如何使用由 class 实现的接口方法，它还扩展了另一个 class？

Question

I have some classes named as "Account", "CurrentAccount", "SavingsAccount". "CurrentAccount" and "SavingsAccount" extends "Account", Also "CurrentAccount" implements an interface "TaxDeduction". "TaxDeduction" has method by name of "deductTax()", whose body is defined in "CurrentAccount".

public class CurrentAccount extends Account implements TaxDeduction {
  public void deductTax() {
   double tax = (super.getBalance() * taxRate) / 100;
    super.setBalance(super.getBalance() - tax);
    }
}

public interface TaxDeduction {
    static double taxRate=8.5;
    void deductTax();
}

Now I made an array of Account[] which stores objects of "CurrentAccount" & "SavingsAccount". When I retrieve a "CurrentAccount" Object in main class and try to use "deductTax()" method then I get error that "deductTax()" method is not resolved in "Account" whereas I can use all other normal methods in "CurrentAccount" class. How can I resolve this issue?

Answer 1

Java is a statically typed language. If you have a variable of type Account , you can only call methods defined in Account (and its superclasses and implemented interfaces). Attempting to call a method not defined in Account will result in a compiletime error, because as far as the compiler is concerned, the value held in the variable is only an Account .

So, the compiler will not allow you to call methods of TaxDeduction , then either Account (or one of its superclasses) must implement it, or you must check if the instance held by the variable is an instance of TaxDeduction (using instanceof ), and then cast to TaxDeduction and call the method.

When you use instanceof you check the actual type at runtime, and the cast tells the compiler that you're sure it's actually a TaxDeduction , so you can call the method(s) defined in TaxDeduction . When you're wrong about the type in the cast, you'll get a runtime exception, ClassCastException (which is why it's recommended to use instanceof before casting).

In other words, something like:

Account[] accounts = ...;
for (Account account : accounts) {
    if (account instanceof TaxDeduction) {
        ((TaxDeduction) account).deductTax();
    }
}

Or in Java 16 and higher ( JEP 394 ):

Account[] accounts = ...;
for (Account account : accounts) {
    if (account instanceof TaxDeduction td) {
        td.deductTax();
    }
}