instanceOf operator question

[thejaka samarakoon]

Why the output is Browser rather than IE.Since at runtime according to VMI it is selected as a IE.

   
public class Browser {
 public static void main(String [] args) {
Browser ref = new IE();
 if(ref instanceof Firefox) {
 System.out.println(“Firefox”);
 } else if(ref instanceof Browser) {
 System.out.println(“Browser”);
 } else if(ref instanceof IE) {
 System.out.println(“IE”);
 } else {
 System.out.println(“None of the above”);
}

public void go() {
 System.out.println(“Inside Browser”);
  }
 }

class Firefox extends Browser {
public void go() {
System.out.println(“Inside Firefox”);
 }
}

class IE extends Browser {
 public void go() {
 System.out.println(“Inside IE”);
 }
  }
}

[Avishkar Nikale]

Can you post the complete code alongwith the class definitions for Browser etc?

We will be able to help you & explain you better.

[Malatesh Karabisti]

instanceof operator checks for IS Relation here. In your code Browser comes before IE in "if else" block change the order and try, I mean put IE before Browser.

I think that helps you

Check out this from K& B book

Look for instanceof questions that test whether an object is an instance of an interface, when the object's class
implements the interface indirectly. An indirect implementation occurs when one of an object's superclasses
implements an interface, but the actual class of the instance does not—for example,
interface Foo { }
class A implements Foo { }
class B extends A { }
...
A a = new A();
B b = new B();
the following are true:
a instanceof Foo
b instanceof A
b instanceof Foo // implemented indirectly
An object is said to be of a particular interface type (meaning it will pass the instanceof test) if any of the
object's superclasses implement the interface.

[thejaka samarakoon]

I have not understand the answer.Can anyone help?

[Malatesh Karabisti]

public class Browser {
 public static void main(String [] args) {
Browser ref = new IE();
 if(ref instanceof Firefox) {
 System.out.println("Firefox");
 } 
 else if(ref instanceof IE) {    // IE is before to Browser
 System.out.println("IE");
 } else if(ref instanceof Browser) {
 System.out.println("Browser");
 } else {
 System.out.println("None of the above");
}
 }
}

class IE extends Browser { }
class Firefox extends Browser { }

Try this and spot the difference. If else block always executes only one condition at time if it executes first one it won't executes second one. In your code "ref instanceof Browser" is true hence it won't even do a check for "ref instanceof IE". I think this time I explained better.

[RamandeepS Singh]

Malatesh Karabisti wrote:

public class Browser {
 public static void main(String [] args) {
Browser ref = new IE();
 if(ref instanceof Firefox) {
 System.out.println("Firefox");
 } 
 else if(ref instanceof IE) {
 System.out.println("IE");
 } else if(ref instanceof Browser) {
 System.out.println("Browser");
 } else {
 System.out.println("None of the above");
}
 }
}

class IE extends Browser { }
class Firefox extends Browser { }

Try this and spot the difference. If else block always executes only one condition at time if it executes first one it won't executes second one. In your code "ref instanceof Browser" is true hence it won't even do a check for "ref instanceof IE". I think this time I explained better.

i have tried this code and its printing IE

please try again

[Malatesh Karabisti]

Yes "IE" is expected output.

[Hebert Coelho]

By the way, there is a few compilations erros in your code! [=

Maybe thats why you got the wrong result.

[Malatesh Karabisti]

thejaka,

Yes as Hebert told your code has the compilation errors. resolve the compilation error and post the source code if you face any problem.

[Abimaran Kugathasan]

thejaka, one advice for you, always use propper code fragments before post the code here, it'll be readable, you'll get quick response from ranchers.

Here your code snaps, I don't know whether, the code you mentioned is same as mine.

public class Browser {
 	public static void main(String [] args) {
		Browser ref = new IE();
 		if(ref instanceof Firefox) {
 			System.out.println("Firefox");
 		} else if(ref instanceof Browser) {
 			System.out.println("Browser");
 		} else if(ref instanceof IE) {
 			System.out.println("IE");
 		} else {
 			System.out.println("None of the above");

		}
	}

	public void go() {
 		System.out.println("Inside Browser");
  	}
}

class Firefox extends Browser {
	public void go() {
		System.out.println("Inside Firefox");
 	}
}

class IE extends Browser {
 	public void go() {
 		System.out.println("Inside IE");
 	}
}

And you are asking, why the output is Browser rather that IE? Is that so, If I ask a question, a ball is dropped from top of a building, and there is guy in the next floor to the top floor, having a big net to catch the ball, and there is another guy, waiting to catch the ball at the next floor the previous guy. Who will catch the ball?

Same behavior here.

BTW, why do you expect the VMI and other stuff here?

[Shanky Sohar]

thejaka samarakoon wrote:Why the output is Browser rather than IE.Since at runtime according to VMI it is selected as a IE.

How you are able to run the program,there is a compiler error.int this code..4
Also please QuoteYourSources from where you get this question

[Sid Dev]

This is from the Khalid Mughal's book "At runtime, it is the type of the actual object denoted by the reference on the left-hand side that is compared with the type specified on the right-hand side. In other words, what matters at runtime is the type of the actual object denoted by the reference, not the declared type of the reference."

try changing code to

Browser ref= new Broser();

you will understand it much better.

I hope this answers your original question.

[Shanky Sohar]

@Sid Dev
Thanks for quoting the source.
Welcome to JavaRanch

[Shanky Sohar]

see the below code..you will easliy figure it out what is happening..

public class Browser {
	 public static void main(String [] args) {
    	  Browser br=new Browser(); 
		 Browser ref = br.new IE();
	      if(ref instanceof Firefox) {
	               System.out.println("firefox");
	      }   else if(ref instanceof Browser) {//this is True
	               System.out.println("Browser");
	      } else if(ref instanceof Browser) {//even this is True
              System.out.println("Browser-2");
           }  else if(ref instanceof Browser) {//even this is True
              System.out.println("Browser-3");
            } 
         else if(ref instanceof Browser) {//even this is True
               System.out.println("Browser-4");
             }else if(ref instanceof Browser) {//even this is True
               System.out.println("Browser-5");
             }  
	      else if(ref instanceof IE) {//even this is true
	               System.out.println("IE");
	      }   else {
	               System.out.println("None of the above");
	      }
	 }
	public void go() {
	 System.out.println("Inside Browser");
	  }

	class Firefox extends Browser {
	public void go() {
	System.out.println("Inside Firefox");
	 }
	}

	class IE extends Browser {
	 public void go() {
	 System.out.println("Inside IE");
	 }
	  }
	}
	
	//if all are True then why are getting the error.i think you will understand now.

If that much if-statements are true why the output is Browser.Think you will easliy understand...

Line no.9 to 23 worth watching