Understanding the Try Catch block behavior

Question

I ran into a strange problem with try catch which got me doubting about my own realization of exception handling fundamentals. As per the basic syntax

try{
      code to  be checked     
   }
catch(Exception e){}
finally{}

However the code below gives me a null pointer exception which I believe should have been caught.

class Ideone
{
public static void main (String[] args) throws java.lang.Exception
{



    try{

            for(Model m: Null Collection coming from DB)
            System.out.println("Inner Block");

      System.out.println("Outer Block");        

    }catch(Exception e){}

}
}

The collection is `null`. You can't iterate over a null collection — TheLostMind, Nov 09 '15 at 06:44
@Vinod Madyalkar the point is why isn't it being caught by the catch Block — aman shivhare, Nov 09 '15 at 06:46
@amanshivhare - How do you know that it is not getting caught? — TheLostMind, Nov 09 '15 at 06:48
@VinodMadyalkar because I am getting an error at that point ? Process: com.example.aman.navtest2, PID: 6207 java.lang.NullPointerException — aman shivhare, Nov 09 '15 at 06:55
Why don't you have a sysout in your catch block to check if it is getting caught or not?. Or debug through your code?. — TheLostMind, Nov 09 '15 at 06:57

score 1 · Accepted Answer · edited May 23 '17 at 11:44

1

The following snippet prints "An exception!"

List<String> strings = null;
try {
    for (String s : strings) {
        System.out.println(s);
    }
} catch (Exception e) {
    System.out.println("An exception!");
}

And as others have pointed out, and you stated yourself, Runtime exceptions are caugth by Exception catches

Have you tried recompiling all your code from scratch? In my team (250.000 lines codebase) using eclipse we sometimes have trouble with bad compiles that can give unexplainable problemes like this. We usually solve them by a complete recompile.

edited May 23 '17 at 11:44

Community

1
1

answered Nov 09 '15 at 07:05

Staffan Runnsjö

76
6

Runnsjo that suprisingly solved the problem. (Rebuilding Project) – aman shivhare Nov 09 '15 at 07:12

score 0 · Answer 2 · answered Nov 09 '15 at 06:51

Are you sure about your issue? I believe there's some clean-up code in your catch which hasn't been initialized yet - hence the exception. Would help if you post your actual code. The following works as expected:

import java.util.List;

public class Main {

public static void main(String[] args) {
    List i=null;
    try{
        for(Object o: i) {
            System.out.println("Inner Block");
        }
        System.out.println("Outer Block");
    }catch(Exception e){}
}

}

Husam · Answer 3 · 2015-11-09T07:36:33.963

Understanding the Try Catch block behavior?

The main reason to use try-catch block is to handle something will goes wrong with your code or something you are unexpected for the normal case and something will throw an exception somehow and to handle it in catch block, and the finally block is used almost to close any opened stream and it will run every time in the code even if the try or catch returned any value (except for system termination).

In your code it seems that there is something you get it from database which is null or not initialized yet or it is already returned from database of null value, Now you cannot use null object if it's not initialized already! you have to insure if it's null or not then use it like this:

class Ideone {
    public static void main(String[] args) throws java.lang.Exception {
        try {
            if(Null Collection coming from DB != null){
                for(Model m: Null Collection coming from DB)
                    System.out.println("Inner Block");
            }
            System.out.println("Outer Block");

        } catch (Exception e) {}
    }
}

The NullPointerException is extends RuntimeException and it's thrown when you try to use a null object.

score -1 · Answer 4 · answered Nov 09 '15 at 06:44

-1

i has not been initialized,

public class Ideone{
public static void main (String[] args) throws java.lang.Exception
{

    int i = 0;

    try{ 


            if(i<2)
            System.out.println("Inner Block");

      System.out.println("Outer Block");        

    }catch(Exception e){}

}
}

answered Nov 09 '15 at 06:44

codedude

1
3

If you changed i = 12, do you know what will happen? – codedude Nov 09 '15 at 06:46
try changing the }catch(NullPointerException e){ System.out.println("nulls exist"}; – codedude Nov 09 '15 at 06:48
Irrespective of whether `i` is initialized, this code wouldn't give a NullPointerException, since `i` is primitive. – Andy Turner Nov 09 '15 at 07:30

Harper Koo · Answer 5 · 2015-11-09T07:05:59.480

-2

Cannot reproduce. I tried this

public static void main (String[] args) throws java.lang.Exception
    {
        Collection c = null ;
        try{ 
            for(Object i:c)
                System.out.println("Inner Block");
            System.out.println("Outer Block");        
        }catch(Exception e){}
    }

It works well.

edited Nov 09 '15 at 07:05

answered Nov 09 '15 at 06:58

Harper Koo

597
5
14

2

It would print "error" in the console whether or not an exception is thrown. – Andy Turner Nov 09 '15 at 07:01
It not gives me a null pointer exception, I delete the last line – Harper Koo Nov 09 '15 at 07:05
1

Maybe try putting the `System.out.println("error");` in the `catch` block, then you'd see the exception. http://ideone.com/pagkGm – Andy Turner Nov 09 '15 at 07:06
I know what you mean. But it really can catch the exception instead of throw it out. – Harper Koo Nov 09 '15 at 07:11
Yes, it catches an exception if one occurs. Your statement that it doesn't give a null pointer exception is simply wrong: a NPE is thrown, however you swallow it without giving any indication that it occurred. – Andy Turner Nov 09 '15 at 07:31
I'm confused now. You mean this func throws a NPE? – Harper Koo Nov 09 '15 at 07:41
An NPE is thrown in this code; however, you can't see that because you swallow the exception. There is no way to discern whether it is thrown or not at runtime. – Andy Turner Nov 09 '15 at 07:44

Understanding the Try Catch block behavior

5 Answers5