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From: Lew <noone@lewscanon.com>
Newsgroups: comp.lang.java.programmer
Subject: Re: About using assertion
Date: Thu, 19 May 2011 09:38:41 -0400
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On 05/19/2011 09:16 AM, Michal Kleczek wrote:
> Lew wrote:
>
>>
>> That is wrong.  That needs to be checked with an 'if' if 'NaN' is not a
>> legitimate input.  'assert' is not meant to correct things, only to make
>> sure
>
> I totally agree with you.
>
>> that the correction is documented and proven.  If 'assert' is turned off
>> in production, you still have the bug if you use 'assert' as the only
>> guard.
>
> I was imprecise - just wanted to praise an example of "having confidence"
> being sooo wrong and how having assertions (even though they look redundant)
> _may_ help you.

Totally correct.

Assertions are a signpost: HERE THERE BE AN INVARIANT!

When subtypes or refactored code violate the invariant, the assertion can help 
catch it.

  public class Foo
  {
    private Attribute attribute;

    public void setAttribute( Attribute attr )
    {
      if ( attr == null )
      {
        throw new IllegalArgumentException( "null argument, dummy" );
      }
      attribute = attr;
      assert attribute != null;
    }

    public Attribute getAttribute()
    {
      assert attribute != null;
      return attribute;
    }
  }

  public class Bar extends Foo()
  {
    public void setAttribute( Attribute attr )
    {
      attribute = attr;
    }
  }

The assertion will help reveal that class 'Bar' violated the invariant.  In 
production, the 'NullPointerException' (in the logs from 'getAttribute()' 
clients) will trigger an investigation, which will include enabling assertions 
for the class that yields the value, and then the invariant checks will help 
find the problem.

-- 
Lew
Honi soit qui mal y pense.
http://upload.wikimedia.org/wikipedia/commons/c/cf/Friz.jpg