CODEWITHSUNDEEP
javaoopinheritance
vladfau
vladfau

Reputation: 1003

Getting clear idea about casting and inheritance in Java

I can't get a clear idea about inheritance in Java. For instance, I have:

class A
{ 
     ...
     public void MethodA();
}

Here is everything is clear. Then I want a (sub)class:

class B extends A
{
    ...
    public void MethodB();
}

Then I create an instance of A. Then I want to get an instance of B having all the stuff from A. I want to make something like aA = new B(); but get an error.

Using this thing is not really comfortable:

A aA = new A(...);
B aB = new B(null,null,...);
aB = aA

How should I make it correctly and get a clear idea about inheritance in Java?

UPD: So how can i get an instance of B having all the stuff of A?

Upvotes: 1

Views: 132

Answers (4)

Vaibhav Raj
Vaibhav Raj

Reputation: 2232

You need to understand the IS-A relationship. Let's discuss an example of Employee and Manager relationship.

class Employee{
    public Employee(){
        System.out.println("Employee obj");
    }
}

class Manager extends Employee{
    public Manager(){
        System.out.println("Manager obj");
    }
}

By extending Employee we are declaring Manager as a specialization of Employee. Therefore, every Manager is an Employee but every Employee is not Manager.

A reference of type Employee can refer to the objects who satisfy the IS-A relationship. Therefore, these are perfectly legal to write:

Employee emp1 = new Employee();
Employee emp2 = new Manager();

However, if we try to write

Manager man1 = new Employee(); //Wrong: Compilation error.

This violates the IS-A relationship. As a result of this we get compile time error. In order to avoid this kind of compilation error we need to add a cast to type Manager. But, casting to type Manager only avoids compilation issues. You will get a java.lang.ClassCastException as a result of of this kind of casting.

Manager man1 = (Manager) new Employee(); //Wrong: avoids compilation error but will generate a ClassCastException at runtime.

I suggest you to go through the Java Language Specification: Section 5.5.1 to get a full idea of casting a reference type.

Upvotes: 1

NPE
NPE

Reputation: 500367

It sounds like you want an instance of B in the first place. Once you have such an instance, you can use it whenever either A or B is required.

Put another way, every instance of B is automatically also an instance of A.

This is called the Liskov Substitution Principle.

Upvotes: 4

Boris Mocialov
Boris Mocialov

Reputation: 3489

In your example, A - superclass, B - subclass.

A subclass possesses all the attributes and operations of its superclass (because a subclass inherited all attributes and operations from its superclass). This means that a subclass object can do whatever its superclass can do. As a result, we can substitute a subclass instance when a superclass instance is expected, and everything shall work fine. This is called substitutability.

Upvotes: 1

Alexey Romanov
Alexey Romanov

Reputation: 170745

Every instance of B has all of stuff from A, because it is an A. But it seems like you actually want to create an instance of B from a given instance of A. This isn't possible, unless you specifically write your class so as to allow it:

public class B extends A {
  public B(A original) { ... }
}

and later

 A a = new A();
 B b = new B(a);

Upvotes: 0

Related Questions