Polymorphism with Java class

Question

suppose we have a class Employee with the following data field and function. The program attempts to see if 2 Employees are equal by comparing their name and address

public class Employee{
  private String name;
  private double hours;
  private double rate;
  private Address address;
    @Override
    public boolean equals(Object obj){
        if(obj == this) return true;
        if(obj == null) return false;
        if(this.getClass() == obj.getClass()){
            Employee other = (Employee) obj;
            return name.equals(other.name) && address.equals(other.address);
        }else{
            return false;
        }
    }

why didn't we do this instead public boolean equals(Employee obj) (different parameters)?

amit · Accepted Answer · 2012-10-15 23:48:53Z

5

(1) if you do new Empolyee().equals(null) - this will happen.

(2) Because the Object class declared equals(Object) - and you cannot put Employee as a parameter in order to override this method. You can only widen the parameter when overriding a method, never narrow it.
What will happen if you then do:

Object o = new Employee();
o.equals(new Object());

The "method" will have no idea how to handle this case, while it is perfectly valid and cannot be known (in the general case) in compile time.

To avoid it, this type of variance is not allowed when overriding a method, and you will get a compilation error if you try to do it.

answered Oct 15, 2012 at 23:48

amit

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7 Comments

user133466 Over a year ago

why would i ever intentionally pass a null to new Empolyee().equals(null). is if(obj == null) return false; written to cover all the possibilities?

Hovercraft Full Of Eels Over a year ago

@user133: it has nothing to do with "intentionally". You can't control how the method is used since it's public.

amit Over a year ago

@user133466: 1st, HoverfraftFullOfEels is correct. 2nd- this scenario is very much possible. Imagine you have a Map<Integer,Employee>. Now you do myEmployee.equals(myMap.get(5));. If myMap does not contain 5 as a key, it will return null.

user133466 Over a year ago

what does this notation mean Map<Integer,Employee> ? and why does it matter if the method is public?

amit Over a year ago

@user133466: This (Map<Integer,Employee>) is a java data structure for a map. You will probably get to it later in your studies - no need to rush. The method must be public because it is how it was declared in Object - and you cannot reduce the visibility of a method.

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