Why Can't Reference to Child Class Object Refer to the Parent Class Object

Why can't reference to child Class object refer to the parent Class object?

Exactly because aChild is a superset of aParent's abilities. You can write:

class Fox : Animal

Because each Fox is an Animal. But the other way is not always true (not every Animal is a Fox).

Also it seems that you have your OOP mixed up. This is not a Parent-Child relationship, because there's no composition/trees involved. This is a Ancestor/Descendant inheritance relation.

Inheritance is "type of" not "contains". Hence it's Fox is a type of Animal, in your case it doesn't sound right -- "Child is a type of Parent" ? The naming of classes was the source of confusion ;).

class Animal {}
class Fox : Animal {}
class Fish : Animal {}

Animal a = new Fox(); // ok!
Animal b = new Fish(); // ok!
Fox f = b; // obviously no!

Why do we assign a parent reference to the child object in Java?

First, a clarification of terminology: we are assigning a Child object to a variable of type Parent. Parent is a reference to an object that happens to be a subtype of Parent, a Child.

It is only useful in a more complicated example. Imagine you add getEmployeeDetails to the class Parent:

public String getEmployeeDetails() {
    return "Name: " + name;
}

We could override that method in Child to provide more details:

@Override
public String getEmployeeDetails() {
    return "Name: " + name + " Salary: " + salary;
}

Now you can write one line of code that gets whatever details are available, whether the object is a Parent or Child:

parent.getEmployeeDetails();

The following code:

Parent parent = new Parent();
parent.name = 1;
Child child = new Child();
child.name = 2;
child.salary = 2000;
Parent[] employees = new Parent[] { parent, child };
for (Parent employee : employees) {
    employee.getEmployeeDetails();
}

Will result in the output:

Name: 1
Name: 2 Salary: 2000

We used a Child as a Parent. It had specialized behavior unique to the Child class, but when we called getEmployeeDetails() we could ignore the difference and focus on how Parent and Child are similar. This is called subtype polymorphism.

Your updated question asks why Child.salary is not accessible when the Childobject is stored in a Parent reference. The answer is the intersection of "polymorphism" and "static typing". Because Java is statically typed at compile time you get certain guarantees from the compiler but you are forced to follow rules in exchange or the code won't compile. Here, the relevant guarantee is that every instance of a subtype (e.g. Child) can be used as an instance of its supertype (e.g. Parent). For instance, you are guaranteed that when you access employee.getEmployeeDetails or employee.name the method or field is defined on any non-null object that could be assigned to a variable employee of type Parent. To make this guarantee, the compiler considers only that static type (basically, the type of the variable reference, Parent) when deciding what you can access. So you cannot access any members that are defined on the runtime type of the object, Child.

When you truly want to use a Child as a Parent this is an easy restriction to live with and your code will be usable for Parent and all its subtypes. When that is not acceptable, make the type of the reference Child.

Why do we assign child class object to parent class reference variable?

It's because you've redeclared the Print method in Child. So at compile time, P.Print() resolves to Parent.Print, but C.Print() resolves to Child.Print(). If you had a virtual method which was overridden in Child instead, they'd both print "Child Method":

public class Parent
{
    // Declare this as virtual, allowing it to be overridden in
    // derived classes. The implementation will depend on the
    // execution-time type of the object it's called on.
    public virtual void Print()
    {
        Console.WriteLine ("Parent Method");
    }
}

public class Child : Parent
{
    // Override Parent.Print, so if Print is called on a reference
    // with compile-time type of Parent, but at execution it
    // refers to a Child, this implementation will be executed.
    public override void Print()
    {
        Console.WriteLine ("Child Method");
    }
}

Related:

• What's the difference between override and new? (C# FAQ entry on MSDN - turns out to have been written by me, but I'd forgotten!)
• Knowing when to use the override and new keywords (MSDN)

Why parent object cannot be assigned to a child reference in java?

As user7 said and Stultuske updated, a Car is a Vehicle, but not all Vehicles are Cars. So this:

Vehicle v = /*...some kind of Vehicle...*/;
Car c = v;

...makes no sense. What if v doesn't refer to a Car instance?

More formally, it would violate the contract. Say we have:

class Vehicle {
}
class Car extends Vehicle {
    public void drive() {
        // ...
    }
}

Suppose this actually worked:

Car c = new Vehicle(); // Doesn't work, but let's pretend it does

What should happen if you call c.drive()? The contract (Car) says it has a drive method, but the object (of type Vehicle) does not.

Even if Car didn't add any (public) features, it's still misleading to have a Car-typed variable (which, by definition, refers to Car instances or instances of a Car subclass) refer to a Vehicle object. The Vehicle isn't (necessarily) a Car.

If you have a Vehicle reference to an object and you know the object is actually a Car, then you can cast it:

Vehicle v = new Car();
Car c = (Car)v;

...since there's no mismatch there; the object actually is a Car. The cast tells the compiler you, the programmer, are taking responsibility for the object that v refers to actually being compatible with Car. (And the JVM checks at runtime, so if you're wrong, you get a ClassCastException.)

Why can't object refer to parent object and child object?

All your constructors require the parent and child to be constructed before the current object can be constructed. That is impossible in your case as the references form a dependency cycle (First -> Fourth -> Third -> Second -> First, therefore First can't be fully created till after First is created, etc.).

One solution would be to make the Node abstract (or an interface) and only fetch the values when they are requested (i.e. in the getters):

interface Node {
    val number: Int
    val parent: Node?
    val child: Node?
}

object First : Node {
    override val number = 1
    override val parent get() = Fourth
    override val child get() = Second
}

// Similar for Second, Third, and Fourth

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If you're interested in why you got that particular pattern of nulls, it's important to remember that objects in Kotlin usually aren't created till the first time they are used.

1. The first object you use (in main) is First, so it's constructor is called.
2. The first* thing its constructor does is get a reference to Fourth, which in turn does Third, which in turn does Second, which in turn tries to do First, but First is already in the process of being created, so its reference is null.
3. Then (we're still in Second) it tries to create Third (as the child), which is in the process of being created, so its reference is also null. At this point Second is done, so any future reference to it will not be null.
4. Third now has a valid reference to Second, and moves on to trying to create Fourth, which is still being created, thus null. Now Third is done.
5. Fourth now has a valid reference to Third, and moves on to First, which is still being created, thus null. Now Forth is done.
6. First now has a valid reference to Fourth, and moves on to Second, which already exists and is valid. Now First is done.

*Technically the second, since the first thing it does is set the number, but it's the first thing that matters for the purposes of this discussion.

Why we can't instantiate child class object from parent class

Because the compiler automatically casts the deriving class to the base class - it just interprets it as an object of the base class, ignoring other properties. But to cast an object to the derived class, it must have been an object of this derived class before and the compiler wants you to explicitly define the cast. But even if you'd define the cast explicitly, it wouldn't work because the object has never been of the derived type.

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