Polymorphism

Polymorphism means “many forms” (poly = many, morph = forms).
It is the ability of an object or method to take different forms and perform a single action in multiple ways.

In Java, polymorphism is categorized into two types:

1. Compile-time Polymorphism (also called Static Polymorphism)

2. Runtime Polymorphism (also called Dynamic Polymorphism)

Polymorphism in Java can be achieved through:

• Method Overloading → Used to implement compile-time polymorphism.
• Method Overriding → Used to implement runtime polymorphism.

Environment Setup

Set the Path :

1.Go on my computer icon and right click, after that click on properties option.

2.Now click on Advance Settings

3.Click on advance option

4.Click on Environment Variable.

5.Click on new button

6.Now one dialog box is appear, now ignore this but don't close

7.Now open my computer open c:/ > Programs Files > Java > jdk1.8.0_144 > bin copy this path

C:\Program Files\Java\jdk1.8.0_144\bin

Now come back on previous open dilogbox and write variable name 'path' and for variable value paste all copied path upto the bin folder. Put .; at the end. It (.) selects all the tools from the bin folder.

What are different ways of iterating over keys, values and entry in Map?

What are different ways of iterating over keys, values and entry in Map?

Answer: 
Create and put key-value pairs in HashMap

package com.tutorial4u; import java.util.HashMap; import java.util.Iterator; import java.util.Map; public class HashMapDemo {      public static void main(String[] args) {            Map hashMap = new HashMap();            hashMap.put(11, "CSS");            hashMap.put(21, "IT");            hashMap.put(31, "ECE");            hashMap.put(41, "EEE");      }      }

Iterate over keys :-

hashMap.keySet().iterator() method returns iterator to iterate over keys in HashMap.

Iterator keyIterator =hashMap.keySet().iterator();            while(keyIterator.hasNext()){            System.out.println(keyIterator.next());        }

/*OUTPUT
21
41
11

31
*/
Iterate over values :-

hashMap.values().iterator() method returns iterator to iterate over keys in HashMap.

Iterator valueIterator=hashMap.values().iterator();            while(valueIterator.hasNext()){            System.out.println(valueIterator.next());        }

/*OUTPUT
IT
EEE
CSS

ECE
*/

Iterate over Entry-

hashMap.entrySet().iterator() method returns iterator to iterate over keys in HashMap.

Iterator entryIterator=hashMap.entrySet().iterator();            while(entryIterator.hasNext()){            System.out.println(entryIterator.next());        }

/*OUTPUT
21=IT
41=EEE
11=CSS
31=ECE

*/

Encapsulation

Encapsulation is a mechanism where you bind your data and code together as a single unit. It also means to hide your data in order to make it safe from any modification. 

Real Time Example :

take a example of news channel. Here the journalist finds the information and he only knows from where he had sourced this information.So it's source is protected inside their workplace.And the public only knows about the news. And the source is hidden.

We can achieve encapsulation in Java by: 

Declaring the variables of a class as private. Providing public setter and getter methods to modify and view the variables values.
Example :
               

package com.tutorial4u; class Employee{ private int empNo; private String empName; private int salary; public int getEmpNo() { return empNo; } public void setEmpNo(int empNo) { this.empNo = empNo; } public String getEmpName() { return empName; } public void setEmpName(String empName) { this.empName = empName; } public int getSalary() { return salary; } public void setSalary(int salary) { this.salary = salary; } } public class EncapsulationDemo{ public static void main(String[] args) {       Employee emp = new Employee();       emp.setEmpNo(101);       emp.setEmpName("Ashish Kumar");       emp.setSalary(1000);       System.out.println("Employee No : "+emp.getEmpNo());       System.out.println("Employee Name is : "+emp.getEmpName());       System.out.println("Employee salary is : "+emp.getSalary()); } } Output : Employee No : 101 Employee Name is : Ashish Kumar Employee salary is : 1000

   
        
The main advantages of encapsulation are:

1. We can achieve security.
2. Enhancement will become very easy.
3. It improves maintainability of the application. 
4. It provides flexibility to the user to use system very easily.

Disadvantage :

The main disadvantage of encapsulation is it increases length of the code and slows down execution.

How many ways create an Object In Java?

Ques : How many ways create an Object In Java?

In interview asked how to create an object in java generally and everyone says
using the “new” operator to create an Object of a Class. But is it the only way to create an Object?

Simple answers is NO, then in how many ways we can create Object of a Class. 

There are four different ways to create objects in java:

• Using New keyword
• Class.forName()
• Using Clone()
• Using Deserilization

1. Using new keyword:

This is the most common way to create an object in java. Almost 99% of objects are created in this way.

MyObject object = new Object();

2. Using Class.forName():

If we know the name of the class & if it has a public default constructor we can create an object in this way.

 

MyObject obj = (MyObject) class.forName("object").newInstance();

3. Using clone():

The clone() can be used to create a copy of an existing object.

             

MyObject obj = new MyObject();   MyObject object = (MyObject )obj.clone();

4. Using Object Deserialization :

Object deserialization is nothing but creating an object from its serialized form.

ObjectInputStream istream = new objectInputStream(some data); MyObject object=(MyObject) instream.readObject();