Class Basics: Below is the bare minimum requirement to create a class and object

•  Defining a class
  (default private) class Class_name   //declaring class name
   {
  //class body starts within these two braces
   //constants, method, fields, 
              }
    public (accessor) class Entry_point //this is required
   {
   Class_name cn = new Class_name();
   // instance of a class and object 
   //created with new keyword
  }
  

•  Inheritance: 
  (public/private) class derived_class 
                  extends super_class 
                  implements custome_interface
  {
  //field, constructor, and method declarations
   }
  
   

•  derived class is like a child class, 
    can inherit from only one caring parent class
    (public or private)  class derived_class extends
     super_class implements custome_interface
        {
  //field, constructor, and method declarations
  private class_nested 
  {
  my_parent.my_self my_object = new my_parent.my_self my();
           }
         }

• Inner class:

    There are two additional types of inner classes.
    Local and Anonymous Inner Classes
    You can declare an inner class within the body of a method.
                  
    public class class_name
     {
     public void a_method(){ class A_local 
     {  //code implementation          } }
      } 
     Such a class is known as a local inner class.
                   
      You can also declare an inner class within the body 
      of a method without naming it.
      These classes are known as 
      anonymous inner classes. 
                  
       class Process 
    {
    public Process() 
    { System.out.println("Process outer constructor");}
    public int n2; String name="";
     public void a_method(String str_method)
    {
    {
     //contents of anonymous class
       name = this.getClass().getName();
     System.out.println("Received at anonymous"
     +" inner class" + str_method);
       }//end of anonymous class
      System.out.println("Retreive class name "+
      "at inner class: " + name);
                   }  //end method_1
                }//end of class process
  

Class accessor : private, public and abstract, if a class is not specified, default is private.

fields : that indicates the type of data is being hold by an object. Also know as instance variable

constructor : keeps track of the object and evoked when an object (new instances of class using new key word) is instantiated. The constructors are created just like other method, but it is named after the class.

• class-name Do_something(); // declaration
• public Do_something() { // no parameter }
• public Do_something(String str, int n1) {
  // with two parameter to handle two arguments from a caller function }; }

method : process client's request. One method can call another with some rules and whether these method would be accessible out side of  the class or not, is determined by the accessor keywords (access modifiers)  like public, protected, private, final or abstract.

• public methods are accessible  with object created in the client's routine, then makes a request using appropriate syntaxes.
• private: If a class is not

• One Line with no argument/parameter pair.
  • Class-Name a-variable = new Class-Name();
• One Line with one  argument/parameter pair.
  • Class-Name a-variable = new Class-Name(feed-argumet-to-a-parameter);

import java.io.*;
// javac simple_client_1.java
public class simple_client_1
{
public static void main(String args[])throws IOException
{
System.out.println("Client Starts");
//creating object where ss is reference to the class simple server_1
simple_server_1 ss = new simple_server_1();
ss.process();
System.out.println("Client ends");
}//end of main
}//end of class
class simple_server_1
{
//constructor
public simple_server_1()

{ System.out.println("Server Constructor Starts as an object is created");}
//these are the fileds
private int n1 = 0;
private int n2 = 0;
public String str2="Call Me if you need";
//method starts
//accesor type public
//return type void
//() place holder for parameters
public void process() throws IOException
{
//implementation with code satrs
InputStreamReader isr = new InputStreamReader(System.in);
BufferedReader br = new BufferedReader(isr);
////////////////
String str= "";
try {
System.out.println("---While Loop---");
while(( n1 !=9)&&(n2 !=11) )
{
System.out.print("Please enter number: ");
str = br.readLine();
int n3 = Integer.parseInt(str);
n1 = n3;
System.out.print("Please enter pin: ");
str= br.readLine();
int n4=Integer.parseInt(str);
n2= n4;
System.out.println("You Entered : " + n1 + n2);
}
}//try
catch(NumberFormatException e)

 { System.out.println("data was blank");}
///
System.out.println();
}//end of main
}//end of class