What is the List interface?
What is a Map ?
- The List interface provides support for ordered collections of objects.
- Lists may contain duplicate elements.
The main implementations of the List interface are as follows :
- ArrayList : Resizable-array implementation of the List interface. The best all-around implementation of the List interface.
- Vector : Synchronized resizable-array implementation of the List interface with additional "legacy methods."
- LinkedList : Doubly-linked list implementation of the List interface. May provide better performance than the ArrayList implementation if elements are frequently inserted or deleted within the list. Useful for queues and double-ended queues (deques).
Some of the advantages ArrayList has over arrays are:
- It can grow dynamically
- It provides more powerful insertion and search mechanisms than arrays.
- ArrayList internally uses and array to store the elements, when that array gets filled by inserting elements a new array of roughly 1.5 times the size of the original array is created and all the data of old array is copied to new array.
- During deletion, all elements present in the array after the deleted elements have to be moved one step back to fill the space created by deletion. In linked list data is stored in nodes that have reference to the previous node and the next node so adding element is simple as creating the node an updating the next pointer on the last node and the previous pointer on the new node. Deletion in linked list is fast because it involves only updating the next pointer in the node before the deleted node and updating the previous pointer in the node after the deleted node.
If you need to support random access,
without inserting or removing elements from any place other than the
end, then ArrayList offers the optimal collection.
If, however, you need to frequently add and remove elements from the
middle of the list and
only access the list elements sequentially, then LinkedList offers the
better implementation.
Why are Iterators returned by ArrayList called Fail Fast ?
Because, if list is structurally
modified at any time after the iterator is created, in any way except
through the iterator's own remove or add methods, the iterator will
throw a ConcurrentModificationException. Thus, in the face of concurrent
modification, the iterator fails quickly and cleanly, rather than
risking arbitrary, non-deterministic behavior at an undetermined time
in the future.
What is an Iterator ?
- The Iterator interface is used to step through the elements of a Collection.
- Iterators let you process each element of a Collection.
- Iterators are a generic way to go through all the elements of a Collection no matter how it is organized.
- Iterator is an Interface implemented a different way for every Collection.
What is the Set interface ?
- The Set interface provides methods for accessing the elements of a finite mathematical set
- Sets do not allow duplicate elements
- Contains no methods other than those inherited from Collection
- It adds the restriction that duplicate elements are prohibited
- Two Set objects are equal if they contain the same elements
The main implementations of the List interface are as follows:
- HashSet
- TreeSet
- LinkedHashSet
- EnumSet
- A HashSet is an unsorted, unordered Set.
- It uses the hashcode of the object being inserted (so the more efficient your hashcode() implementation the better access performance you’ll get).
- Use this class when you want a collection with no duplicates and you don’t care about order when you iterate through it.
TreeSet is a Set implementation that
keeps the elements in sorted order. The elements are sorted according
to the natural order of elements or by the comparator provided at
creation time.
What is an EnumSet ?
An EnumSet is a specialized set for
use with enum types, all of the elements in the EnumSet type that is
specified, explicitly or implicitly, when the set is created.
Difference between HashSet and TreeSet ?
| HashSet | TreeSet |
|---|---|
| HashSet is under set interface i.e. it does not guarantee for either sorted order or sequence order. | TreeSet is under set i.e. it provides elements in a sorted order (acceding order). |
| We can add any type of elements to hash set. | We can add only similar types of elements to tree set. |
What is a Map ?
- A map is an object that stores associations between keys and values (key/value pairs).
- Given a key, you can find its value. Both keys and values are objects.
- The keys must be unique, but the values may be duplicated.
- Some maps can accept a null key and null values, others cannot.
The main implementations of the List interface are as follows:
- HashMap
- HashTable
- TreeMap
- EnumMap
TreeMap actually implements the
SortedMap interface which extends the Map interface. In a TreeMap the
data will be sorted in ascending order of keys according to the natural
order for the key's class, or by the comparator provided at creation
time. TreeMap is based on the Red-Black tree data structure.
How do you decide when to use HashMap and when to use TreeMap ?
For inserting, deleting, and locating
elements in a Map, the HashMap offers the best alternative. If,
however, you need to traverse the keys in a sorted order, then TreeMap
is your better alternative. Depending upon the size of your collection,
it may be faster to add elements to a HashMap, then convert the map to a
TreeMap for sorted key traversal.
Difference between HashMap and Hashtable ?
| HashMap | Hashtable |
|---|---|
| HashMap lets you have null values as well as one null key. | HashTable does not allows null values as key and value. |
| The iterator in the HashMap is fail-safe (If you change the map while iterating, you’ll know). | The enumerator for the Hashtable is not fail-safe. |
| HashMap is unsynchronized. | Hashtable is synchronized. |
Note: Only one
NULL is allowed as a key in HashMap. HashMap does not allow multiple
keys to be NULL. Nevertheless, it can have multiple NULL values.
What Are the different Collection Views That Maps Provide?
Maps Provide Three Collection Views.
- Key Set - allow a map's contents to be viewed as a set of keys.
- Values Collection - allow a map's contents to be viewed as a set of values.
- Entry Set - allow a map's contents to be viewed as a set of key-value mappings.
Create an implementation of the java.lang.Comparable interface that knows how to order your objects and pass it to java.util.Collections.sort(List, Comparator).
What is the Comparable interface ?
The Comparable interface is used to sort collections and arrays of objects using the
he Comparable interface in the generic form is written as follows:
Collections.sort() and java.utils.Arrays.sort() methods respectively. The objects of the class implementing the Comparable interface can be ordered.interface Comparable<T>
where T is the name of the type parameter.
All classes implementing the Comparable interface must implement the
All classes implementing the Comparable interface must implement the
compareTo() method that has the return type as an integer. The signature of the compareTo() method is as follows:int i = object1.compareTo(object2)
- If object1 < object2: The value of i returned will be negative.
- If object1 > object2: The value of i returned will be positive.
- If object1 = object2: The value of i returned will be zero.
| Comparable | Comparator |
|---|---|
| It uses the compareTo() method. int objectOne.compareTo(objectTwo). |
t uses the compare() method. int compare(ObjOne, ObjTwo) |
| It is necessary to modify the class whose instance is going to be sorted. | A separate class can be created in order to sort the instances. |
| Only one sort sequence can be created. | Many sort sequences can be created. |
| It is frequently used by the API classes. | It used by third-party classes to sort instances. |
Why Collection doesn’t extend Cloneable and Serializable interfaces?
Collection interface specifies group of Objects known as elements. How the elements are maintained is left up to the concrete implementations of Collection. For example, some Collection implementations like List allow duplicate elements whereas other implementations like Set don’t. A lot of the Collection implementations have a public clone method. However, it does’t really make sense to include it in all implementations of Collection. This is because Collection is an abstract representation. What matters is the implementation.
The semantics and the implications of either cloning or serializing come into play when dealing with the actual implementation; so concrete implementation should decide how it should be cloned or serialized, or even if it can be cloned or serialized.
So mandating cloning and serialization in all implementations is actually less flexible and more restrictive. The specific implementation should make the decision as to whether it can be cloned or serialized.
Why Map interface doesn’t extend Collection interface?
Although Map interface and it’s implementations are part of Collections Framework, Map are not collections and collections are not Map. Hence it doesn’t make sense for Map to extend Collection or vice versa.
If Map extends Collection interface, then where are the elements? Map contains key-value pairs and it provides methods to retrieve list of Keys or values as Collection but it doesn’t fit into the “group of elements” paradigm.
What is Collections Class?
Collection interface specifies group of Objects known as elements. How the elements are maintained is left up to the concrete implementations of Collection. For example, some Collection implementations like List allow duplicate elements whereas other implementations like Set don’t. A lot of the Collection implementations have a public clone method. However, it does’t really make sense to include it in all implementations of Collection. This is because Collection is an abstract representation. What matters is the implementation.
The semantics and the implications of either cloning or serializing come into play when dealing with the actual implementation; so concrete implementation should decide how it should be cloned or serialized, or even if it can be cloned or serialized.
So mandating cloning and serialization in all implementations is actually less flexible and more restrictive. The specific implementation should make the decision as to whether it can be cloned or serialized.
Why Map interface doesn’t extend Collection interface?
Although Map interface and it’s implementations are part of Collections Framework, Map are not collections and collections are not Map. Hence it doesn’t make sense for Map to extend Collection or vice versa.
If Map extends Collection interface, then where are the elements? Map contains key-value pairs and it provides methods to retrieve list of Keys or values as Collection but it doesn’t fit into the “group of elements” paradigm.
What is Collections Class?
java.util.Collections is
a utility class consists exclusively of static methods that operate on
or return collections. It contains polymorphic algorithms that operate
on collections, “wrappers”, which return a new collection backed by a
specified collection, and a few other odds and ends.This class contains
methods for collection framework algorithms, such as binary search,
sorting, shuffling, reverse etc
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