Java FAQ's for Exerienced and Beginners - Interview questions with answers

Java FAQ's for Exerienced and Beginners - Interview questions with answers

What is the difference between an Interface and an Abstract class ? 

Java provides and supports the creation both of abstract classes and interfaces. Both implementations share some common characteristics, but they differ in the following features:

All methods in an interface are implicitly abstract. On the other hand, an abstract class may contain both abstract and non-abstract methods.
A class may implement a number of Interfaces, but can extend only one abstract class.
In order for a class to implement an interface, it must implement all its declared methods. However, a class may not implement all declared methods of an abstract class. Though, in this case, the sub-class must also be declared as abstract.
Abstract classes can implement interfaces without even providing the implementation of interface methods.
Variables declared in a Java interface is by default final. An abstract class may contain non-final variables.
Members of a Java interface are public by default. A member of an abstract class can either be private, protected or public.
An interface is absolutely abstract and cannot be instantiated. An abstract class also cannot be instantiated, but can be invoked if it contains a main method.

What is Annotation in Java?

An annotation, in the java programming language is a special form of syntactic metadata that can be added to Java Source Code.
Classes, methods, variables parameters and packages may be annotated.
Unlike Java doc tags, Java annotation are reflective, in that they are embedded in class files generated by the compiler and may be retained by the java VM to make retrievable at run-time. Annotation is basically to attach metadata to method, class or package. Metadata is used by the compiler to perform some basic compile-time checking.

Difference between Thread and Process in Java?

Thread is subset of Process, in other words one process can contain multiple threads. Two process runs on different memory space, but all threads share same memory space. Don’t confuse this with stack memory, which is different for different thread and used to store local data to that thread.

How do you implement Thread in Java?

At language level, there are two ways to implement Thread in Java. An instance of java.lang.Thread represent a thread but it need a task to execute, which is instance of interface java.lang.Runnable. Since Thread class itself implement Runnable, you can override run() method either by extending Thread class or just implementing Runnable interface.

When to use Runnable vs Thread in Java?
This is follow-up of previous multi-threading interview question. As we know we can implement thread either by extending Thread class or implementing Runnable interface, question arise, which one is better and when to use one? This question will be easy to answer, if you know that Java programming language doesn’t support multiple inheritance of class, but it allows you to implement multiple interface. Which means, its better to implement Runnable than extends Thread, if you also want to extend another class e.g. Canvas or CommandListener.

Difference between Runnable and Callable in Java?

Both Runnable and Callable represent task which is intended to be executed in separate thread. Runnable is there from JDK 1.0, while Callable was added on JDK 1.5. Main difference between these two is that Callable’s call() method can return value and throw Exception, which was not possible with Runnable’s run() method. Callable return Future object, which can hold result of computation.

What is difference between Array and ArrayList ? When will you use Array over ArrayList ?

The Array and ArrayList classes differ on the following features:
Arrays can contain primitive or objects, while an ArrayList can contain only objects.
Arrays have fixed size, while an ArrayList is dynamic.
An ArrayListprovides more methods and features, such as addAll, removeAll, iterator, etc.
For a list of primitive data types, the collections use autoboxing to reduce the coding effort. However, this approach makes them slower when working on fixed size primitive data types.

What is difference between ArrayList and LinkedList ? 

Both the ArrayList and LinkedList classes implement the List interface, but they differ on the following features:
An ArrayList is an index based data structure backed by an Array. It provides random access to its elements with a performance equal to O(1). On the other hand, a LinkedList stores its data as list of elements and every element is linked to its previous and next element. In this case, the search operation for an element has execution time equal to O(n).
The Insertion, addition and removal operations of an element are faster in a LinkedList compared to an ArrayList, because there is no need of resizing an array or updating the index when an element is added in some arbitrary position inside the collection.
A LinkedList consumes more memory than an ArrayList, because every node in a LinkedList stores two references, one for its previous element and one for its next element.

Difference between CyclicBarrier and CountDownLatch in Java? 

Though both CyclicBarrier and CountDownLatch wait for number of threads on one or more events, main difference between them is that you can not re-use CountDownLatch once count reaches to zero, but you can reuse same CyclicBarrier even after barrier is broken.

What is volatile variable in Java?

volatile is a special modifier, which can only be used with instance variables. In concurrent Java programs, changes made by multiple threads on instance variables is not visible to other in absence of any synchronizers e.g. synchronized keyword or locks. Volatile variable guarantees that a write will happen before any subsequent read, as stated “volatile variable rule” in previous question.

What is race condition in Java? Given one example? 

Race condition are cause of some subtle programming bugs when Java programs are exposed to concurrent execution environment. As name suggests, race condition occurs due to race between multiple threads, if a thread which is supposed to execute first lost the race and executed second, behaviour of code changes, which surface as non-deterministic bugs. This is one of the hardest bugs to find and re-produce because of random nature of racing between threads. One example of race condition is out-of-order processing,

What happens when an Exception occurs in a thread?

In simple words, If not caught thread will die, if an uncaught exception handler is registered then it will get a call back. Thread.UncaughtExceptionHandler is an interface, defined as nested interface for handlers invoked when a Thread abruptly terminates due to an uncaught exception. When a thread is about to terminate due to an uncaught exception the Java Virtual Machine will query the thread for its UncaughtExceptionHandler using Thread.getUncaughtExceptionHandler() and will invoke the handler’s uncaughtException() method, passing the thread and the exception as arguments.

What is FutureTask in Java?

FutureTask represents a cancellable asynchronous computation in concurrent Java application. This class provides a base implementation of Future, with methods to start and cancel a computation, query to see if the computation is complete, and retrieve the result of the computation. The result can only be retrieved when the computation has completed; the get methods will block if the computation has not yet completed. A FutureTask object can be used to wrap a Callable or Runnable object. Since FutureTask
also implements Runnable, it can be submitted to an Executor for execution.

Difference between interrupted and isInterrupted method in Java?

Main difference between interrupted() and isInterrupted() is that former clears the interrupt status while later does not. The interrupt mechanism in Java multi-threading is implemented using an internal flag known as the interrupt status. Interrupting a thread by calling Thread.interrupt() sets this flag. When interrupted thread checks for an interrupt by invoking the static method Thread.interrupted(), interrupt status is cleared. The non-static isInterrupted() method, which is used by one thread to query the interrupt status of another, does not change the interrupt status flag. By convention, any method that exits by throwing an InterruptedException clears interrupt status when it does so. However, it’s always possible that interrupt status will immediately be set again, by another thread invoking interrupt.

Difference between synchronized and concurrent collection in Java?

Though both synchronized and concurrent collection provides thread-safe collection suitable for multi-threaded and concurrent access, later is more scalable than former. Before Java 1.5, Java programmers only had synchronized collection which becomes source of contention if multiple thread access them concurrently, which hampers scalability of system. Java 5 introduced concurrent collections like ConcurrentHashMap, which not only provides thread-safety but also improves scalability by using modern techniques like lock stripping and partitioning internal table.

Difference between livelock and deadlock in Java?

This question is extension of previous interview question. A livelock is similar to a deadlock, except that the states of the threads or processes involved in the livelock constantly change with regard to one another, without any one progressing further. Livelock is a special case of resource starvation. A real-world example of livelock occurs when two people meet in a narrow corridor, and each tries to be polite by moving aside to let the other pass, but they end up swaying from side to side without making any progress because they both repeatedly move the same way at the same time. In short, main difference between livelock and deadlock is that in former state of process change but no progress is made.

How do you take thread dump in Java?

There are multiple ways to take thread dump of Java process depending upon operating system. When you take thread dump, JVM dumps state of all threads in log files or standard error console. In windows you can use Ctrl + Break key combination to take thread dump, on Linux you can use kill -3 command for same. You can also use a tool called jstack for taking thread dump, it operate on process id, which can be found using another tool called jps.

Which JVM parameter is used to control stack size of thread?

This is the simple one, -Xss parameter is used to control stack size of Thread in Java. You can see this list of JVM options to learn more about this parameter.

Difference between synchronized and ReentrantLock in Java?

There were days when only way to provide mutual exclusion in Java was via synchronized keyword, but it has several shortcomings e.g. you can not extend lock beyond a method or block boundary, you can not give up trying for a lock etc. Java 5 solves this problem by providing more sophisticated control via Lock interface. ReentrantLock is a common implementation of Lock interface and provides re-entrant mutual exclusion Lock with the same basic behaviour and semantics as the implicit monitor lock accessed using synchronized methods and statements, but with extended capabilities.

How to create Immutable object in Java?

This question might not look related to multi-threading and concurrency, but it is. Immutability helps to simplify already complex concurrent code in Java. Since immutable object can be shared without any synchronization its very dear to Java developers. Core value object, which is meant to be shared among thread should be immutable for performance and simplicity. Unfortunately there is no @Immutable annotation in Java, which can make your object immutable, hard work must be done by Java developers. You need to keep basics like initializing state in constructor, no setter methods, no leaking of reference, keeping separate copy of mutable object to create Immutable object.

What is busy spin in multi-threading?

Busy spin is a technique which concurrent programmers employ to make a thread wait on certain condition. Unlike traditional methods e.g. wait(), sleep() or yield() which all involves relinquishing CPU control, this method does not relinquish CPU, instead it just runs empty loop. Why would someone do that? to preserve CPU caches. In multi core system, its possible for a paused thread to resume on different core, which means rebuilding cache again. To avoid cost of rebuilding cache, programmer prefer to wait for much smaller time doing busy spin.

What is fork join framework in Java?

The fork join framework, introduced in JDK 7 is a powerful tool available to Java developer to take advantage of multiple processors of modern day servers. It is designed for work that can be broken into smaller pieces recursively. The goal is to use all the available processing power to enhance the performance of your application. One significant advantage of The fork/join framework is that it uses a work-stealing algorithm. Worker threads that run out of things to do can steal tasks from other threads that are still busy.

How does thread synchronization occurs inside a monitor ? What levels of synchronization can you apply ? 

The JVM uses locks in conjunction with monitors. A monitor is basically a guardian that watches over a sequence of synchronized code and ensuring that only one thread at a time executes a synchronized piece of code. Each monitor is associated with an object reference. The thread is not allowed to execute the code until it obtains the lock.

Why Collection doesn’t extend Cloneable and Serializable interfaces ? 

The Collection interface specifies groups of objects known as elements. Each concrete implementation of a Collection can choose its own way of how to maintain and order its elements. Some collections allow duplicate keys, while some other collections don’t. The semantics and the implications of either cloning or serialization come into play when dealing with actual implementations. Thus, the concrete implementations of collections should decide how they can be cloned or serialized.

What is difference between fail-fast and fail-safe ? 

The Iterator's fail-safe property works with the clone of the underlying collection and thus, it is not affected by any modification in the collection. All the collection classes in java.util package are fail-fast, while the collection classes in java.util.concurrent are fail-safe. Fail-fast iterators throw a ConcurrentModificationException, while fail-safe iterator never throws such an exception.

What is the importance of hashCode() and equals() methods ? 

In Java, a HashMap uses the hashCode and equals methods to determine the index of the key-value pair and to detect duplicates. More specifically, the hashCode method is used in order to determine where the specified key will be stored. Since different keys may produce the same hash value, the equals method is used, in order to determine whether the specified key actually exists in the collection or not. Therefore, the implementation of both methods is crucial to the accuracy and efficiency of the HashMap.

What differences exist between HashMap and Hashtable ? 

Both the HashMap and Hashtable classes implement the Map interface and thus, have very similar characteristics. However, they differ in the following features:
A HashMap allows the existence of null keys and values, while a Hashtable doesn’t allow neither null keys, nor null values.
A Hashtable is synchronized, while a HashMap is not. Thus, HashMap is preferred in single-threaded environments, while a Hashtable is suitable for multi-threaded environments.
A HashMap provides its set of keys and a Java application can iterate over them. Thus, a HashMap is fail-fast. On the other hand, a Hashtable provides an Enumeration of its keys.
The Hashtable class is considered to be a legacy class.

What is Enum in Java?

Enum was introduced in Java 1.5 as a new type whose fields consists of fixed set of constants. For example, in Java we can create Direction as enum with fixed fields as EAST, WEST, NORTH, SOUTH.
enum is the keyword to create an enum type and similar to class. Enum constants are implicitly static and final.

What is the benefit of Composition over Inheritance?

One of the best practices of java programming is to “favor composition over inheritance”. Some of the possible reasons are:
Any change in the superclass might affect subclass even though we might not be using the superclass methods. For example, if we have a method test() in subclass and suddenly somebody introduces a method test() in superclass, we will get compilation errors in subclass. Composition will never face this issue because we are using only what methods we need.
Inheritance exposes all the super class methods and variables to client and if we have no control in designing superclass, it can lead to security holes. Composition allows us to provide restricted access to the methods and hence more secure.
We can get runtime binding in composition where inheritance binds the classes at compile time. So composition provides flexibility in invocation of methods.

What are different types of classloaders?

Java Classloader is the program that loads byte code program into memory when we want to access any class. We can create our own classloader by extending ClassLoader class and overriding loadClass(String name) method
There are three types of built-in Class Loaders in Java:
Bootstrap Class Loader – It loads JDK internal classes, typically loads rt.jar and other core classes.
Extensions Class Loader – It loads classes from the JDK extensions directory, usually $JAVA_HOME/lib/ext directory.
System Class Loader – It loads classes from the current classpath that can be set while invoking a program using -cp or -classpath command line options.

What is Serialization and Deserialization?

We can convert a Java object to an Stream that is called Serialization. Once an object is converted to Stream, it can be saved to file or send over the network or used in socket connections.
The object should implement Serializable interface and we can use java.io.ObjectOutputStream to write object to file or to any OutputStream object. Read more at Java Serialization.
The process of converting stream data created through serialization to Object is called deserialization.