What is difference between Set and List in Java?
Most of Java programmer knows that Set doesn't allowed duplicate while List does and List maintains insertion order while Set doesn't. What is key here is to show interviewer that you can decide which collection is more suited based on requirements.
Difference between HashMap and Hashtable?
HashMap is not synchronized while hashtalbe is not or hashmap is faster than hashtable etc. What could go wrong is that if he placed another followup question like how hashMap works in Java or can you replace Hashtable with ConcurrenthashMap etc. See HashTable vs HashMap in Java for detailed answer of this interview question.
What is difference between Synchronized Collection and ConcurrentCollection?
Java5 has added several new ConcurrentCollection classes e.g. ConcurrentHashMap, CopyOnWriteArrayList, BlockingQueue etc, which has made Interview questions on Java Collection even trickier. Java Also provided way to get Synchronized copy of collection e.g. ArrayList, HashMap by using Collections.synchronizedMap() Utility function.One Significant difference is that ConccurentCollections has better performance than synchronized Collection because they lock only a portion of Map to achieve concurrency and Synchronization.
What is difference between fail-fast and fail-safe Iterators?
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
What is difference between Iterator and Enumeration?
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.
When do you use ConcurrentHashMap in Java?
ConcurrentHashMap is better suited for situation where you have multiple readers and one Writer or fewer writers since Map gets locked only during write operation. If you have equaled number of reader and writer than ConcurrentHashMap will perform in line of hashtable or synchronized hashMap.
How do you Sort objects on collection?
Sorting is implemented using Comparable and Comparator in Java and when you call Collections.sort() it gets sorted based on natural order specified in CompareTo method while Collections.sort(Comparator) will sort objects based on compare() method of Comparator.
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