Java Part-04

31. What is the tradeoff between using an unordered array versus an ordered array ? 

The major advantage of an ordered array is that the search times have time complexity of O(log n), compared to that of an unordered array, which is O (n). The disadvantage of an ordered array is that the insertion operation has a time complexity of O(n), because the elements with higher values must be moved to make room for the new element. Instead, the insertion operation for an unordered array takes constant time of O(1).

32. What are some of the best practices relating to the Java Collection framework ?

·         Choosing the right type of the collection to use, based on the application’s needs, is very crucial for its performance. For example if the size of the elements is fixed and know a priori, we shall use an Array, instead of an ArrayList.

·         Some collection classes allow us to specify their initial capacity. Thus, if we have an estimation on the number of elements that will be stored, we can use it to avoid rehashing or resizing.

·         Always use Generics for type-safety, readability, and robustness. Also, by using Generics you avoid theClassCastException during runtime.

·         Use immutable classes provided by the Java Development Kit (JDK) as a key in a Map, in order to avoid the implementation of the hashCode and equals methods for our custom class.

·         Program in terms of interface not implementation.

·         Return zero-length collections or arrays as opposed to returning a null in case the underlying collection is actually empty.

33. What’s the difference between Enumeration and Iterator interfaces ? 

Enumeration is 

twice as fast as compared to an Iterator and uses very less memory. However, the Iterator is much safer compared to Enumeration, because other threads are not able to modify the collection object that is currently traversed by the iterator. Also,Iteratorsallow the caller to remove elements from the underlying collection, something which is not possible withEnumerations.

34. What is the difference between HashSet and TreeSet ? 

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).

Garbage Collectors

35. What is the purpose of garbage collection in Java, and when is it used ? 

The purpose of garbage collection is to identify and discard those objects that are no longer needed by the application, in order for the resources to be reclaimed and reused.

36. What does System.gc() and Runtime.gc() methods do ?

 These methods can be used as a hint to the JVM, in order to start a garbage collection. However, this it is up to the Java Virtual Machine (JVM) to start the garbage collection immediately or later in time.

37. When is the finalize() called ? What is the purpose of finalization ?

 The finalize method is called by the garbage collector, just before releasing the object’s memory. It is normally advised to release resources held by the object inside the finalize method.

38. If an object reference is set to null, will the Garbage Collector immediately free the memory held by that object ? 

No, the object will be available for garbage collection in the next cycle of the garbage collector.

39. What is structure of Java Heap ? What is Perm Gen space in Heap ?

 The JVM has a heap that is the runtime data area from which memory for all class instances and arrays is allocated. It is created at the JVM start-up. Heap memory for objects is reclaimed by an automatic memory management system which is known as a garbage collector. Heap memory consists of live and dead objects. Live objects are accessible by the application and will not be a subject of garbage collection. Dead objects are those which will never be accessible by the application, but have not been collected by the garbage collector yet. Such objects occupy the heap memory space until they are eventually collected by the garbage collector.

40. What is the difference between Serial and Throughput Garbage collector ? 

The throughput garbage collector uses a parallel version of the young generation collector and is meant to be used with applications that have medium to large data sets. On the other hand, the serial collector is usually adequate for most small applications (those requiring heaps of up to approximately 100MB on modern processors).