java.md

Java

Practical tasks

• Fibonacci recursive, iterative, stream.
• Factorial recursive, iterative, stream.
• Get rid of instanceOf() from code sample.
• Reverse string recursive, iterative, stream.
• Reverse array recursive, iterative, stream.

Basic tasks

• Implement custom Stack / List / Queue.
• Implement custom Stack / List / Queue using pure TDD.
• Implement Stack / List / Queue methods using only recursion.
• Fibonacci recursive, iterative, stream.
• Factorial recursive, iterative, stream.
• Get rid of instanceOf() from code sample.
• Reverse string recursive, iterative, stream.
• Reverse array recursive, iterative, stream.

Core

• How can we construct a new object in Java (i.e. a new operator, cloning, deserialization, reflection -> Class.forName())
• What primitive types do you know? What is the size of each?
• Describe what is the meaning of following keywords: protected, private, public, no keyword.
• Overloading vs overriding vs hiding
• What are rules of overloading?
• What is the difference between override and overload + static and runtime polymorphism.
• How can you prevent class from being subclassed?
• Transient keyword, what is it used for?
• How to make an object immutable?
• What do we gain from class immutability?
• What is a defensive copying?
• Hard, soft, weak, phantom references. What are differences? How to make a cache from weak references? What can be used instead of finalize method?
• How to make a memory leak in Java? How to analyze memory leaks in Java? What tools you can use?
• What is new in JDK7/8/9/10...?
• Checked vs unchecked exceptions. Can we catch Error? How to make a good API?
• Exceptions translation
• Can static method be overloaded? What is hiding? What will happen if extending class will define the non-static method with the same name?
• Difference between object and primitive?
• How to observe polymorphism in Java? Provide an example.
• Can interface have methods implemented?
• Can interface have private methods?
• What are final, finalize, finally?
• When finally is not going to be executed?
• Will finally be executed when we override SecurityManager behavior?
• What is Boxing/Unboxing/Autoboxing and what are the performance implications?
• What is try-with-resources?
• What does it mean that a stream (from Java8) is lazy?
• What are terminal vs non-terminal operations in streams? What are types returned from such operations?
• For loop performance vs Stream performance for 100 items.
• How to analyze the app performance? What tools you can use?
• What good pratices for benchmarking in Java do you know?
• Clone in Java (Cloneable interface, clone() method). How to implement a clone mechanism properly? Shall we use that at all?
• How to serialize object in Java? What is versionUID?
• What is "transient" keyword?
• Can Java return a covariant type?
• String in in Java. intern() method. Security of a String pool
• Can we extend a String?
• How many compilers are there in the JDK distribution?
• What is immutable object pattern? Is it used in java (JDK)? Describe immutable types
• What design patterns are used in JDK? (i.e. Decorator in I/O, Strategy in Comparators, Singleton while getting runtime, Builder in StringBuilder)
• What methods are defined in Object class in Java?
• There are two methods with same name - one takes the argument of Object, another one of String. Which one will be invoked if we pass null and why? (The "Most Specific" Principle)
• Is it a good practice to use "Optional" as a method argument? How about serialization?
• What do you think about returning a Stream from a method?
• There is set of users. Each user contains set of languages he speaks. How to get all languages spoken by all users? (stream with flatmap and distincts)
• Fields in lambdas are effective final. How we can bypass that to i.e. increment a value inside (use objects like i.e. AtomicInteger to increment)
• What is a classloader? What is a hierarchy of classloaders (Bootstrap, Platform, Application, Custom)?
• How to create an annotation?
• Is it possible to have a final constructor and why?
• Do we need to override hashcode if we override equals?
• What is a ThreadLocal? Where can it be helpful?
• What is a difference between Inner (non-static) and Nested (static) Class?
• When we need a private constructor?
• How can we make a XOR in Java?
• How can we create a copy constructor?
• What is a Dynamic Binding?
• Explain when default methods in interfaces can be handy.

Generics

• What is a "type erasure"? Why it was introduced? What version of Java introduced generics?
• PECS ("Producer extends, Consumer super") - when to use super, extends. What limitations they have? (i.e. putting/getting from collections, cast required etc.)
• invariance vs covariance vs contravariance
• Are arrays covariant?
• What is the difference between i.e. List<?> vs List<Object>? What is the purpose of the wildcard here? Where such construct can be used?
• Can we have generic fields in a non-generic class?

Multithreading

• What is deadlock, race-condition, data-race, livelock, starvation?
• What is the thread lifecycle (states) in JVM?
• What is double-checked-locking? How to make it right?
• How to implement singleton in the safe way? Enums as singletons.
• i++ How many operations must jvm perform in such line?
• What methods should be overridden in HttpServlet, what can we tell about thread safety of fields in servlet?
• java.util.concurrent -> CountDownLatch, Semaphores, Read/Write locks (ReentrantLock, ReentrantReadWriteLock), Atomic types etc.
• synchronized based on method, object, class etc. Pros/Cons of each solution
• How Atomic types work? What is CAS?
• How volatile works?
• How to create and start a new thread?
• How to stop a thread? (stop is deprecated)
• Runnable vs Callable
• Describe Features
• What is the difference between synchronization of static and non-static methods?
• What does it mean that lock is reentrant?
• Write a code to make a deadlock
• How to debug deadlocks?
• What is thread dump? How to analyze that? What tools you can use to analyze a thread dump?
• What does it mean that synchronized methods are reentrant synchronization?
• What is the difference between wait and sleep?
• wait/notify/notifyAll, sleep, yield - what are they for?
• What happens to a thread when it sleeps? How and from where to call the wait method?
• What is POJO and where we should use it?
• What is JNI (Java Native Interface)?
• What are thread pools? What framework have you used to build such thread pool?
• Why I/O resources are most likely exposed to deadlocks?
• Where we can use Callable interface? What is Future an object in Callable interface?
• How can we determine how may cores we can utilize in current environment to build our thread pool?
• How we can generate ThreadDump on the machine we have only console/CLI access?
• What is a critical section?
• What is thread pools? What framework have you used to build such thread pool?
• Why Concurrency Utilities were introduced? What data structures it implements? What other mechanisms it provides?
• What thread safe structures do you know in Java?
• What concurrency patterns / good practices do you know?

Multithreading tasks

• Implement a good singleton (with serialization / classloading and instantiation problems solved).
• Implement double check idiom.

Collections

• Draw diagram of Java collections.

Maps

• How does Map interface correspond with Collection interface?
• Map vs Switch statement performance?
• Would you use Switch with String or Map instead?
• What types can be used in switch (in different Java versions)?

HashMap

• What is HashMap initial size?
• How is initial size calculated. Why is bit-shift preferred over modulo for calculations like this?
• What is HashMap loadFactor?
• What is default value of load factor for HashMap?
• What is bucket?
• What data structures are used for buckets?
• Does put() method have return type or is it void method?
• Does get() method have return type of is it void method?
• How is item stored in HashMap when put() is called.
• How is item retrieved when get() is called.
• Where are nulls stored in HashMap ( with index )?
• What data structure is HashMap build upon?
• What are optimisations for storing items in HashMap? (i.e. LinkedList becomes a Tree if object has Comparable implemented)
• Describe what rehashing is and when it occurs.
• What data retrieval speed you would get from JDK 7 HashMap if someone implemented malicious hashcode that returns same value for all items put to map. Would such thing even work? If yes why if no why? What about JDK 8+ ?
• Does Java HashMap support perfect hashing?
• What needs to be implemented to make hash based collection working (hashcode/equals)
• Synchronized map vs ConcurrentHashMap
• How to make a good key in Map?

Lists

• Can you put primitive types to List? Why?
• When would you use ArrayList and when LinkedList?
• How can we check whether the LinkedList has a loop?

ArrayList

• What data structure is used for array list.
• What is default size of underlying array if we don't specify initial size of ArrayList?
• Is it good idea to define initial size of ArrayList and why?
• What happens if underlying array gets filled up to much? ( ensureCapacity() )
• What specific interface ArrayList implements that LinkedList does not?
• What means that ArrayList implements RandomAccess?
• Big O for each ArrayList methods.
• Is ArrayList thred safe?
• What other data structure would you use in place of ArrayList in multithreaded environment?
• How many thread simple program that prints "Hello world" to console has?

Queues

• Types of queues in Java

Others

• FIFO vs LIFO
• Big O for collections (Hash, Tree, Linked, Array based)
• When to use which types of collections? (hash vs tree vs linked)
• Map vs Set vs Lists vs Queues
• What is EnumSet?
• What needs to be implemented to make i.e. tree based collection working (Comparable interface)
• PECS - when to use super, extends. What limitations they have? (i.e. putting/getting from collections, cast required etc.)

Collections - practial tasks:

• Implement custom Stack / List / Queue.
• Implement custom Stack / List / Queue using pure TDD.
• Implement Stack / List / Queue methods using only recursion.

JVM, Garbage Collector, JIT

• Java memory model (heap vs stack, young generation vs old generation, metaspace etc.)
• What flags do you know for tuning JVM/Garbage Collector?
• How many thread simple program that prints "Hello world" to console has?
• Describe Java memory model
• How can we measure performance of the application? What tools do you know for such purpose?
• Difference between PermGen and Metaspace (i.e. which is newer, which has dynamic space allocation)?
• What is full/major/minor GC? How to avoid full GC?
• Young vs Old generation
• How GC works? (root, phases etc.)
• Do you know difference between survivor 1 and survivor 2 in Java?
• What is the structure of JVM heap (Eden, Survivor Space, Old Gen)?
• How we can influence garbage collection in JVM?
• Tools provided with JDK
• What is profiling?
• What is JIT?
• Describe how the code is compiled in Java (javac -> JIT)
• How the code is compiled in java? bytecode vs native code compiled by JIT
• Is Java a compiled or interpreted language?
• What optimizations do you know that are performed by JIT? (inlining, branch prediction,loop enrolling moving some objects from heap to stack etc.)
• What is the difference between server/client JIT compiler?
• What is "Stop the world" in GC? When it is run?
• What is memory fragmentation and why this can effect JVM-based applications performance or possibility to work?
• If JVM cannot allocate memory for big collection of data but we have more than enough memory what can be an issue on level of operation system that we can expect?
• How to bypass memory limitations of JVM using JNI interface? Why we can do that?
• How you will secure JVM application from perspective of performance, when you're developing socket application?
• How to create a second instance of a singleton object?
• What are the GC phases?
• What Garbage Collectors do you know? What are the main differnces between them?
• What is a difference between heap vs thread vs core dump? How to analyze these files? What tools can you use for that?
• What is jcmd? How to use that?

Libraries

• How can we provide logging mechanisms inside Java application? What logging frameworks do you know?
• Why it's better to use logging frameworks instead of output to System.out?

JMX

• What is JMX?
• Is JMX one way or two way communication protocol (read, write or read & write)?

JMS

• Topic vs Queue in JMS

XML, JAXB

• Sax vs DOM vs JAXB in XML parsing. What is the difference with the approach?
• What is JAXB? What tools (i.e wsimport) do you know?
• How we convert Java object to XML? What are steps in JAXB marshalling?
• How transient keyword works in JAXB (XmlTransient)?
• What is XSLTs?
• WHat is a difference between DTD and XML Schema?