Jist of Rules

[Sherry Jacob]

Hi All,
Can anyone give me a jist of important rules to be kept in mind while casting object references and using the instanceof operator.

I don't seem to understand the logic behind the instanceof operator.

Also please clarify...
Is the instanceof operator used to check what object a reference will denote at runtime ?

[Mark Spritzler]

Simply put, is the reference an instance of a certain type, Can that reference be assigned to another reference of that type.

It is all about inheritence.

So, if you do instanceof Object, to any oreference variable, then that will always be true.

So let's make up a Hierarchy.

Object
|
Vehicle
/ \
Car Truck
| |
Jetta S10

So the following return true

Car a = new Car();
a is an instance of Car, Vehicle, and Object. But not an instance of Jetta.
Car a = new Jetta();
a is an instance of Car, Vehicle, and Object. And is an instance of Jetta.

So the actual type, not the reference type actually determines the instance of. So anything that is of that type or of a super type of that type will pass instanceof.

Hope that helps.

Mark

[Marcelo Ortega]

boolean b = (ref1 instanceof ClassType);

The above expression is simply read as follows:

if ref1 is actually an INSTANCE of ClassType, then return true, if not then you guessed it... false.

But remeber than ref1's type and ClassType must be found in the same class heirarchy if not there will be a compile-time error.

If ClassType were an InterfaceType, then there will be no problem.

[Marcelo Ortega]

Well put Mark.

[Sherry Jacob]

Originally posted by Mark Spritzler:
Simply put, is the reference an instance of a certain type, Can that reference be assigned to another reference of that type.

It is all about inheritence.

So, if you do instanceof Object, to any oreference variable, then that will always be true.

So let's make up a Hierarchy.

Object
|
Vehicle
/ \
Car Truck
| |
Jetta S10

So the following return true

Car a = new Car();
a is an instance of Car, Vehicle, and Object. But not an instance of Jetta.
Car a = new Jetta();
a is an instance of Car, Vehicle, and Object. And is an instance of Jetta.

So the actual type, not the reference type actually determines the instance of. So anything that is of that type or of a super type of that type will pass instanceof.

Hope that helps.

Mark

Okay Mark...I seem to have understood something.

Now, if I say,
Car a = new Car();
Jetta b = new Jetta();

and then I say
b = a;

What am I doing ? What would (b instanceof a) give and why ?

[Mark Spritzler]

Now, if I say,
Car a = new Car();
Jetta b = new Jetta();

and then I say
b = a;

well you can't assign a to b, because a is a Car, and b is a Jetta. You do not know that a is a Jetta because the instance is of Car and the reference is of Car

if you had

Car a = new Jetta();

then you can assign a to b, but it will require for an explicit cast in the line as in

Jetta b = (Jetta)a;

And as far as (b instanceof a) You can't do that because you have to have a Class not a reference on the right side of instanceof.

but b is an instanceof Car.

Mark