations on hand (as well as examples of sites you think are super effective) going into an interview. Answering this question comprehensively shows interviewers that you aren’t signing on to mindlessly write code—you understand what makes good sites good and how to make bad sites better.
Tip: This question is another one where you might be tempted to make the interviewer happy. But you know what’s guaranteed to make YOU unhappy? Working for a manager whose style you can’t stand. Be as flexible and as open minded as you can when describing your preferred management style, but if there’s something that’s a complete deal-breaker for you (or that you particularly appreciate), don’t be shy about making it known.
Tip: Your clear and concise summary of a web developer role shows how you think about the web development process in general, and lets interviewers know what specific developer strengths and interests you bring to the job.
Tip: Managing your time and estimating how long individual tasks will take is critical to your success as a web developer. If you’re already good at time management and estimation, revisit what works for you and build on it ahead of this question. And if your current time management approach isn’t working? Now’s a great time to implement a new system and get the most out of your work day.
Tip: Soft skills can be a difference maker. If it’s a choice between a skilled programmer and a skilled programmer who can write well or who has experience with project management, most employers will pick the latter. So have a list of your own soft skills ready, but also have examples of how they’ll be relevant to a web developer job. It’s not enough to say you’re good at written and verbal communication. You need to explain how your excellent written and verbal communication skills will help you relay project details to team members and stakeholders.
Tip: Yes, you’re interviewing for a web developer job, but remember to look to other experiences in your life for inspiration. Examples like the time you helped improve the ordering system at the cafe you worked at or put together a volunteer fundraising effort to save the music program at your kids’ school all speak to the breadth of your problem solving abilities and experiences.
Tip: Be transparent about what a real web development project looks like for you. Highlight your wins, of course, but don’t shy away from being real about the challenges. Interviewers aren’t looking to hear that you never have setbacks (that’s not realistic). They want to hear how you get past setbacks and ultimately succeed.
Tip: Similar to the last question, interviewers are looking for honesty here. Sometimes projects go badly, and that’s OK. What’s important is how you respond to failures and learn from them so they don’t happen next time.
Tip: This question is pretty straightforward—let the interviewer know which languages you’re familiar with and how you use them. Ideally, you’ve scoped out the job beforehand and know that your experience syncs with what the employer needs. At the same time, have some new languages in mind that you’d like to learn. This highlights your willingness to keep growing professionally.
Tip: It’s a common pitfall to interview for a job and never explicitly say WHY you want to work in this specific field or for this particular employer/company. Even if the question doesn’t get asked, find a way to touch on it during the interview.
Tip: You may be tempted to say whatever you think the interviewer is looking for, but it’s way better to be honest. If the team you’ll be working with has a work style that’s completely outside of your comfort zone, then this particular job might not be a good fit for you. That being said, most development teams are dynamic and flexible, and if your employer knows what kind of environment suits you best, they can help find a spot on the team that WILL work for you.
Tip: You DO keep up with industry news, don’t you? If so, simply rattle off your list of favorite news sources and why they’re effective for keeping you in the know. And if tech news is something you’ve overlooked while being in the weeds of learning tech skills, take a few minutes to find a few suitable news blogs and tech Twitter accounts to put in your hip pocket (and be ready to bust them out at your next interview).
Tip: The gist here is to demonstrate that you understand the importance of keeping clients and stakeholders up to date, and that you have ideas for establishing systems of communication (or that you’re willing to work with systems like Agile or Scrum if they’re used by your employer).
Tip: Having an effective communication strategy with stakeholders doesn’t mean you won’t sometimes receive negative feedback. So how do you respond? Do you get defensive? Shut down? Give up? Or do you find creative ways to accept that feedback and address client or shareholder concerns? Interviewers are looking for candidates who can adapt to and recover from hard times, so either think of a real example that you can share, or develop a client appeasement gameplan that you’ll use when the time comes.
Tip: As a web developer you’ll find yourself in situations where you need to talk “tech” with non-techies. Making your work make sense to people who have no idea what it is you actually do is a valuable skill. Take off your developer hat for a day and think of some ways to describe web development to someone who doesn’t know Java from JavaScript.
Tip: Interviewers may ask you to provide an example of a website you think is less than stellar, then ask you to describe what you think is lacking and what you’d do to improve it. It’s a good idea to have examples and explanations on hand (as well as examples of sites you think are super effective) going into an interview. Answering this question comprehensively shows interviewers that you aren’t signing on to mindlessly write code—you understand what makes good sites good and how to make bad sites better.
Tip: This question is another one where you might be tempted to make the interviewer happy. But you know what’s guaranteed to make YOU unhappy? Working for a manager whose style you can’t stand. Be as flexible and as open minded as you can when describing your preferred management style, but if there’s something that’s a complete deal-breaker for you (or that you particularly appreciate), don’t be shy about making it known.
Tip: Your clear and concise summary of a web developer role shows how you think about the web development process in general, and lets interviewers know what specific developer strengths and interests you bring to the job.
Tip: Managing your time and estimating how long individual tasks will take is critical to your success as a web developer. If you’re already good at time management and estimation, revisit what works for you and build on it ahead of this question. And if your current time management approach isn’t working? Now’s a great time to implement a new system and get the most out of your work day.
Tip: Soft skills can be a difference maker. If it’s a choice between a skilled programmer and a skilled programmer who can write well or who has experience with project management, most employers will pick the latter. So have a list of your own soft skills ready, but also have examples of how they’ll be relevant to a web developer job. It’s not enough to say you’re good at written and verbal communication. You need to explain how your excellent written and verbal communication skills will help you relay project details to team members and stakeholders.
Tip: Yes, you’re interviewing for a web developer job, but remember to look to other experiences in your life for inspiration. Examples like the time you helped improve the ordering system at the cafe you worked at or put together a volunteer fundraising effort to save the music program at your kids’ school all speak to the breadth of your problem solving abilities and experiences.
From Wikipedia, the free encyclopediaJump to navigationJump to search"Factory pattern" redirects here. For the GoF design patterns using factories, see factory method pattern and abstract factory pattern.Factory Method in LePUS3
