instanceof doubt...

[Siri Naray]

Hai ranchers
could any one explain me the use of instanceof?

If B inherits A
which one is true
1. A instanceof B
2. B instanceof A
3. Or both work fine?

Thank you

[Deepak Bala]

If b is the subclass then b instanceof A is true

[Keith Lynn]

Originally posted by Sirishaaaaa Ghatty:
Hai ranchers
could any one explain me the use of instanceof?

If B inherits A
which one is true
1. A instanceof B
2. B instanceof A
3. Or both work fine?

Thank you

Actually neither is syntactically correct.

The left-hand operand of instanceof must be a reference name, and the right-hand side must be a reference type.

Also, if the class type of the reference name is not a subclass nor a superclass of the class type on the right-hand side, then a compile-time error occurs.

So, for example,

String s = "Hello";
System.out.println(s instanceof String);

will print true.

But

String s = "Hello";
System.out.println(s instanceof Thread);

will cause a compile-time error.

http://java.sun.com/docs/books/jls/third_edition/html/expressions.html#15.20.2

[marc weber]

Consider an example: Cat extends Animal.

An Animal is always an Animal.
A Cat is always a Cat.
Furthermore, a Cat is always an Animal.

But is an Animal always a Cat? Sometimes, but not always. This is where instanceof is useful.

(Note that the left side must be a reference, and the right side must be a type. You cannot compare two types using instanceof, as your example implies.)

[Siri Naray]

Hi John,
Can you please explain me this

class Animal
{
void makeNoise(){System.out.println("generic noise");}
}
class Dog extends Animal
{
void makeNoise() {System.out.println("bark");}
void playDead() {System.out.println( "roll over");}
}
class CastTest2
{
public static void main(String [] args)
{
Animal [] a = {new Animal(), new Dog(), new Animal()};
for(Animal animal : a)
{
animal.makeNoise();
if(animal instanceof Dog)
animal.playDead();
}
}
}

When i compile this code compiler gives an error at animal.playDead()
Cannot find symbol : method playDead() at class Animal animal.playDead();

Does it mean at compile time "animal instanceof Dog" is true?
Only when "if" statement is true does the compiler go to next statement.
Can you please explain.
Thank You

[Siri Naray]

Thank you i got the answer. But what about runtime?
When i modified the code i posted above
if(animal instanceof Dog)
System.out.println(" animal is an instanceof Dog" );

output
-------
generic noise
bark
animal is an instanceof Dog
generic noise

But when "animal instanceof Dog" is true output should be

generic noise
animal is an instanceof Dog
bark
animal is an instanceof Dog
generic noise
animal is an instanceof Dog

I am confused.. please help
Thank you

[Keith Lynn]

The problem is that the reference is of type Animal, and not of type Dog, even though the runtime type of the object is Dog.

You cannot call methods defined in the subclass (and not defined in the superclass) using a superclass reference.

[rohitt kumarr]

You need to cast it to the type Dog if you want to invoke the playdead method