Call a child class method from a parent class variable without using instanceof or casting

Question

I have 4 classes

public class User {
    private Account account;

    public void setAccount(Account account) {
        this.account = account;
    }

    public Account getAccount() {
        return this.account;
    }
}

public abstract class Account {
    private double balance;
    private String password;
    private String userName;
    private String mobileNumber;
    private Provider provider;
// some getters and setters
    
}

public class BankAccount extends Account {
 
        // this not in the parent class
    private String accountNumber;

    public String getAccountNumber() {
        return accountNumber;
    }

    
    public void setAccountNumber(String accountNumber) {
        this.accountNumber = accountNumber ;
    }
}

public class WalletAccount extends Account {

}

How I redesign this to make like this

Account account = new BankAcccount() ;
account.getAccountNumber() ;

without using instanceof or casting like

((BankAccount) account)getAccountNumber() ;

Redesign the classes to allow call of the BankAccount methods without using instanceof or casting using design patterns or add more classes any approach.

Answer 1

In the simple case you can add an abstract method to the parent class.

public abstract class Account {
    private double balance;
    private String password;
    private String userName;
    private String mobileNumber;
    private Provider provider;
// some getters and setters
    public abstract String getAccountNumber();
}

This makes an accountNumber property to be abstract. Every non-abstract subclass should implement the abstract method of Account class.

Then you can use

Account account = new BankAcccount() ;
account.getAccountNumber() ;

without using instanceof or casting like

((BankAccount) account)getAccountNumber() ;

This is allowed because abstract methods can use polymorphism.

---

Another way is to use interface to access the property of some subclass. Another subclass might not implement this interface if it doesn't have such property. When you need to access the property then you upcast to the interface without instanceof.

public interface AccountNumberAware {
  public String getAccountNumber();
}

public abstract class Account implements AccountNumberAware {
    private double balance;
    private String password;
    private String userName;
    private String mobileNumber;
    private Provider provider;
// some getters and setters

    // dummy implementation of interface
    public String getAccountNumber() {
        return null;
    }
    
}
public class BankAccount extends Account {
 
        // this not in the parent class
    private String accountNumber;

    public String getAccountNumber() {
        return accountNumber;
    }

    
    public void setAccountNumber(String accountNumber) {
        this.accountNumber = accountNumber ;
    }
}

Then you can use

Account account = new BankAcccount() ;
account.getAccountNumber() ;