Why do we assign a parent reference to the child object in Java

Question

I am asking a quite simple question  but I am bit confused in this  Suppose I have a class Parent  public class Parent        int name     And have another class Child  public class Child extends Parent       int salary     And finally my Main java class public class Main        public static void main String   args                Parent parent   new Child            parent name   quot abcd quot            If I make a child object like Child child   new Child     Then child object can access both name and salary variables  My question is  Parent parent   new Child     gives the access of only name variable of Parent class  So what is the exact use of this line    Parent parent   new Child     And also when it is using dynamic polymorphism then why the variable of child class is not accessible after doing this Parent parent   new Child

User · Answer

It allows you to access all subclasses through a common parent interface. This is beneficial for running common operations available on all subclasses. A better example is needed:

public class Shape
{
  private int x, y;
  public void draw();
}

public class Rectangle extends Shape
{ 
  public void draw();
  public void doRectangleAction();
}

Now if you have:

List<Shape> myShapes = new ArrayList<Shape>();

You can reference every item in the list as a Shape, you don't have to worry if it is a Rectangle or some other type like let's say Circle. You can treat them all the same; you can draw all of them. You can't call doRectangleAction because you don't know if the Shape is really a rectangle.

This is a trade of you make between treating objects in a generic fashion and treating the specifically.

Really I think you need to read more about OOP. A good book should help: http://www.amazon.com/Design-Patterns-Explained-Perspective-Object-Oriented/dp/0201715945

User · Answer

When you compile your program the reference variable of the base class gets memory and compiler checks all the methods in that class  So it checks all the base class methods but not the child class methods  Now at runtime when the object is created  only checked methods can run   In case a method is overridden in the child class that function runs  Child class other functions aren t run because the compiler hasn t recognized them at the compile time

User · Answer

It s simple     Parent parent   new Child      In this case the type of the object is Parent  Ant Parent has only one properties  It s name   Child child   new Child      And in this case the type of the object is Child  Ant Child has two properties  They re name and  salary   The fact is that there s no need to initialize non-final field immediately at the declaration  Usually this   s done at run-time because often you cannot know exactly what exactly implementation will you need  For example imagine that you have a class hierarchy with class Transport at the head  And three subclasses  Car  Helicopter and Boat  And there s another class Tour which has field Transport  That is   class Tour      Transport transport        As long as an user hasn t booked a trip and hasn t chosen a particular type of transport you can t initialize this field  It s first    Second  assume that all of these classes must have a method go   but with a different implementation  You can define a basic implementation by default in the superclass Transport and own unique implementations in each subclass  With this initialization Transport tran  tran   new Car    you can call the method tran go   and get result without worrying about specific implementation  It   ll call overrided method from particular subclass    Moreover you can use instance of subclass everywhere where instance of superclass is used  For example you want provide opportunity to rent your transport  If you don t use polymorphism  you have to write a lot of methods for each case  rentCar Car car   rentBoat Boat boat  and so forth  At the same time polymorphism allows you to create one universal method rent Transport transport   You can pass in it object of any subclass of Transport  In addition  if over time your logic will increase up and you ll need to create another class in the hierarchy  When using polymorphism you don t need to change anything  Just extend class Transport and pass your new class into the method    public class Airplane extends Transport         implementation     and rent new Airplane     And new Airplane   go   in second case

User · Answer

I think all explanations above are a bit too technical for the people who are new to Object Oriented Programming  OOP   Years ago  it took me a while to wrap my head around this  as Jr Java Developer  and I really did no understand why we use a parent class or an interface to hide the actual class we are actually calling under the covers    The immediate reason why is to hide complexity  so that the caller does not need to change often  be hacked and jacked in laymen s terms   This makes a lot of sense  especially if you goals is to avoid creating bugs  And the more you modify code  the more likely it is that you will have some of them creep up on you  On the other hand  if you just extend code  it is way less likely that you will have bugs because you concentrate on one thing at a time and your old code does not change or changes just a bit  Imagine that you have simple application that allows the employees in the medical profession to create profiles  For simplicity  let s assume that we have only GeneralPractitioners  Surgeons  and Nurses  in reality there are many more specific professions  of course   For each profession  you want to store some general information and some specific to that professional alone  For example  a Surgeon may have general fields like firstName  lastName  yearsOfExperience as general fields but also specific fields  e g  specializations stored in an list instance variable  like List with contents simiar to  Bone Surgery    Eye Surgery   etc  A Nurse would not have any of that but may have list procedures they are familiar with  GeneralPractioners would have their own specifics  As a result  how you save a profile of a specifics  However  you don t want your ProfileManager class to know about these differences  as they will inevitably change and increase over time as your application expands its functionality to cover more medical professions  e g  Physio Therapist  Cardiologist  Oncologist  etc  All you want your ProfileManger to do is just say save    no matter whose profile it is saving  Thus  it is common practice to hide this behind and Interface  and Abstract Class  or a Parent Class  if you plan to allow creating a general medical employee   In this case  let s choose a Parent class and call it MedicalEmployee  Under the covers  it can reference any of the above specific classes that extend it  When the ProfileManager calls myMedicalEmployee save   the save   method will be polymorphically  many-structurally  be resolved to the correct class type that was used to create the profile originally  for example Nurse and call the save   method in that class  In many cases  you don t really know what implementation you will need at runtime  From the example above  you have no idea if a GeneralPractitioner  a Surgeon  or a Nurse would create a profile  Yet  you know that you need to save that profile once completed  no matter what  MedicalEmployee profile   does exactly that  It is replicated  overridden  by each specific type of MedicalEmployee - GeneralPractitioner  Surgeon  Nurse   The result of  1  and  2  above is that you now can add new medical professions  implement save   in each new class  thereby overriding the save   method in MedicalEmployee  and you don t have to modify ProfileManager at all

User · Answer

First  a clarification of terminology  we are assigning a Child object to a variable of type Parent  Parent is a reference to an object that happens to be a subtype of Parent  a Child   It is only useful in a more complicated example  Imagine you add getEmployeeDetails to the class Parent   public String getEmployeeDetails         return  Name      name      We could override that method in Child to provide more details    Override public String getEmployeeDetails         return  Name      name     Salary      salary      Now you can write one line of code that gets whatever details are available  whether the object is a Parent or Child   parent getEmployeeDetails      The following code   Parent parent   new Parent    parent name   1  Child child   new Child    child name   2  child salary   2000  Parent   employees   new Parent     parent  child    for  Parent employee   employees        employee getEmployeeDetails        Will result in the output   Name  1 Name  2 Salary  2000   We used a Child as a Parent  It had specialized behavior unique to the Child class  but when we called getEmployeeDetails   we could ignore the difference and focus on how Parent and Child are similar  This is called subtype polymorphism   Your updated question asks why Child salary is not accessible when the Childobject is stored in a Parent reference  The answer is the intersection of  polymorphism  and  static typing   Because Java is statically typed at compile time you get certain guarantees from the compiler but you are forced to follow rules in exchange or the code won t compile  Here  the relevant guarantee is that every instance of a subtype  e g  Child  can be used as an instance of its supertype  e g  Parent   For instance  you are guaranteed that when you access employee getEmployeeDetails or employee name the method or field is defined on any non-null object that could be assigned to a variable employee of type Parent  To make this guarantee   the compiler considers only that static type  basically  the type of the variable reference  Parent  when deciding what you can access  So you cannot access any members that are defined on the runtime type of the object  Child   When you truly want to use a Child as a Parent this is an easy restriction to live with and your code will be usable for Parent and all its subtypes  When that is not acceptable  make the type of the reference Child

User · Answer

I know this is a very old thread but I came across the same doubt once   So the concept of Parent parent   new Child    has something to do with early and late binding in java   The binding of private  static and final methods happen at the compile as they cannot be overridden and the normal method calls and overloaded methods are example of early binding   Consider the example   class Vehicle       int value   100      void start             System out println  Vehicle Started               static void stop             System out println  Vehicle Stopped             class Car extends Vehicle        int value   1000        Override     void start             System out println  Car Started               static void stop             System out println  Car Stopped               public static void main String args                  Car extends Vehicle         Vehicle vehicle   new Car            System out println vehicle value           vehicle start            vehicle stop              Output  100  Car Started  Vehicle Stopped  This is happening because stop   is a static method and cannot be overridden  So binding of stop   happens at compile time and start   is non-static is being overridden in child class  So  the information about type of object is available at the run time only late binding  and hence the start   method of Car class is called   Also in this code the vehicle value gives us 100 as the output because variable initialization doesn t come under late binding  Method overriding is one of the ways in which java supports run time polymorphism    When an overridden method is called through a superclass reference  Java determines which version superclass subclasses  of that method is to be executed based upon the type of the object being referred to at the time the call occurs  Thus  this determination is made at run time  At run-time  it depends on the type of the object being referred to  not the type of the reference variable  that determines which version of an overridden method will be executed   I hope this answers where Parent parent   new Child    is important and also why you weren t able to access the child class variable using the above reference

User · Answer

You declare parent as Parent  so java will provide only methods and attributes of the Parent class    Child child   new Child      should work  Or  Parent child   new Child      Child child  salary   1

User · Answer

This situation happens when you have several implementations  Let me explain  Supppose you have several sorting algorithm and you want to choose at runtime the one to implement  or you want to give to someone else the capability to add his implementation  To solve this problem you usually create an abstract class  Parent  and have different implementation  Child   If you write   Child c   new Child      you bind your implementation to Child class and you can t change it anymore  Otherwise if you use   Parent p   new Child      as long as Child extends Parent you can change it in the future without modifying the code   The same thing can be done using interfaces  Parent isn t anymore a class but a java Interface    In general you can use this approch in DAO pattern where you want to have several DB dependent implementations  You can give a look at FactoryPatter or AbstractFactory Pattern  Hope this can help you

User · Answer

Let s say you d like to have an array of instances of Parent class  and a set of child classes Child1  Child2  Child3 extending Parent  There re situations when you re only interested with the parent class implementation  which is more general  and do not care about more specific stuff introduced by child classes

User · Answer

If you assign parent type to a subclass it means that you agree with to use the common features of the parent class    It gives you the freedom to abstract from different subclass implementations  As a result limits you with the parent features    However  this type of assignment is called upcasting   Parent parent   new Child        The opposite is downcasting   Child child    Child parent    So  if you create instance of Child and downcast it to Parent you can use that type attribute name  If you create instance of Parent you can do the same as with previous case but you can t use salary because there s not such attribute in the Parent  Return to the previous case that can use salary but only if downcasting to Child      There s more detail explanation

User · Answer

for example we have a class Employee     int getsalary     return 0    String getDesignation       return    default            class Manager extends Employee     int getsalary       return 20000      String getDesignation       return    Manager           class SoftwareEngineer extends Employee     int getsalary       return 20000      String getDesignation       return    Manager           now if you want to set or get salary and designation of all employee  i e software enginerr manager etc   we will take an array of Employee and call both method getsalary   getDesignation Employee arr   new Employee 10    arr 1  new SoftwareEngieneer     arr 2  new Manager     arr n           for int i i gt arr length i        System out println arr i  getDesignation        arr i  getSalary        now its an kind of loose coupling because you can have different types of employees ex softeware engineer manager hr pantryEmployee etc so you can give object to the parent reference irrespective of different employee object

[java] Why do we assign a parent reference to the child object in Java?

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