how to convert this multiple inheritance to interfaces in Java?

Question

I want to implement the following multiple inheritance case:

(took from http://www.learncpp.com/cpp-tutorial/117-multiple-inheritance/ )

Person           Employee
   |_________________|
              |
           Nurse

at the end I want to print the salaries from Employee and Nurse. In C++ it is easy to be done by using multiple inheritance, but I have problems with Java.

I have the following codes:

public class Person{
    protected String name;
    protected int age;
    public Person(String name,int age){
        this.name=name;
        this.age=age;
    }
}

and the interface:

public interface Employee{
    public double getSalary();
}

and the class Nurse:

public class Nurse extends Person implements Employee{
    private double salary;
    public Nurse(String name, int age, double salary){
        super(name,age);
        this.salary=salary;
    }
    @Override
    public double getSalary(){
        return salary;
    }
}

but I do not have a clue how to make the Employee to print its salary, because it is an interface. I do not want to use other abstract class called Employee. How to fix this?

Thanks

Answer 1

Assuming that all employees are people (unless you are hiring monkeys!!), could you not chain the classes?

public class Person {
    private String name;
    private int age;

    public Person(String name,int age){
        this.name = name;
        this.age = age;
    }
}

public class Employee extends Person {
    private double salary;

    public double getSalary(){
        return salary;
    }
}

public class Nurse extends Employee {
}

Answer 2

Java allows multiple inheritance of interfaces only, so in order to implement that you have to define an interface for every class you'd like to combine and compose them in a single class by delegating all methods derived from the partial interfaces to their implementation.

Person interface - as you've defined it

PersonImpl class :

public final class PersonImpl implements Person {
    private final String name;
    private final int age;

    public PersonImpl(String name, int age) {
        this.name = name;
        this.age = age;
    }

    @Override
    public String getName() {
        return name;
    }

    @Override
    public int getAge() {
        return age;
    }
}

EmployeeImpl class :

public final class EmployeeImpl implements Employee {
    private final double salary;

    public EmployeeImpl(double salary) {
        this.salary = salary;
    }

    @Override
    public double getSalary() {
        return salary;
    }
}

Nurse class - composed from to other classes and delegates the functionality to them:

public class Nurse implements Employee, Person {

    private final Employee employee;
    private final Person person;

    public Nurse(String name, int age, double salary) {
        person = new PersonImpl(name, age);
        employee = new EmployeeImpl(salary);
    }

    @Override
    public double getSalary() {
        return employee.getSalary();
    }

    @Override
    public String getName() {
        return person.getName();
    }

    @Override
    public int getAge() {
        return person.getAge();
    }
}

It's advisable to define an interface for every class you code in order to able that approach in the future.