Calling child method, when cast to parent type in java

Question

I am having problems with some course work i'm trying to finish off and any help would be appreciated!

I have 3 types of accounts which extend an abstract type "Account".. [CurrentAccount, StaffAccount and MortgageAccount].

I am trying to read in some data from file and create account objects along with user objects to add to hashmaps stored within the program.

When I create account objects I use a temporary variable of type Account and define its subtype dependant on the data read in.

for example:

Account temp=null;

if(data[i].equalsIgnoreCase("mortgage"){
    temp= new MortgageAccount;
}

Problem is when I try to call a method belonging to type MortgageAccount..

Do I need a temp variable of each type, StaffAccount MortgageAccount and CurrentAccount and use them coresspondingly in order to use their methods?

Thanks in advance!

Answer 1

If all your account objects have the same interface, meaning they declare the same methods and they only differ in how they are implemented then you do not need a variable for each type.

However, if you want to call a method that is specific to the sub-type then you would need a variable of that type, or you would need to cast the reference before you would be able to call the method.

class A{
    public void sayHi(){ "Hi from A"; }
}


class B extends A{
    public void sayHi(){ "Hi from B"; 
    public void sayGoodBye(){ "Bye from B"; }
}


main(){
  A a = new B();

  //Works because the sayHi() method is declared in A and overridden in B. In this case
  //the B version will execute, but it can be called even if the variable is declared to
  //be type 'A' because sayHi() is part of type A's API and all subTypes will have
  //that method
  a.sayHi(); 

  //Compile error because 'a' is declared to be of type 'A' which doesn't have the
  //sayGoodBye method as part of its API
  a.sayGoodBye(); 

  // Works as long as the object pointed to by the a variable is an instanceof B. This is
  // because the cast explicitly tells the compiler it is a 'B' instance
  ((B)a).sayGoodBye();

}