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0 pointsandy_wrote7y ago0 comments

I really like your last point "whatever the computer is telling is true from the computer point of view".

In a surprising number of coding interviews that I've conducted, I've seen a candidate write an incorrect solution that generates an exception and then attempt to figure out the error almost on first principles, rather than actually reading the exception carefully and thinking about what coding errors could have caused it to happen. The exception message is a _huge_ hint!

It's a red flag to see someone try to debug without paying attention to it. Conversely it's very positive to see someone encounter one and then think carefully about it. Writing an incorrect solution but then testing it out and showing the ability to debug and fix it systematically and expediently is no worse than getting it right on the first try, in my book.

0 comments

fhood7y ago

It isn't just a huge hint, most of the time it tells you exactly what the damn issue is. This discussion is blowing my mind.

I am honestly speechless. IT GIVES YOU THE LINE NUMBER AND THE FUNCTION CALL AND THE REASON! Why would you ever even attempt to debug without parsing it?

eitland7y ago

Wait wait wait.

Not so fast.

i'm one of those programmers who read manuals and stack traces but I have to admit that I've seen my share of both manuals as well as stack traces that didn't make much sense.

kickopotomus7y ago

This is my experience with Clojure stack traces ~60-75% of the time. Perhaps it is a symptom of functional programming? A lot of function passing and anonymous functions make it difficult to generate a meaningful stack trace.

3 more replies

dave_sid7y ago

Yeah I agree. The way this thread is going you would think that all errors are easy and all you have to do is what the computer tells you and you’re done. I wish I’d known it was all that simple!

TimTheTinker7y ago

> stack traces that didn't make much sense.

Those can be especially prevalent in systems that utilize a lot of indirection.

_mdpn7y ago

Especially when you end up in C++ template hell.

2 more replies

dba7dba7y ago

That's exactly what I felt as I was spending time on MSDN/Microsoft support website, reading about weird errors messages on Windows 95/98/NT/XP in 1990s. Some generic desc about the Windows error messages and no resolution.

tyrex20177y ago

as in tensorflow stack traces

dsr_7y ago

And in the opposite direction: remember when you are writing error messages to include all the relevant details, and never ever write "this shouldn't happen" or "document later".

fhood7y ago

What about, log_panic("If we have somehow managed to hit this code path, this project if FUBAR. I would recommend giving up and starting over.")

2 more replies

pletnes7y ago

If used correctly, «this should not happen» can be very useful, indicating e.g that the program is in some illegal state, as opposed to the input being incorrectly formatted.

1 more reply

scarface747y ago

That’s only true if the developer of the class module let the entire stack trace get through to the consumer.

In C# it’s the difference between...

try

{

....

}catch(Exception e)

{

//Do stuff

   throw new MyCustomException(“Something Bad happened”);

}

And

try

{

....

}catch(Exception e)

{

  //do stuff

   throw;

}

moron4hire7y ago

Erh, exception types should include an inner exception parameter, for the generic one they wrap. So neither of your two examples are all that great.

1 more reply

debaserab27y ago

In what case would you ever want the second example?

3 more replies

bmhin7y ago

I remember once encountering an error that had all of the above plus it presented the solution to the issue right in the error message. It was amazing and why I still remember seeing that to this day.

avip7y ago

Let's try that! I was trying to do some trivial task on a .gov website, but was greeted with the following, which I'm pasting here for your convenience. Could you spot the problem? Because their customer service can't.

---

Error Page Exception SRVE0260E: The server cannot use the error page specified for your application to handle the Original Exception printed below.

Original Exception: Error Message: javax.servlet.ServletException: /verification.xhtml at line 66 and column 34 action="#{verificationAction.authenticateClient()}": java.lang.NullPointerException Error Code: 500 Target Servlet: Faces Servlet Error Stack: javax.el.ELException: /verification.xhtml at line 66 and column 34 action="#{verificationAction.authenticateClient()}": java.lang.NullPointerException at org.apache.myfaces.view.facelets.el.TagMethodExpression.invoke(TagMethodExpression.java:95) at javax.faces.component._MethodExpressionToMethodBinding.invoke(_MethodExpressionToMethodBinding.java:88) at org.apache.myfaces.application.ActionListenerImpl.__AW_processAction(ActionListenerImpl.java:100) at org.apache.myfaces.application.ActionListenerImpl.processAction(ActionListenerImpl.java) at javax.faces.component.UICommand.broadcast(UICommand.java:120) at javax.faces.component.UIViewRoot._broadcastAll(UIViewRoot.java:995) at javax.faces.component.UIViewRoot.broadcastEvents(UIViewRoot.java:278) at javax.faces.component.UIViewRoot._process(UIViewRoot.java:1307) at javax.faces.component.UIViewRoot.processApplication(UIViewRoot.java:733) at org.apache.myfaces.lifecycle.InvokeApplicationExecutor.execute(InvokeApplicationExecutor.java:34) at org.apache.myfaces.lifecycle.LifecycleImpl.__AW_executePhase(LifecycleImpl.java:172) at org.apache.myfaces.lifecycle.LifecycleImpl.executePhase(LifecycleImpl.java) at org.apache.myfaces.lifecycle.LifecycleImpl.__AW_execute(LifecycleImpl.java:119) at org.apache.myfaces.lifecycle.LifecycleImpl.execute(LifecycleImpl.java) at javax.faces.webapp.FacesServlet.__AW_service(FacesServlet.java:189) at javax.faces.webapp.FacesServlet.service(FacesServlet.java) at com.ibm.ws.webcontainer.servlet.ServletWrapper.service(ServletWrapper.java:1233) at com.ibm.ws.webcontainer.servlet.ServletWrapper.handleRequest(ServletWrapper.java:782) at com.ibm.ws.webcontainer.servlet.ServletWrapper.handleRequest(ServletWrapper.java:481) at com.ibm.ws.webcontainer.servlet.ServletWrapperImpl.handleRequest(ServletWrapperImpl.java:178) at com.ibm.ws.webcontainer.filter.WebAppFilterChain.invokeTarget(WebAppFilterChain.java:136) at com.ibm.ws.webcontainer.filter.WebAppFilterChain.doFilter(WebAppFilterChain.java:97) at com.icbc.licensing.filter.ThreadContextFilter.__AW_doFilter(ThreadContextFilter.java:92) at com.icbc.licensing.filter.ThreadContextFilter.doFilter(ThreadContextFilter.java) at com.ibm.ws.webcontainer.filter.FilterInstanceWrapper.doFilter(FilterInstanceWrapper.java:195) at com.ibm.ws.webcontainer.filter.WebAppFilterChain.doFilter(WebAppFilterChain.java:91) at org.apache.logging.log4j.web.Log4jServletFilter.__AW_doFilter(Log4jServletFilter.java:71) at org.apache.logging.log4j.web.Log4jServletFilter.doFilter(Log4jServletFilter.java) at com.ibm.ws.webcontainer.filter.FilterInstanceWrapper.doFilter(FilterInstanceWrapper.java:195) at com.ibm.ws.webcontainer.filter.WebAppFilterChain.doFilter(WebAppFilterChain.java:91) at com.ibm.ws.webcontainer.filter.WebAppFilterManager.doFilter(WebAppFilterManager.java:967) at com.ibm.ws.webcontainer.filter.WebAppFilterManager.invokeFilters(WebAppFilterManager.java:1107) at com.ibm.ws.webcontainer.servlet.CacheServletWrapper.handleRequest(CacheServletWrapper.java:87) at com.ibm.ws.webcontainer.WebContainer.handleRequest(WebContainer.java:949) at com.ibm.ws.webcontainer.WSWebContainer.handleRequest(WSWebContainer.java:1817) at com.ibm.ws.webcontainer.channel.WCChannelLink.ready(WCChannelLink.java:200) at com.ibm.ws.http.channel.inbound.impl.HttpInboundLink.handleDiscrimination(HttpInboundLink.java:463) at com.ibm.ws.http.channel.inbound.impl.HttpInboundLink.handleNewRequest(HttpInboundLink.java:530) at com.ibm.ws.http.channel.inbound.impl.HttpInboundLink.processRequest(HttpInboundLink.java:316) at com.ibm.ws.http.channel.inbound.impl.HttpInboundLink.ready(HttpInboundLink.java:287) at com.ibm.ws.tcp.channel.impl.NewConnectionInitialReadCallback.sendToDiscriminators(NewConnectionInitialReadCallback.java:214) at com.ibm.ws.tcp.channel.impl.NewConnectionInitialReadCallback.complete(NewConnectionInitialReadCallback.java:113) at com.ibm.ws.tcp.channel.impl.AioReadCompletionListener.futureCompleted(AioReadCompletionListener.java:175) at com.ibm.io.async.AbstractAsyncFuture.invokeCallback(AbstractAsyncFuture.java:217) at com.ibm.io.async.AsyncChannelFuture$1.__AW_run(AsyncChannelFuture.java:205) at com.ibm.io.async.AsyncChannelFuture$1.run(AsyncChannelFuture.java) at com.ibm.ws.util.ThreadPool$Worker.run(ThreadPool.java:1892) Caused by: java.lang.NullPointerException at com.icbc.licensing.clio.action.VerificationAction.DbAuthInsertandCount(VerificationAction.java:118) at com.icbc.licensing.clio.action.VerificationAction.__AW_authenticateClient(VerificationAction.java:161) at com.icbc.licensing.clio.action.VerificationAction.authenticateClient(VerificationAction.java) at sun.reflect.GeneratedMethodAccessor440.invoke(Unknown Source) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:56) at java.lang.reflect.Method.invoke(Method.java:620) at org.apache.el.parser.AstValue.__AW_invoke(AstValue.java:268) at org.apache.el.parser.AstValue.invoke(AstValue.java) at org.apache.el.MethodExpressionImpl.invoke(MethodExpressionImpl.java:278) at org.apache.myfaces.view.facelets.el.TagMethodExpression.invoke(TagMethodExpression.java:83) ... 46 more

Error Page Exception: Error Message: javax.servlet.ServletException: WebBeans context with scope type annotation @RequestScoped does not exist within current thread Error Code: 0 Target Servlet: Error Stack: javax.enterprise.context.ContextNotActiveException: WebBeans context with scope type annotation @RequestScoped does not exist within current thread at org.apache.webbeans.container.BeanManagerImpl.__AW_getContext(BeanManagerImpl.java:363) at org.apache.webbeans.container.BeanManagerImpl.getContext(BeanManagerImpl.java) at org.apache.webbeans.intercept.NormalScopedBeanInterceptorHandler.getContextualInstance(NormalScopedBeanInterceptorHandler.java:125) at org.apache.webbeans.intercept.NormalScopedBeanInterceptorHandler.invoke(NormalScopedBeanInterceptorHandler.java:96) at org.apache.webbeans.conversation.ConversationImpl_$$_javassist_0.isTransient(ConversationImpl_$$_javassist_0.java) at ...

s3696107y ago

verifcationAction.authenticateClient() call hit a null ref exception (called from the template at /verification.xhtml) and their error page is trying to use a webBean context which hasn't been setup or activated yet to display the error.

The original site of the null ref error seems to be the DBAuthInsert method at com.icbc.licensing.clio.action.VerificationAction.DbAuthInsertandCount(VerificationAction.java:118) so maybe a getConnection is returning a null and not being checked, or some config variable is borked. Would start digging at line 118 to work out what variables are being dereferenced and where their values come from.

emmelaich7y ago

This? https://stackoverflow.com/questions/25685895/webbeans-contex...

rad_gruchalski7y ago

The server process running in a thread other than the router? Dunno, I’m sure I could figure it out having access to the code. Support most likely does not employ software engineers / SREs.

TeMPOraL7y ago

In my experience, the progression of skill comes almost full circle. Juniors don't read error messages and try to debug "from first principles". Seniors pay attention to error messages. Experts often don't (initially) read the error message, because the very fact that there is an error immediately clues them in on its likely cause.

sethammons7y ago

I had a candidate once. They were having trouble getting the message body on a 400 level error during the technical interview. They had chosen to solve it in Java. I'm not a Java developer (and I let the candidate know ahead of time and that I could only help with naive suggestions). Their exception message was pretty clear. After they prodded around for a bit getting nowhere, I pointed out the exception message. He ignored my suggestions multiple times (multiple angles of "your exception says blah"). After the interview, I took his solution and fixed it per the exception message. Boom. Worked. After the interview, his feedback to our recruiter was that it was obvious I did not know Java. Some people are bad at reading and listening :(. Also, this was a former Googler. It helped shatter an (unconscious?) bias that somehow Googlers were a breed above the rest.

btschaegg7y ago

In principle, I couldn't agree more. I get this from time to time, someone is stumped on an exception and after seeing it, I usually go "well, it's a null pointer exception in this method, so from looking at the code, it could come from these 3 places".

The pattern I see there is that a) in release builds, exceptions on many platforms suck. Things get inlined, parts of the stack trace are missing etc. If you understand why that is, that's not so much of a problem. But: b) people often have this Voodoo-like mentality about this. They often see the 100th stack trace that doesn't make much sense and try to wing it, because that's obviously always the case. The best thing you can do look at them together: Lead by example, think aloud and you might just teach someone how to read it in a better way.

It's rather important you have a consistent and accurate mental model of why your stack traces are mangled, lest you end up like one of those beasts that got random treats in B.F. Skinner's experiments and ended up with the strangest behaviors possible :-)

thrower1237y ago

People never read error messages. Most of them click straight through 8 milliseconds after the error appears, without taking the slightest opportunity to understand.

It's rage-inducing when troubleshooting over laggy screensharing sessions. If enough frames drop, I may not even see that the error was raised.

j / k navigate · click thread line to collapse

0 comments

fhood7y ago

It isn't just a huge hint, most of the time it tells you exactly what the damn issue is. This discussion is blowing my mind.

I am honestly speechless. IT GIVES YOU THE LINE NUMBER AND THE FUNCTION CALL AND THE REASON! Why would you ever even attempt to debug without parsing it?

eitland7y ago

Wait wait wait.

Not so fast.

i'm one of those programmers who read manuals and stack traces but I have to admit that I've seen my share of both manuals as well as stack traces that didn't make much sense.

kickopotomus7y ago

3 more replies

dave_sid7y ago

Yeah I agree. The way this thread is going you would think that all errors are easy and all you have to do is what the computer tells you and you’re done. I wish I’d known it was all that simple!

TimTheTinker7y ago

> stack traces that didn't make much sense.

Those can be especially prevalent in systems that utilize a lot of indirection.

_mdpn7y ago

Especially when you end up in C++ template hell.

2 more replies

dba7dba7y ago

tyrex20177y ago

as in tensorflow stack traces

dsr_7y ago

And in the opposite direction: remember when you are writing error messages to include all the relevant details, and never ever write "this shouldn't happen" or "document later".

fhood7y ago

What about, log_panic("If we have somehow managed to hit this code path, this project if FUBAR. I would recommend giving up and starting over.")

2 more replies

pletnes7y ago

If used correctly, «this should not happen» can be very useful, indicating e.g that the program is in some illegal state, as opposed to the input being incorrectly formatted.

1 more reply

scarface747y ago

That’s only true if the developer of the class module let the entire stack trace get through to the consumer.

In C# it’s the difference between...

try

{

....

}catch(Exception e)

{

//Do stuff

   throw new MyCustomException(“Something Bad happened”);

}

And

try

{

....

}catch(Exception e)

{

  //do stuff

   throw;

}

moron4hire7y ago

Erh, exception types should include an inner exception parameter, for the generic one they wrap. So neither of your two examples are all that great.

1 more reply

debaserab27y ago

In what case would you ever want the second example?

3 more replies

bmhin7y ago

avip7y ago

---

Error Page Exception SRVE0260E: The server cannot use the error page specified for your application to handle the Original Exception printed below.

s3696107y ago

emmelaich7y ago

This? https://stackoverflow.com/questions/25685895/webbeans-contex...

rad_gruchalski7y ago

The server process running in a thread other than the router? Dunno, I’m sure I could figure it out having access to the code. Support most likely does not employ software engineers / SREs.

TeMPOraL7y ago

sethammons7y ago

btschaegg7y ago

thrower1237y ago

People never read error messages. Most of them click straight through 8 milliseconds after the error appears, without taking the slightest opportunity to understand.

It's rage-inducing when troubleshooting over laggy screensharing sessions. If enough frames drop, I may not even see that the error was raised.

j / k navigate · click thread line to collapse