Call a child class method from a parent class object

Question

I have the following classes  class Person       private String name      void getName          class Student extends Person      String class      void getClass           class Teacher extends Person      String experience      void getExperience            This is just a simplified version of my actual schema  Initially I don t know the type of person that needs to be created  so the function that handles the creation of these objects takes the general Person object as a parameter    void calculate Person p         Now I want to access the methods of the child classes using this parent class object  I also need to access parent class methods from time to time so I CANNOT MAKE IT ABSTRACT      I guess I simplified too much in the above example  so here goes   this is the actual structure    class Question        private attributes       private QuestionOption option       getters and setters for private attributes       public QuestionOption getOption             class QuestionOption            class ChoiceQuestionOption extends QuestionOption   private boolean allowMultiple   public boolean getMultiple             class Survey    void renderSurvey Question q                       Depending on the type of question  choice  dropdwn or other  I have to render           the question on the UI  The class that calls this doesnt have compile time            knowledge of the type of question that is going to be rendered  Each question            type has its own rendering function  If this is for choice   I need to access            its functions using q                  if q getOption   getMultiple                           The if statement says  cannot find getMultiple for QuestionOption   OuestionOption has many more child classes that have different types of methods that are not common among the children  getMultiple is not common among the children

User · Answer

A parent class should not have knowledge of child classes. You can implement a method calculate() and override it in every subclass:

class Person {
    String name;
    void getName(){...}
    void calculate();
}

and then

class Student extends Person{
    String class;
    void getClass(){...}

    @Override
    void calculate() {
        // do something with a Student
    }
}

and

class Teacher extends Person{
    String experience;
    void getExperience(){...}

    @Override
    void calculate() {
        // do something with a Student
    }

}

By the way. Your statement about abstract classes is confusing. You can call methods defined in an abstract class, but of course only of instances of subclasses.

In your example you can make Person abstract and the use getName() on instanced of Student and Teacher.

User · Answer

Many of the answers here are suggesting implementing variant types using  Classical Object-Oriented Decomposition    That is  anything which might be needed on one of the variants has to be declared at the base of the hierarchy   I submit that this is a type-safe  but often very bad  approach   You either end up exposing all internal properties of all the different variants  most of which are  invalid  for each particular variant  or you end up cluttering the API of the hierarchy with tons of procedural methods  which means you have to recompile every time a new procedure is dreamed up    I hesitate to do this  but here is a shameless plug for a blog post I wrote that outlines about 8 ways to do variant types in Java   They all suck  because Java sucks at variant types   So far the only JVM language that gets it right is Scala   http   jazzjuice blogspot com 2010 10 6-things-i-hate-about-java-or-scala-is html  The Scala creators actually wrote a paper about three of the eight ways   If I can track it down  I ll update this answer with a link   UPDATE  found it here

User · Answer

NOTE  Though this is possible  it is not at all recommended as it kind of destroys the reason for inheritance  The best way would be to restructure your application design so that there are NO parent to child dependencies  A parent should not ever need to know its children or their capabilities   However   you should be able to do it like   void calculate Person p          Student p  method        a safe way would be   void calculate Person p        if p instanceof Student    Student p  method

User · Answer

Why don t you just write an empty method in Person and override it in the children classes  And call it  when it needs to be   void caluculate Person p     p dotheCalculate        This would mean you have to have the same method in both children classes  but i don t see why this would be a problem at all

User · Answer

class Car extends Vehicle           protected int numberOfSeats   1           public int getNumberOfSeats                 return this numberOfSeats                      public void  printNumberOfSeats                   return this numberOfSeats              System out println numberOfSeats                         Parent class    class Vehicle           protected String licensePlate   null           public void setLicensePlate String license                this licensePlate   license              System out println licensePlate                  public static void main String   args           Vehicle c   new Vehicle           c setLicensePlate  LASKF12341        Used downcasting to call the child method from the parent class     Downcasting   It   s the casting from a superclass to a subclass         Vehicle d   new Car            Car  d  printNumberOfSeats

User · Answer

I had the same situation and I found a way around with a bit of engineering as follows - -   You have to have your method in parent class without any parameter and use - -  Class lt   extends Person gt  cl   this getClass       inside parent class  Now  with  cl  you can access all child class fields with their name and  initialized values by using - -  cl getDeclaredFields    cl getField  myfield       and many more  In this situation your  this  pointer will reference your child class object if you are calling parent method through your child class object  Another thing you might need to use is Object obj   cl newInstance      Let me know if still you got stucked somewhere

User · Answer

One possible solution can be   class Survey    void renderSurvey Question q               Depending on the type of question  choice  dropdwn or other  I have to render       the question on the UI  The class that calls this doesnt have compile time        knowledge of the type of question that is going to be rendered  Each question        type has its own rendering function  If this is for choice   I need to access        its functions using q          if q getOption   instanceof ChoiceQuestionOption          ChoiceQuestionOption choiceQuestion    ChoiceQuestionOption q getOption        boolean result   choiceQuestion getMultiple          do something with result

[java] Call a child class method from a parent class object

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