balamt/ December 9, 2018/ hello-world, How-To, Java

OOPs

              Object oriented Programming Concepts

  • Abstraction
  • Encapsulation
  • Polymorphism
  • Inheritance
  • Association
  • Composition
  • Aggregation

Abstraction

              Hiding the internal details and describing things in simple terms, there are many ways to achieve abstraction in OOPs, such as encapsulation and inheritance.

              For Example: a method that adds two integers, the method internal process is hidden from outer world.

Encapsulation

              It is a technique to implement abstraction in OOPs, used for access restriction to class method and members.

              Access modifier keywords are used for encapsulation in OOPs.

              For Example: encapsulation is achieved using the keywords private, public, and protected.

Polymorphism

              It is the concept where an object behaves differently in different situation.

              Two type: runtime polymorphism and compile time polymorphism.

              Compile time polymorphism: achieved by method overloading, (meaning same method name, but different arguments).

              Runtime polymorphism: This is implemented when we have IS-A relationship between objects. Called method overriding.

                             Example:Same method being used by another class which implements the super class which has this method declaration.

public interface shape{
            public void draw();
}
public class circle implements shape{
            @Override
            Public void draw(){
                //Method overridden from shape interface              
             }
}

Inheritance

              It is concept where object is based on another object. Inheritance is the mechanism of code reuse, Super class and sub class.

              We use extends keyword to implement the inheritance.

Association

              It the concept to define the relationship between objects, One to many relationship in Teachers and students object similarly students can have one to many relation with teacher class,

Aggregation

              Object has its own life cycle, but there is an ownership. Whenever we have HAS-A relationship between objects and ownership then it’s Aggregation.

Composition

              Composition is more restrictive form of Aggregation, when the contained object in HAS-A relationship cant exist in it own.

              Example: house has-a room, well room cannot exist without house.

Java Concurrency (Multi-threading)

              It is the ability to run several programs or several parts of a program in parallel, an time consuming task can be performed asynchronously or in parallel.

Process Vs Thread

Process runs independently and isolated of other processes, it cannot directly access shared data in other processes. OS provides the resources such as memory, CPU time.

Thread is so called lightweight process, it has its own call stack, but it can access shared data of other threads in the same process. Thread has its own memory cache, the thread shared data is stored in the memory cache. Thread can re-read the shared data.

Java application runs by default in once process, within the java application multiple threads to achieve parallel processing or asynchronous behavior.

Issues with Concurrency

              Since thread has its own call stack and also can access the shared data, because of this we have two problem, Visibility and access problem.

              Visibility problem: when thread A reads shared data and which is later modified by the thread B and thread A does not know about the change.

              Access problem: when many threads try to change the shared data at the same time

How to avoid multiple thread call same method?

              By using the synchronized keyword, which allows only one thread to enter into the method at the same time.

Collections

              Collections in java is a framework that provides an architecture to store and manipulate the group of objects.

              We can perform the following operations on the collection of data, Searching,sorting, insertion, deletion and manipulation.

              In Java collections we have few interfaces (Set, List, Queue, Deque) and Classes(ArrayList, Vector, LinkedList, PriorityQueue, HashSet, TreeSet)

All these interface and classes are presented in the java.util package in java

Hierarchy of Java Collection framework

Methods of Collection Interface

  1. public boolean add(E e) – Used to add element in the collection
  2. public boolean addAll(Collection<?Extends E> c) – used to add one collection to another.
  3. public boolean remove(Object element) – used to remove the object from the collection.
  4. public boolean removeAll(Collection<?>c) – used to remove specific collection in a collection
  5. default boolean removeIf(Predicate<? Super E> filter) – Used to remove the element based on the predicate.
  6. public boolean retainAll(Collection<?> c)  – used to remove all other element apart from the collection passes as argument.
  7. public int size() – gives the size of the collection
  8. public void clear() – it removed the total number of elements in the collection
  9. public boolean contains(Object element) – used to check whether the passed element argument is available in the collection.
  10. public boolean containsAll(Collection<?>c) – used to check whether the passed collection element are available in the collection.
  11. public Iterator iterator() –  used to provide the Iterator for the collection.
  12. public Object[] toArray() – used to convert the collection to an Array
  13. public <T> T[] toArray(T[] a)– it converts collection into array, it has runtime type of returned array.
  14. public boolean isEmpty()  – used to check whether the collection is empty.
  15. public boolean equals(Object element) – used to check whether the element matches with the collection.
  16. public int hashCode() – it returns the hashcode of the collection.

Iterator Interface

              It provides facility to iterate over the collection in forward direction only.

Method of iterator interface:
  • public boolean hashNext() – checks whether it has any more element.
    • public Object next() – it gives the item from the collection and moved the cursor to next element.
    • public void remove() – it removed the last element returned by the iterator, it is used less.

How to iterate over the collection

List sample = new ArrayList();
sample.add(“Hello”);
sample.add(“world”);
sample.add(“test”);

Iterator itr = sample.iterator();
while(itr.hasNext()){
System.out.println(itr.next());
}

Iterable Interface

              It is the root interface for all the collection classes, The Collection interface extends the Iterable interface.

              Hence all the classes and sub class of collection also implements the Iterable interface.

              Iterable interface contains only one abstract method (Iterator<T> iterator())

Collection Interface

              This is the interface which is implemented by all the classes in Collection framework.

List Interface

              It is the child interface of Collection interface, It inhibits a list data type structure in which we can store the ordered collection of objects. It can have duplicate values.

              List interface is implemented by the following classes,

  • ArrayList – List<T> list1 = newArrayList();
  • Stack  –List<T> list1 = new Stack();
  • Vector – List<T> list1 = new Vector();
  • LinkesList – List<T> list1 = newLinkedList();

Set Interface

              It represents the unordered set of elements which does not allow us to store duplicate elements. We can store at most one NULL value in Set.

              Set interface is implemented by the following classes,

  • HashSet  – Set<T>set1 = new HashSet(); – this uses the HashTable for storage, it contains uniqueelements.
  • LinkedHashSet – Set<T> set2 = new LinkedHashSet();- LinkedList implementation of Set Interface and extends HashSet, So It maintainsthe insertion order , permits null element and contains unique elements.
  • TreeSet – SortedSet<T> set3 = new TreeSet(); – Itis SortedSet, it is arranged in ascending order, TreeSet is fastwhen accessing compared to the HashSet.

Map Interface

              Map is based on Key, Value pair collection. Each key and value is know as entity. Map contains unique key. And it does not contains duplicate keys.

              There are two interface implementing Map and SortedMap and three classes

  • HashMap – It is implemented from Map and does not maintain any order
  • TreeMap – It is implemented from Mapand SortedMap, it maintains ascending order.
  • LinkedHashMap – It is implemented fromMap and it inherits HashMap class, it maintains the insertion order.
Method Description
V put(Object key, Object value) It is used to insert an entry in the map.
void putAll(Map map) It is used to insert the specified map in the map.
V putIfAbsent(K key, V value) It inserts the specified value with the specified key in the map only if it is not already specified.
V remove(Object key) It is used to delete an entry for the specified key.
boolean remove(Object key, Object value) It removes the specified values with the associated specified keys from the map.
Set keySet() It returns the Set view containing all the keys.
Set<Map.Entry<K,V>> entrySet() It returns the Set view containing all the keys and values.
void clear() It is used to reset the map.
V compute(K key, BiFunction<? super K,? super V,? extends V> remappingFunction) It is used to compute a mapping for the specified key and its current mapped value (or null if there is no current mapping).
V computeIfAbsent(K key, Function<? super K,? extends V> mappingFunction) It is used to compute its value using the given mapping function, if the specified key is not already associated with a value (or is mapped to null), and enters it into this map unless null.
V computeIfPresent(K key, BiFunction<? super K,? super V,? extends V> remappingFunction) It is used to compute a new mapping given the key and its current mapped value if the value for the specified key is present and non-null.
boolean containsValue(Object value) This method returns true if some value equal to the value exists within the map, else return false.
boolean containsKey(Object key) This method returns true if some key equal to the key exists within the map, else return false.
boolean equals(Object o) It is used to compare the specified Object with the Map.
void forEach(BiConsumer<? super K,? super V> action) It performs the given action for each entry in the map until all entries have been processed or the action throws an exception.
V get(Object key) This method returns the object that contains the value associated with the key.
V getOrDefault(Object key, V defaultValue) It returns the value to which the specified key is mapped, or defaultValue if the map contains no mapping for the key.
int hashCode() It returns the hash code value for the Map
boolean isEmpty() This method returns true if the map is empty; returns false if it contains at least one key.
V merge(K key, V value, BiFunction<? super V,? super V,? extends V> remappingFunction) If the specified key is not already associated with a value or is associated with null, associates it with the given non-null value.
V replace(K key, V value) It replaces the specified value for a specified key.
boolean replace(K key, V oldValue, V newValue) It replaces the old value with the new value for a specified key.
void replaceAll(BiFunction<? super K,? super V,? extends V> function) It replaces each entry’s value with the result of invoking the given function on that entry until all entries have been processed or the function throws an exception.
Collection values() It returns a collection view of the values contained in the map.
int size() This method returns the number of entries in the map.
Java Map Hierarchy
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