41.Difference between Iterator and Enumeration?
Iterators differ from enumerations in three ways:
- Iterators allow the caller to remove elements from the underlying collection during the iteration with its remove() method. You can not add/remove elements from a collection when using enumerator.
- Enumeration is available in legacy classes i.e Vector/Stack etc. whereas Iterator is available in all modern collection classes.
- Another minor difference is that Iterator has improved method names e.g. Enumeration.hasMoreElement() has become Iterator.hasNext(), Enumeration.nextElement() has become Iterator.next() etc.
42.Difference between HashMap and HashSet?
HashMap is collection of key-value pairs whereas HashSet is un-ordered collection of unique elements. That’s it. No need to describe further.
43.Difference between Iterator and ListIterator?
There are three Differences are there:
- We can use Iterator to traverse Set and List and also Map type of Objects. But List Iterator can be used to traverse for List type Objects, but not for Set type of Objects.
- By using Iterator we can retrieve the elements from Collection Object in forward direction only whereas List Iterator, which allows you to traverse in either directions using hasPrevious() and previous() methods.
- ListIterator allows you modify the list using add() remove() methods. Using Iterator you can not add, only remove the elements.
44.Difference between TreeSet and SortedSet?
SortedSet is an interface which TreeSet implements. That’ it !!
45.Difference between ArrayList and LinkedList?
- LinkedList store elements within a doubly-linked list data structure. ArrayList store elements within a dynamically resizing array.
- LinkedList allows for constant-time insertions or removals, but only sequential access of elements. In other words, you can walk the list forwards or backwards, but grabbing an element in the middle takes time proportional to the size of the list. ArrayLists, on the other hand, allow random access, so you can grab any element in constant time. But adding or removing from anywhere but the end requires shifting all the latter elements over, either to make an opening or fill the gap.
- LinkedList has more memory overhead than ArrayList because in ArrayList each index only holds actual object (data) but in case of LinkedList each node holds both data and address of next and previous node.
46.How to make a collection read only?
Use following methods:
- Collections.unmodifiableList(list);
- Collections.unmodifiableSet(set);
- Collections.unmodifiableMap(map);
These methods takes collection parameter and return a new read-only collection with same elements as in original collection.
47.How to make a collection thread safe?
Use below methods:
- Collections.synchronizedList(list);
- Collections.synchronizedSet(set);
- Collections.synchronizedMap(map);
Above methods take collection as parameter and return same type of collection which are synchronized and thread safe.
48.Why there is not method like Iterator.add() to add elements to the collection?
The sole purpose of an Iterator is to enumerate through a collection. All collections contain the add() method to serve your purpose. There would be no point in adding to an Iterator because the collection may or may not be ordered. And add() method can not have same implementation for ordered and unordered collections.
49.What are different ways to iterate over a list?
You can iterate over a list using following ways:
- Iterator loop
- For loop
- For loop (Advance)
- While loop
50.What do you understand by iterator fail-fast property?
Fail-fast Iterators fail as soon as they realized that structure of Collection has been changed since iteration has begun. Structural changes means adding, removing or updating any element from collection while one thread is Iterating over that collection.
Fail-fast behavior is implemented by keeping a modification count and if iteration thread realizes the change in modification count it throws ConcurrentModificationException.
51.What is difference between fail-fast and fail-safe?
You have understood fail-fast in previous question. Fail-safe iterators are just opposite to fail-fast. They never fail if you modify the underlying collection on which they are iterating, because they work on clone of Collection instead of original collection and that’s why they are called as fail-safe iterator.
Iterator of CopyOnWriteArrayList is an example of fail-safe Iterator also iterator written by ConcurrentHashMap keySet is also fail-safe iterator and never throw ConcurrentModificationException.
52.How to avoid ConcurrentModificationException while iterating a collection?
You should first try to find another alternative iterator which are fail-safe. For example if you are using List and you can use ListIterator. If it is legacy collection, you can use enumeration.
If above options are not possible then you can use one of three changes:
- If you are using JDK1.5 or higher then you can use ConcurrentHashMap and CopyOnWriteArrayList classes. It is the recommended approach.
- You can convert the list to an array and then iterate on the array.
- You can lock the list while iterating by putting it in a synchronized block.
Please note that last two approaches will cause a performance hit.
53.What is UnsupportedOperationException?
This exception is thrown on invoked methods which are not supported by actual collection type.
For example, if you make a read-only list list using “Collections.unmodifiableList(list)” and then call add() or remove() method, what should happen. It should clearly throw UnsupportedOperationException.
54.Which collection classes provide random access of it’s elements?
ArrayList, HashMap, TreeMap, Hashtable classes provide random access to it’s elements.
55.What is BlockingQueue?
A Queue that additionally supports operations that wait for the queue to become non-empty when retrieving an element, and wait for space to become available in the queue when storing an element.
BlockingQueue methods come in four forms: one throws an exception, the second returns a special value (either null or false, depending on the operation), the third blocks the current thread indefinitely until the operation can succeed, and the fourth blocks for only a given maximum time limit before giving up.
Read the example usage of blocking queue in post : How to use blocking queue?
56.What is Queue and Stack, list down their differences?
A collection designed for holding elements prior to processing. Besides basic Collection operations, queues provide additional insertion, extraction, and inspection operations.
Queues typically, but do not necessarily, order elements in a FIFO (first-in-first-out) manner.
Stack is also a form of Queue but one difference, it is LIFO (last-in-first-out).
Whatever the ordering used, the head of the queue is that element which would be removed by a call to remove() or poll(). Also note that Stack and Vector are both synchronized.
Usage: Use a queue if you want to process a stream of incoming items in the order that they are received.Good for work lists and handling requests.
Use a stack if you want to push and pop from the top of the stack only. Good for recursive algorithms.
57.What is Comparable and Comparator interface?
In java. all collection which have feature of automatic sorting, uses compare methods to ensure the correct sorting of elements. For example classes which use sorting are TreeSet, TreeMap etc.
To sort the data elements a class needs to implement Comparator or Comparable interface. That’s why all Wrapper classes like Integer,Double and String class implements Comparable interface.
Comparable helps in preserving default natural sorting, whereas Comparator helps in sorting the elements in some special required sorting pattern. The instance of comparator if passed usually as collection’s constructor argument in supporting collections.
58.What are Collections and Arrays classes?
Collections and Arrays classes are special utility classes to support collection framework core classes. They provide utility functions to get read-only/ synchronized collections, sort the collection on various ways etc.
Arrays also helps array of objects to convert in collection objects. Arrays also have some functions which helps in copying or working in part of array objects.
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