What is difference between ArrayList and vector? 

Ans: ) 

1) Synchronization - ArrayList is not thread-safe whereas Vector is thread-safe. In Vector class each method like add(), get(int i) is surrounded with a synchronized block and thus making Vector class thread-safe. 

2) Data growth - Internally, both the ArrayList and Vector hold onto their contents using an Array. When an element is inserted into an ArrayList or a Vector, the object will need to expand its internal array if it runs out of room. A Vector defaults to doubling the size of its array, while the ArrayList increases its array size by 50 percent. 
 
How can Arraylist be synchronized without using Vector? 

Ans) Arraylist can be synchronized using: 

• Collection.synchronizedList(List list) 

Other collections can be synchronized: 

• Collection.synchronizedMap(Map map) 

• Collection.synchronizedCollection(Collection c) 
 
If an Employee class is present and its objects are added in an arrayList. Now I want the list to be sorted on the basis of the employeeID of Employee class. What are the steps?

Ans) 1) Implement Comparable interface for the Employee class and override the compareTo(Object obj) method in which compare the employeeID 

2) Now call Collections.sort() method and pass list as an argument.

Now consider that Employee class is a jar file. 

1) Since Comparable interface cannot be implemented, create Comparator and override the compare(Object obj, Object obj1) method .

2) Call Collections.sort() on the list and pass comparator as an argument.
 
What is difference between HashMap and HashTable? 

Ans) Both collections implements Map. Both collections store value as key-value pairs. The key differences between the two are

1. Access to the Hashtable is synchronized on the table while access to the HashMap isn't. You can add it, but it isn't there by default.

2. Another difference is that iterator in the HashMap is fail-safe while the enumerator for the Hashtable isn't. If you change the map while iterating, you'll know. • Fail-safe - “if the Hashtable is structurally modified at any time after the iterator is created, in any way except through the iterator's own remove method, the iterator will throw a ConcurrentModificationException”

3. HashMap permits null values and only one null key, while Hashtable doesn't allow key or value as null.
 
What is difference between Arrays and ArrayList ? 

Ans) Arrays are created of fix size whereas ArrayList is of not fix size. It means that once array is declared as : 

int [] intArray= new int[6]; 
intArray[7]   // will give ArraysOutOfBoundException. 
Also the size of array cannot be incremented or decremented. But with arrayList the size is variable.
Once the array is created elements cannot be added or deleted from it. But with ArrayList the elements can be added and deleted at runtime. 
List list = new ArrayList();
list.add(1);
list.add(3);
list.remove(0) // will remove the element from the 1st location.

ArrayList is one dimensional but array can be multidimensional. 
            int[][][] intArray= new int[3][2][1];   // 3 dimensional array     
 
When to use ArrayList or LinkedList ?

Ans)  Adding new elements is pretty fast for either type of list. For the ArrayList, doing  random lookup using "get" is fast, but for LinkedList, it's slow. It's slow because there's no efficient way to index into the middle of a linked list. When removing elements, using ArrayList is slow. This is because all remaining elements in the underlying array of Object instances must be shifted down for each remove operation. But here LinkedList is fast, because deletion can be done simply by changing a couple of links. So an ArrayList works best for cases where you're doing random access on the list, and a LinkedList works better if you're doing a lot of editing in the middle of the list. 

Source : Read More - from java.sun 
 
Consider a scenario. If an ArrayList has to be iterate to read data only, what are the possible ways and which is the fastest? 

Ans) It can be done in two ways, using for loop or using iterator of ArrayList. The first option is faster than using iterator. Because value stored in arraylist is indexed access. So while accessing the value is accessed directly from the index. 
 
Now another question with respect to above question is if accessing through iterator is slow then why do we need it and when to use it.

Ans) For loop does not allow the updation in the array(add or remove operation) inside the loop whereas Iterator does. Also Iterator can be used where there is no clue what type of collections will be used because all collections have iterator. 
 
Which design pattern Iterator follows?

Ans) It follows Iterator design pattern. Iterator Pattern is a type of behavioral pattern. The Iterator pattern is one, which allows you to navigate through a collection of data using a common interface without knowing about the underlying implementation. Iterator should be implemented as an interface. This allows the user to implement it anyway its easier for him/her to return data. The benefits of Iterator are about their strength to provide a common interface for iterating through collections without bothering about underlying implementation.

Example of Iteration design pattern - Enumeration The class java.util.Enumeration is an example of the Iterator pattern. It represents and abstract means of iterating over a collection of elements in some sequential order without the client having to know the representation of the collection being iterated over. It can be used to provide a uniform interface for traversing collections of all kinds. 
 
Why is it preferred to declare: List<String> list = new ArrayList<String>(); instead of ArrayList<String> = new ArrayList<String>();

Ans) It is preferred because:

If later on code needs to be changed from ArrayList to Vector then only at the declaration place we can do that. 
The most important one – If a function is declared such that it takes list. E.g void showDetails(List list);
When the parameter is declared as List to the function it can be called by passing any subclass of List like ArrayList,Vector,LinkedList making the function more flexible 
 
How to sort list in reverse order? 

Ans) To sort the elements of the List in the reverse natural order of the strings, get a reverse Comparator from the Collections class with reverseOrder(). Then, pass the reverse Comparator to the sort() method. 

List list = new ArrayList(); 

Comparator comp = Collections.reverseOrder();

Collections.sort(list, comp) 

How to sort list of strings - case insensitive? 

Ans) using Collections.sort(list, String.CASE_INSENSITIVE_ORDER);
 
Can a null element added to a set ? 

Ans) A null element can be added only if the set contains one element because when a second element is added then as per set defination a check is made to check duplicate value and comparison with null element will throw NullPointerException.

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