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Ans. Step 1 - We can create a Registry Class having Map of all created objects as key and References list as value.
Step 2 - Whenever we create an object , we should update the Registry in the constructor to add the new object.
Step 3 - Whenever we assign a new reference to the object , we need to update the entry in Map. Similarly if the reference get's removed ( end of scope etc ), we need to remove the entry of reference from the list.
Step 4 - We can have threaded code to monitor the Map to see if any object looses all it's references and should call the method to destroy object and clean the memory.
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Ans. HashTable has been deprecated. As an alternative, ConcurrentHashMap has been provided. It uses multiple buckets to store data and hence much better performance than HashTable. Moreover, there is already a raw type HashMap. The only difference between the HashTable and HashMap is that Hashtable is synchronized whereas HashMap is not. Most of the synchronized collections have been deprecated and their raw alternative have been presented as preferred.Synchronization has a cost. Using synchronized collection in places where there is no need of it leads to useless utilization of resources. As these collections are rarely used in a static context or shared among threads, Java might have thought it better to just provide the raw collection and let developers implement synchronization if he feels the need to do so. HashMap is now presented as the default and the preferred way of using Map with read optimized hashing, and ConcurrentHashMap has been provided for synchronized access which provides better performance than HashTable. Because of this, Java thought it right to deprecate the use of HashTable.'
Synchronization has a cost. Using synchronized collection at a place where there is hardly any need of it would means useless utilization of resources. As these collections are rarely used in static context or shared among threads, Java might have thought it better to just provide the raw collection and let developer implement synchronization if he feels the need to do so.
As HashMap has been presented as default and preferred way of using Map with read optimized hashing, and ConcurrentHashMap has been provided for synchronized access which provides better performance than HashTable, Java thought it right to deprecate the use of HashTable.
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LikeDiscussCorrect / Improve hashtable synchronized collections Why synchronized collections have been deprecated Why HashTable has been deprecated HashTable vs HashMap expert
Ans. 1. HashSet doesnt maintain its elements in any specific order and is all random whereas TreeSet maintains elements in natural order 9 order defined by the equals method of TreeSet element type )
2. TreeSet doesnt allow null elements whereas HashMap does.
3. As TreeSet orders elements and is hence insertion is comparatively slower.
4. HashSet performs basic operations like add(), remove(), contains(), size() etc. in a constant size time. A TreeSet performs these operations at the order of log(n) time.
5. HashMap in Java internally backs a HashSet. A NavigableMap backs a TreeSet internally.
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Ans. List - Members are stored in sequence in memory and can be accessed through index.
Set - There is no relevance of sequence and index. Sets doesn't contain duplicates whereas multiset can have duplicates.
Map - Contains Key , Value pairs.
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Ans. Garbage Collector wont remove a strong reference.
A soft reference will only get removed if memory is low.
A weak reference will get removed on the next garbage collection cycle.
A phantom reference will be finalized but the memory will not be reclaimed. Can be useful when you want to be notified that an object is about to be collected.
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Ans. Collections can only hold objects, It can't hold primitive data types.
Collections have performance overheads as they deal with objects and offer dynamic memory expansion. This dynamic expansion could be a bigger overhead if the collection class needs consecutive memory location like Vectors.
Collections doesn't allow modification while traversal as it may lead to concurrentModificationException.
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Q23. Can we call the garbage collector explicitly ?
Ans. Yes, We can call garbage collector directly but it doesn't guarantees that the gc will start executing immediately. This gc( ) method appears in both Runtime and System classes of java.lang package.
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Ans. The purpose of comparator interface is to compare objects of the same class to identify the sorting order. Sorted Collection Classes ( TreeSet, TreeMap ) have been designed such to look for this method to identify the sorting order, that is why class need to implement Comparator interface to qualify its objects to be part of Sorted Collections.
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Ans. BlockingQueue is a Queue that supports operations that wait for the queue to become non-empty when retrieving and removing an element, and wait for space to become available in the queue when adding an element.
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Ans. Collections class is a utility class having static methods for doing operations on objects of classes which implement the Collection interface. For example, Collections has methods for finding the max element in a Collection.
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Ans. Comparable interface is used for single sequence sorting i.e.sorting the objects based on single data member where as comparator interface is used to sort the object based on multiple data members.
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Ans. Collections are re-sizable in nature. We can increase or decrease the size as per recruitment.
Collections can hold both homogeneous and heterogeneous data's.
Every collection follows some standard data structures.
Collection provides many useful built in methods for traversing,sorting and search.
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Q37. Difference between ArrayList and LinkedList ?
Ans. LinkedList and ArrayList are two different implementations of the List interface. LinkedList implements it with a doubly-linked list. ArrayList implements it with a dynamically resizing array.
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