A: Java is a high-level programming language originally developed by Sun Microsystems and released in 1995. Java runs on a variety of platforms, such as Windows, Mac OS, and the various versions of UNIX.
Q2: What are the supported platforms by Java Programming Language?
A: Java runs on a variety of platforms, such as Windows, Mac OS, and the various versions of UNIX/Linux like HP-Unix, Sun Solaris, Redhat Linux, Ubuntu, CentOS, etc.
Q3: What is JVM? Why Java is called the Platform Independent Programming Language?
A: A Java virtual machine (JVM) is a process virtual machine that can execute Java bytecode. Each Java source file is compiled into a bytecode file, which is executed by the JVM.
Java was designed to allow application programs to be built that could be run on any platform, without having to be rewritten or recompiled by the programmer for each separate platform. A Java virtual machine makes this possible, because it is aware of the specific instruction lengths and other particularities of the underlying hardware platform.
Q4: What is the difference between JDK and JRE?A: The JDK is the Java Development Kit. I.e., the JDK is bundle of software that you can use to develop Java based software.
The JRE is the Java Runtime Environment. I.e., the JRE is an implementation of the Java Virtual Machine which actually executes Java programs.
Typically, each JDK contains one (or more) JRE’s along with the various development tools like the Java source compilers, bundling and deployment tools, debuggers, development libraries, etc.
Q5: What does ‘static’ keyword mean? Can you override private or static method in Java?A: The static keyword denotes that a member variable or method can be accessed, without requiring an instantiation of the class to which it belongs.
A user cannot override static methods in Java, because method overriding is based upon dynamic binding at runtime and static methods are statically binded at compile time. A static method is not associated with any instance of a class so the concept is not applicable.
Q6: Can you access non-static variable in static context?
A: A static variable in Java belongs to its class and its value remains the same for all its instances. A static variable is initialized when the class is loaded by the JVM. If your code tries to access a non-static variable, without any instance, the compiler will complain, because those variables are not created yet and they are not associated with any instance.
Q7: What are the Data Types supported by Java? What is Autoboxing and Unboxing?
A: The eight primitive data types supported by the Java programming language are:
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Byte
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Short
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Int
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Long
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Float
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Double
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Boolean
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Char
Autoboxing is the automatic conversion made by the Java compiler between the primitive types and their corresponding object wrapper classes. For example, the compiler converts an int to an Integer, a double to a Double, and so on. If the conversion goes the other way, this operation is called unboxing.
Q8: What is Function Overriding and Overloading in Java?A: Method overloading in Java occurs when two or more methods in the same class have the exact same name, but different parameters. On the other hand, method overriding is defined as the case when a child class redefines the same method as a parent class. Overridden methods must have the same name, argument list, and return type. The overriding method may not limit the access of the method it overrides.
Q9: What is a Constructor, Constructor Overloading in Java and Copy-Constructor?
A: A constructor gets invoked when a new object is created. Every class has a constructor. In case the programmer does not provide a constructor for a class, the Java compiler (Javac) creates a default constructor for that class.
The constructor overloading is similar to method overloading in Java. Different constructors can be created for a single class. Each constructor must have its own unique parameter list.
Finally, Java does support copy constructors like C++, but the difference lies in the fact that Java doesn’t create a default copy constructor if you don’t write your own.
Q10: Does Java support multiple inheritance?
A: No, Java does not support multiple inheritance. Each class is able to extend only on one class, but is able to implement more than one interfaces.
Q11: Why does Java not support operator overloading?A: One more similar category of tough Java question. C++ supports operator overloading than why not Java? This is the argument Interviewer will give to you and sometime even say that + operator is overloaded in Java for String concatenation. Don’t be fooled with such arguments.
Q12: How do you decide when to use ArrayList and LinkedList?
A: If you need to frequently add and remove elements from the middle of the list and only access the list elements sequentially, then LinkedList should be used. If you need to support random access, without inserting or removing elements from any place other than the end, then ArrayList should be used.
Q13: How HashMap works in Java?
A: A HashMap in Java stores key-value pairs. The HashMap requires a hash function and uses hashCode and equals methods, in order to put and retrieve elements to and from the collection respectively. When the put method is invoked, the HashMap calculates the hash value of the key and stores the pair in the appropriate index inside the collection. If the key exists, its value is updated with the new value. Some important characteristics of a HashMap are its capacity, its load factor and the threshold resizing.
Q14: What is the importance of hashCode() and equals() methods?
A: A HashMap in Java uses the hashCode and equals methods to determine the index of the key-value pair. These methods are also used when we request the value of a specific key. If these methods are not implemented correctly, two different keys might produce the same hash value and thus, will be considered as equal by the collection. Furthermore, these methods are also used to detect duplicates. Thus, the implementation of both methods is crucial to the accuracy and correctness of the HashMap.
Q15: What differences exist between HashMap and Hashtable?
A: Both the HashMap and Hashtable classes implement the Map interface and thus, have very similar characteristics. However, they differ in the following features:
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A HashMap allows the existence of null keys and values, while a Hashtable doesn’t allow neither null keys nor null values.
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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.
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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.
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The Hashtable class is considered to be a legacy class
Q16: What is the difference between poll() and remove() method of Queue interface?
A: Though both poll() and remove() method from Queue is used to remove object and returns head of the queue, there is subtle difference between them. If Queue is empty() then a call to remove() method will throw Exception, while a call to poll() method returns null. By the way, exactly which element is removed from the queue depends upon queue’s ordering policy and varies between different implementation, for example PriorityQueue keeps lowest element as per Comparator or Comparable at head position.Q17: What is the difference between fail-fast and fail-safe Iterators?
A: This is relatively new collection interview questions and can become trick if you hear the term fail-fast and fail-safe first time. Fail-fast Iterators throws ConcurrentModificationException when one Thread is iterating over collection object and other thread structurally modify Collection either by adding, removing or modifying objects on underlying collection. They are called fail-fast because they try to immediately throw Exception when they encounter failure. On the other hand fail-safe Iterators works on copy of collection instead of original collection.
Q18: What is difference between Iterator and Enumeration?
A: This is a beginner level collection interview questions and mostly asked during interviews of Junior Java developer up to experience of 2 to 3 years Iterator duplicate functionality of Enumeration with one addition of remove() method and both provide navigation functionally on objects of Collection. Another difference is that Iterator is more safe than Enumeration and doesn’t allow another thread to modify collection object during iteration except remove() method and throws ConcurrentModificaitonException.
Q19: What is the difference between HashSet and TreeSet?
A: The HashSet is Implemented using a hash table and thus, its elements are not ordered. The add, remove, and contains methods of a HashSet have constant time complexity O(1).
On the other hand, a TreeSet is implemented using a tree structure. The elements in a TreeSet are sorted, and thus, the add, remove, and contains methods have time complexity of O(logn).
Q20: What are the two types of Exceptions in Java? What are the differences between them?A: Java has two types of exceptions: checked exceptions and unchecked exceptions. Unchecked exceptions do not need to be declared in a method or a constructor’s throws clause, if they can be thrown by the execution of the method or the constructor, and propagate outside the method or constructor boundary. On the other hand, checked exceptions must be declared in a method or a constructor’s throws clause. See here for tips on Java exception handling.
Q21: What is the difference between Exception and Error in Java?
A: Exception and Error classes are both subclasses of the Throwable class. The Exception class is used for exceptional conditions that a user’s program should catch. The Error class defines exceptions that are not excepted to be caught by the user program.
Q22: What is CopyOnWriteArrayList, how it is different than ArrayList and Vector?A: CopyOnWriteArrayList is new List implementation introduced in Java 1.5 which provides better concurrent access than Synchronized List. Better concurrency is achieved by Copying ArrayList over each write and replace with original instead of locking. Also CopyOnWriteArrayList doesn’t throw any ConcurrentModification Exception. Its different than ArrayList because its thread-safe and ArrayList is not thread safe and its different than Vector in terms of Concurrency. CopyOnWriteArrayList provides better Concurrency by reducing contention among readers and writers.
Q23: What is the difference between Set, List and Map Collection classes?
A: java.util.Set, java.util.List and java.util.Map defines three of most popular data structure support in Java. Set provides uniqueness guarantee i.e.g you cannot store duplicate elements on it, but it’s not ordered. On the other hand List is an ordered Collection and also allowes duplicates. Map is based on hashing and stores key and value in an Object called entry. It provides O(1) performance to get object, if you know keys, if there is no collision. Popular impelmentation of Set is HashSet, of List is ArrayList and LinkedList, and of Map are HashMap, Hashtable and ConcurrentHashMap. Another key difference between Set, List and Map are that Map doesn’t implement Collection interface, while other two does.
Q24: What is BlockingQueue, how is it different from other collection classes?A: BlockingQueue is a Queue implementation available in java.util.concurrent package. It’s one of the concurrent Collection class added on Java 1.5, main difference between BlockingQueue and other collection classes is that apart from storage, it also provides flow control. It can be used in inter thread communication and also provides built-in thread-safety by using happens-before guarantee. You can use BlockingQueue to solve Producer Consumer problem, which is what is needed in most of concurrent applications.
Q25: What is the use of the ‘SimpleDateFormat’ and how can you use it to display the current system date in ‘yyyy/MM/DD HH:mm:ss’ format?
A: SimpleDateFormat is one such concrete class which is widely used by Java developers for parsing and formatting of dates. This is also used to convert Dates to String and vice-versa.