Inheritance Constructor Invocation And super keyword

As we know instance variable can v initialized by constructor while creating Object
Now we need to know:

How to initialize the instance variables of both parent class and child class using constructors ?

Constructor Invocation

• Base class constructor can not be inherited.
• To initialize the base class member variable which are to be used by derived class Base class constructor must be invoked,during the Object creation of derived class.
• Base class constructor can be Default/parameterised constructor
• If Base class has default constructor then it is invoked by creating the object of Base/Derived class.
• For parameterised constructor derived class has to  invoke base class constructor explicitly from the Derived class constructor only

Note: Base class constructor can be invoked form derived class by "super" keyword

Super keyword

• Using super we can invoke base class constructor from derived class constructor
• It is used to prevent instance variable hiding
• It is used to refer member variable of base class
• Some of the point need to keep in mind while using super
• super can be only used in derived class constructor not in any other method
• super call must be the first line of derived class constructor.
• If the derived class does not explicitly call a base class constructor then Java automatically invoke the base class default constructor just befor the code inside the derived class constructor executes

class Customer{

  private int customerId;

  private String    private long contactNos[]=new long[3];

  private static int counter=1000;

  public Customer(String customerName, long teleNo1, long teleNo2,     long teleNo3){

  customerId=++counter;

  this.customerName=customerName;

   …….      

  }

  public int getCustomerId(){

  return customerId;

  }

}

class RegularCustomer extends Customer{

       private float discount;

       public RegularCustomercustomerName, long teleNo1,              long teleNo2, long teleNo3,float discount){

         super(customerName,teleNo1,teleNo2,teleNo3);

         this.discount=discount;

         }

         public float getDiscount(){

          return discount;

         }

}

class PrivilegedCustomer extends Customer{

       private String memCardType;

       public PrivilegedCustomer (String customerName, long teleNo1, long teleNo2, long teleNo3,String cardType){

       super(customerName,teleNo1,teleNo2,teleNo3);

       this.memCardType=cardType;

       }

       public String getMemCardType(){

         return memCardType;

       }

}

class Retail{

public static void main(String args[]){

    RegularCustomer regObj=new RegularCustomer("John",9980788712L,9886124566L,9496781256L,20.0f);
 PrivilegedCustomer  prvObj=new PrivilegedCustomer("Jennifer",2544871,989789956,949629145,"Silver");

  System.out.println("Regular Customer Details");

  System.out.println("Customer Id:" +regObj.getCustomerId());

  System.out.println("Discount:"+regObj.getDiscount());

  System.out.println("Privileged Customer Details");

  System.out.println("Customer Id:"+prvObj.getCustomerId());

  System.out.println("Membership Card Type:"+prvObj.getMemCardType());

}

}

Output:

Regular Customer Details

Customer Id:1001

Discount:20.0

Privileged Customer Details

Customer Id:1002

Membership Card Type:Silver