Exception not being caught

[kiennjal shah]

Hello,

I have the following code which compiles fine, but doesnt work:

class BadFoodException extends Exception {}
 
public class BadFood
{
public static void main(String [] args)
{
try
{
checkFood(args[0]);
}
catch(BadFoodException e)
{
System.out.println("Exception caught!");
e.printStackTrace();
}
 
}
 
public static void checkFood(String food) throws BadFoodException
{
if(food == "peas")
throw new BadFoodException();
else
System.out.println("Yummy!!! I like this food: " + food);
}
}
  

this is actually a problem from one of the JCP prep books. it compiles fine, but everytime I run it, regardless of whatever i put as command line arg, it always prints only: Yummy!!! I like this food

Can anyone suggest as to why this is not working?

Thanks

-Kiennjal

[Stefan Willi]

Hi kiennjal !

Please pay attention to your comparison of the string Object. You compare the reference of the string literal with the reference of your argument object. that cannot be equal...
To identify, if two strings are equal, use the equal() ore equalIgnoreCase() methods.

public static void checkFood(String food) throws BadFoodException
{
if(food.equalsIgnoreCase("peas"))
throw new BadFoodException();
else
System.out.println("Yummy!!! I like this food: " + food);
}

Stefan

[Nicholas Cheung]

"==" operator checks whether the variable food using the same memory address as "peas". Obviously, they are using different memory addresses and thus, the comparsion always return false.

Nick

[Ravi Kishore]

Hi kiennjal,

what nicholas and stefan has said is correct. "==" operator always compares the references (addresses) but not the contents pointed by the addresses. So for string comparisions it's advisable to use equals() or equalIgnoreCase() functions.

keep in mind that while comparing a string object aganist a string constant always proceed as follows:

String str = "peas";
if("beans".equals(str))
{
//do something
}
else
{
//do something
}

but if it is done as follows:

if(str.equals("beans"))
{
//do something
}
else
{
//do something
}

a new dummy object is created for the string constant "beans" and then the comparision takes place. which will consume memory unnecessarly.

[Joel McNary]

Originally posted by Ravi Kishore:
a new dummy object is created for the string constant "beans" and then the comparision takes place. which will consume memory unnecessarly.

It shouldn't matter. The "beans" is a string literal; as such, an object is always created. At runtime, there's no difference between a "string object" and a "string literal;" both are instances of java.lang.String. (OK, the string literal is stored in the literal pool...)

The primary reason for placing the string literal first is to avoid NullPointerExceptions.:

food.equals("peas")

could throw a NPE if food were null.

"peas".equals(food)

returns false if food were null.

[Nicholas Cheung]

By taking about the handling of NullPointerException for null strings, currently, we try to use an alternative approach.

We used the APIs provided by Jakarta Common, the StringUtils. This package provides lots of functions for string manipulations, like isEmpty(), isBlank(), equals(), etc. And the APIs have been internally handled the null cases, i.e., if x is null, StringUtils.isEmpty(x) returns true instead of throwing the NullPointerException.

Nick

[kiennjal shah]

Thank you all for your help. That was so very silly of me!!!

Joel, thanks for pointing out the effect of placing str before the method: equals.

Have a good one!

-Kiennjal