Java Help - Casting to Integer on Stack, classcastexception

Author

Casting to Integer on Stack, classcastexception

Jeffrey Spoon

2004-03-27, 12:30 am

Hello, I'm getting a ClassCastException and I'm not sure why. The code
for the casting - op1 = ((Integer)newStack.pop()).intValue(); - as I
understand it, is supposed to cast the object returned by pop to an
Integer object and then the actual primitive value is grabbed by
intValue and assigned to the in op1. Or have I got that wrong? Here's
the full listing:

Tokenise.java:

import java.util.*;

public class Tokenise extends Stack {

protected Object item;
protected Object item2;

public int evaluate (String expr) {

int op1, op2, result = 0;
String token;
StringTokenizer tokenizer = new StringTokenizer (expr);

Stack newStack = new Tokenise(); // evaluation stack

while (tokenizer.hasMoreTokens())
{
token = tokenizer.nextToken();

if (isOperator(token) == true)
{
//...pop top two
operands here then operate on them

op1 = ((Integer)newStack.pop()).intValue();
op2 = ((Integer)newStack.pop()).intValue();

System.out.println("op1" + op1);
System.out.println("op2" + op2);

}else{
//...push operand onto stack
System.out.println("pushing: " +
newStack.push(token));

}

}

//look at top of stack

System.out.println("p

: " + newStack.p

());

return result; //...

} // method evaluate

public boolean isOperator(String token) {

if (token.equals("+")){
return true;

}else if (token.equals("-")){
return true;

}else if (token.equals("*")){
return true;
}else if (token.equals("/")){
return true;
}

return false;

}// method isOperator

public boolean empty() {

return true;

}//method empty

public Object p

() throws EmptyStackException {

return super.p

();

}//method p

public Object push(Object item) {

return super.push(item);

// System.out.println("itempush: " + item);

}//method push

public Object pop() throws EmptyStackException {

return super.pop();

//System.out.println("itempop");
//return item;

}// method pop

public int search(Object obj) {

return 0; //for now.

}//method search

} // class Tokenise

Main.java:

public class Main {

public static void main(String [] args) {

Tokenise tok = new Tokenise();
tok.evaluate("7 4 -3 * 1 5 + / *");

} // method: main

} // class: Main

--
Jeffrey Spoon

Lothar Kimmeringer

2004-03-27, 12:30 am

On Thu, 25 Mar 2004 20:58:56 +0000, Jeffrey Spoon wrote:

> Hello, I'm getting a ClassCastException and I'm not sure why.

You push a String on the stack and cast it to Integer when
getting it back. This lead to the exception. You have to
create an Integer out of the String (e.g. with new Integer(token))
and push that instead.

Regards, Lothar
--
Lothar Kimmeringer E-Mail: spamfang@kimmeringer.de
PGP-encrypted mails preferred (Key-ID: 0x8BC3CD81)

Always remember: The answer is forty-two, there can only be wrong
questions!

Eric Sosman

2004-03-27, 12:30 am

Jeffrey Spoon wrote:
>
> Hello, I'm getting a ClassCastException and I'm not sure why. [...]

Because you push each operand onto the stack as a String
object. When an operator comes along, you pop them off and
try to cast them to Integer objects. Strings aren't Integers.

--
Eric.Sosman@sun.com

Jeffrey Spoon

2004-03-28, 10:36 pm

In message <ahuxuoaa4q8w.dlg@kimmeringer.de>, Lothar Kimmeringer
<news200403@kimmeringer.de> writes
>On Thu, 25 Mar 2004 20:58:56 +0000, Jeffrey Spoon wrote:
>
>
>You push a String on the stack and cast it to Integer when
>getting it back. This lead to the exception. You have to
>create an Integer out of the String (e.g. with new Integer(token))
>and push that instead.
>
>
>Regards, Lothar

Doh... Thanks.

--
Jeffrey Spoon

Jeffrey Spoon

2004-03-28, 10:36 pm

In message <406359B9.7626D734@sun.com>, Eric Sosman
<Eric.Sosman@sun.com> writes
>Jeffrey Spoon wrote:
>
> Because you push each operand onto the stack as a String
>object. When an operator comes along, you pop them off and
>try to cast them to Integer objects. Strings aren't Integers.
>

Yep, that'll be why then. Cheers.

--
Jeffrey Spoon

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