IGNOU BCA MCA Students - VIVA Question Answer for JAVA or JSP Part IV

IGNOU BCA MCA Students - VIVA Question Answer for JAVA or JSP

JAVA or JSP Selected Question Answer - PART IV

15. 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.

16. What’s a deadlock ? A condition that occurs when two processes are waiting for each other to complete, before proceeding. The result is that both processes wait endlessly.

17. How do you ensure that N threads can access N resources without deadlock ? A very simple way to avoid deadlock while using N threads is to impose an ordering on the locks and force each thread to follow that ordering. Thus, if all threads lock and unlock the mutexes in the same order, no deadlocks can arise.

Java Collections

18. What are the basic interfaces of Java Collections Framework ? Java Collections Framework provides a well designed set of interfaces and classes that support operations on a collections of objects. The most basic interfaces that reside in the Java Collections Framework are:

·         Collection, which represents a group of objects known as its elements.

·         Set, which is a collection that cannot contain duplicate elements.

·         List, which is an ordered collection and can contain duplicate elements.

·         Map, which is an object that maps keys to values and cannot contain duplicate keys.

19. 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.

20. What is an Iterator ? The Iterator interface provides a number of methods that are able to iterate over anyCollection. Each Java Collection contains the iterator method that returns an Iterator instance. Iterators arecapable of removing elements from the underlying collection during the iteration. 21. What differences exist between Iterator and ListIterator ? The differences of these elements are listed below:

·         An Iterator can be used to traverse the Set and List collections, while the ListIterator can be used to iterate only over Lists.

·         The Iterator can traverse a collection only in forward direction, while the ListIterator can traverse a List in both directions.

·         The ListIterator implements the Iterator interface and contains extra functionality, such as adding an element, replacing an element, getting the index position for previous and next elements, etc.

22. 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 aConcurrentModificationException, while fail-safe iterator never throws such an exception.

23. How HashMap works in Java ? 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 aHashMap are its capacity, its load factor and the threshold resizing.

24. What is the importance of hashCode() and equals() methods ? In Java, a HashMap uses the hashCode andequals methods to determine the index of the key-value pair and to detect duplicates. More specifically, the hashCodemethod 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.