Sorry to all you Groovy dudes if this is a bit of a noob question.
In SOAPUI, i can create a Groovy script where i can define an arbitrary variable to the run context to retrieve at a later time.
context.previouslyUndefinedVariable = 3
def num = context.previouslyUndefinedVariable
What feature of Groovy allows previously undefined variables to be added to an object like this? I would like to learn more about it.
Many thanks in advance!
Groovy has the ability to dynamically add methods to a class through metaprogramming.
To learn more, see:
What is Groovy's MetaClass used for?
Groovy Goodness: Add Methods Dynamically to Classes with ExpandoMetaClass
Runtime and compile-time metaprogramming
The accepted answer is a bit of a poor explanation for how SoapUI is doing it.
In this case, context is always an instance of some SoapUI library java class (such as WsdlTestRunContext), and these are all implementations of Map. You can check context.getClass() and assert context in Map.
When you look up a property on a Map, Groovy uses the getAt and putAt methods. There are various syntaxes you can use. All of these are equivalent:
context.someUndef
context.'someUndef'
context[someUndef]
context['someUndef']
context.getAt('someUndef')
And
context.someUndef = 3
context.'someUndef' = 3
context[someUndef] = 3
context['someUndef'] = 3
context.putAt('someUndef', 3)
I like to use any of the above that include quote marks, so that Groovy-Eclipse doesn't flag it as a missing property.
It's also interesting that Groovy looks for a getAt() method before it checks for a get method being referred to as a property.
For example, consider evaluating "foo".class. The String instance doesn't have a property called class and it also doesn't have a method getAt(String), so the next thing it tries is to look for a "get" method with that name, i.e. getClass(), which it finds, and we get our result: String.
But with a map, ['class':'bar'].class refers to the method call getAt('class') first, which will be 'bar'. If we want to know what type of Map it is, we have to be more specific and write in full: ['class':'bar'].getClass() which will be LinkedHashMap.
We still have to specify getClass() even if that Map doesn't have a matching key, because ['foo':'bar'].class will still mean ['foo':'bar'].getAt('class'), which will be null.
I have an object stored in the ViewScope: ObjectName (valueA:one, valueB:two)
I stored the values using Java:
ObjectObject location = new ObjectObject();
location.put("valueA", FBSUtility.wrap("one"));
location.put("valueB", FBSUtility.wrap("two"));
Utils.setViewScope("ObjectName", location);
How would I go about retrieving these values from the ViewScope? I've tried doing something like the following:
ObjectObject location;
location = (ObjectObject) ExtLibUtil.getViewScope().get("ObjectName");
but I'm not sure what methods to use to get the values or if this is even the correct path. Thanks in advance for any help.
It's a bit unusual to go out of your way to use the FBS classes, but that path is reasonable enough to accomplish what you want. As long as the latter code is executed after the format, that should retrieve the same object and properly cast it to ObjectObject. After that, you could use location.get("valueA"), etc. to get the values by name, and then whatever methods of FBSValue are appropriate (I'd guess stringValue()).
Incidentally, unless you have a specific need to use these internal classes (like if you're doing something fancy with SSJS functions), it may make sense to use just a normal HashMap<String, Object> instead. SSJS and EL can work with those quite well.
I am simply using a cftry/cfcatch block for handling any exception. Take this simple exception:
<cftry>
<cfset abc = 1/0>
<cfcatch>
<cfdump var="#cfcatch.getClass().getName()#">
<cfdump var="#isStruct(cfcatch)#">
<cfdump var="#isObject(cfcatch)#">
<cfdump var="#structKeyExists(cfcatch, 'type')#">
</cfcatch>
</cftry>
And the above code's output is like this:
coldfusion.runtime.DivideByZeroException
NO
YES
YES
My question is:
Why structKeyExists is not throwing an error as cfcatch is not of type struct?
And on dumping cfcatch it seems like it is a struct.
Any suggestions.
I think what is confusing you is that you need to remember that ColdFusion is a typeless language.
ColdFusion documentation on data types
Data types
ColdFusion is often referred to as typeless because you do not assign types to variables and ColdFusion does not associate a type with the variable name. However, the data that a variable represents does have a type, and the data type affects how ColdFusion evaluates an expression or function argument. ColdFusion can automatically convert many data types into others when it evaluates expressions. For simple data, such as numbers and strings, the data type is unimportant until the variable is used in an expression or as a function argument.
ColdFusion variable data belongs to one of the following type categories:
Simple One value. Can use directly in ColdFusion expressions. Include numbers, strings, Boolean values, and date-time values.
Binary Raw data, such as the contents of a GIF file or an executable program file.
Complex ** A container for data. Generally represent more than one value. ColdFusion built-in complex data types include arrays, structures, queries, and XML document objects. You cannot use a complex variable, such as an array, directly in a ColdFusion expression, but you can use simple data type elements of a complex variable in an expression. For example, with a one-dimensional array of numbers called myArray, you cannot use the expression myArray * 5. However, you could use an expression myArray3 * 5 to multiply the third element in the array by five.
Objects Complex constructs. Often encapsulate both data and functional operations. The following table lists the types of objects that ColdFusion can use, and identifies the chapters that describe how to use them:
So the code within the <cfcatch> block contains an object that can be referred to as a "structure". By default the name of that structure is cfcatch. You can override that name by specifying the name attribute within the <cfcatch> tag.
The easiest way to see everything that is available to you is to <cfdump> the entire structure within the <cfcatch> block.
<cfcatch>
<cfdump var="#cfcatch#">
</cfcatch>
CFCatch documentation on cfcatch
The cfcatch variables provide the following exception information:
cfcatch variable Content
cfcatch.type Type: Exception type, as specified in cfcatch.
cfcatch.message Message: Exceptions diagnostic message, if provided; otherwise, an empty string; in the cfcatch.message variable.
cfcatch.detail Detailed message from the CFML interpreter or specified in a cfthrow tag. When the exception is generated by ColdFusion (and not cfthrow), the message can contain HTML formatting and can help determine which tag threw the exception.
cfcatch.tagcontext An array of tag context structures, each representing one level of the active tag context at the time of the exception.
cfcatch.NativeErrorCode Applies to type = "database". Native error code associated with exception. Database drivers typically provide error codes to diagnose failing database operations. Default value is -1.
cfcatch.SQLState Applies to type = "database". SQLState associated with exception. Database drivers typically provide error codes to help diagnose failing database operations. Default value is 1.
cfcatch.Sql Applies to type = "database". The SQL statement sent to the data source.
cfcatch.queryError Applies to type ="database". The error message as reported by the database driver.
cfcatch.where Applies to type= "database". If the query uses the cfqueryparam tag, query parameter name-value pairs.
cfcatch.ErrNumber Applies to type = "expression". Internal expression error > number.
cfcatch.MissingFileName Applies to type = "missingInclude". Name of file that could not be included.
cfcatch.LockName Applies to type = "lock". Name of affected lock (if the lock is unnamed, the value is "anonymous").
cfcatch.LockOperation Applies to type = "lock". Operation that failed (Timeout, Create Mutex, or Unknown).
cfcatch.ErrorCode Applies to type = "custom". String error code.
cfcatch.ExtendedInfo Applies to type = "application" and "custom". Custom error message; information that the default exception handler does not display.
(Too long for comments..)
Adding to Miguel-F's comments about CF's "typelessness"... according to the documentation, IsStruct uses the following rules (emphasis mine):
Returns True, if variable is a ColdFusion structure or is a Java
object that implements the java.lang.Map interface. The return value
is False if the object in variable is a user-defined function (UDF).
CFCatch does not meet that criteria. Hence why IsStruct returns false.
If you dump the cfcatch object, and examine the class hierarchy, you will see it is actually implemented as a subclass of java.lang.Exception:
coldfusion.runtime.DivideByZeroException
coldfusion.runtime.ExpressionException
coldfusion.runtime.NeoException
java.lang.RuntimeException
java.lang.Exception
java.lang.Throwable
java.lang.Object
... not coldfusion.runtime.struct ie CF structure:
coldfusion.runtime.Struct
coldfusion.util.FastHashtable
coldfusion.util.CaseInsensitiveMap
java.lang.Object
So as Miguel-F said, though it can be used like a structure (as can most complex object), technically it is not a CF structure, which explains why IsStruct returns false.
As an aside, the reason you can access its properties using dot notation, like with CF structures, is probably because the cfcatch class uses the JavaBean pattern:
ColdFusion can automatically invoke get_PropertyName_() and
set_PropertyName_(value) methods if a Java class conforms to the
JavaBeans pattern. As a result, you can set or get the property by
referencing it directly, without having to explicitly invoke a method.
So for example, you can access the "message" property of cfcatch using:
cfcatch.message
.. instead of invoking its "getter" method for that property:
cfcatch.getMessage()
cfcatch object acts like a struct, but it is not one. This is a special case.
What you can do is make a duplicate of cfcatch object and try isStruct method on it, it will return true.
for example-
<cftry>
<cfset abc = 1/0>
<cfcatch>
<cfset dup = duplicate(cfcatch)>
<cfdump var="#isStruct(dup)#">
<cfdump var="#isStruct(cfcatch)#">
<cfdump var="#isObject(cfcatch)#">
</cfcatch>
</cftry>
The output would be like
YES
NO
YES
I've often wondered why languages with a null representing "no value" don't differentiate between the passive "I don't know what the value is" and the more assertive "There is no value.".
There have been several cases where I'd have liked to differentiate between the two (especially when working with user-input and databases).
I imagine the following, where we name the two states unknown and null:
var apple;
while (apple is unknown)
{
askForApple();
}
if (apple is null)
{
sulk();
}
else
{
eatApple(apple);
}
Obviously, we can get away without it by manually storing the state somwhere else, but we can do that for nulls too.
So, if we can have one null, why can't we have two?
Isn't is bad enough that we have one null?
In my programming, I recently adopted the practice of differentiating "language null" and "domain null".
The "language null" is the special value that is provided by the programming language to express that a variable has "no value". It is needed as dummy value in data structures, parameter lists, and return values.
The "domain null" is any number of objects that implement the NullObject design pattern. Actually, you have one distinct domain null for each domain context.
It is fairly common for programmers to use the language null as a catch-all domain null, but I have found that it tends to make code more procedural (less object oriented) and the intent harder to discern.
Every time to want a null, ask yourself: is that a language null, or a domain null?
In most programming languages null means "empty" or "undefined". "Unknown" on the other hand is something different. In essence "unknown" describes the state of the object. This state must have come from somewhere in your program.
Have a look at the Null Object pattern. It may help you with what you are trying to achieve.
javascript actually has both null and undefined (http://www.w3schools.com/jsref/jsref_undefined.asp), but many other languages don't.
It would be easy enough to create a static constant indicating unknown, for the rare cases when you'd need such a thing.
var apple = Apple.Unknown;
while (apple == Apple.Unknown) {} // etc
Existence of value:
Python: vars().has_key('variableName')
PHP: isset(variable)
JavaScript: typeof(variable) != 'undefined'
Perl: (variable != undef) or if you wish: (defined variable)
Of course, when variable is undefined, it's not NULL
Why stop at two?
When I took databases in college, we were told that somebody (sorry, don't remember the name of the researcher or paper) had looked at a bunch of db schemas and found that null had something like 17 different meanings: "don't know yet", "can't be known", "doesn't apply", "none", "empty", "action not taken", "field not used", and so on.
In haskell you can define something like this:
data MaybeEither a b = Object a
| Unknown b
| Null
deriving Eq
main = let x = Object 5 in
if x == (Unknown [2]) then putStrLn ":-("
else putStrLn ":-)"
The idea being that Unknown values hold some data of type b that can transform them into known values (how you'd do that depends on the concrete types a and b).
The observant reader will note that I'm just combining Maybe and Either into one data type :)
The Null type is a subtype of all reference types - you can use null instead of a reference to any type of object - which severely weakens the type system. It is considered one of the a historically bad idea by its creator, and only exists as checking whether an address is zero is easy to implement.
As to why we don't have two nulls, is it down to the fact that, historically in C, NULL was a simple #define and not a distinct part of the language at all?
Within PHP Strict you need to do an isset() check for set variables (or else it throws a warning)
if(!isset($apple))
{
askForApple();
}
if(isset($apple) && empty($apple))
{
sulk();
}
else
{
eatApple();
}
In .net langages, you can use nullable types, which address this problem for value types.
The problem remains, however, for reference types. Since there's no such thing as pointers in .net (at least in 'safe' blocks), "object? o" won't compile.
Note null is an acceptable, yet known condition. An unknown state is a different thing IMO. My conversation with Dan in the comments' section of the top post will clarify my position. Thanks Dan!.
What you probably want to query is whether the object was initialized or not.
Actionscript has such a thing (null and undefined). With some restrictions however.
See documentation:
void data type
The void data type contains only one value, undefined. In previous versions of ActionScript, undefined was the default value for instances of the Object class. In ActionScript 3.0, the default value for Object instances is null. If you attempt to assign the value undefined to an instance of the Object class, Flash Player or Adobe AIR will convert the value to null. You can only assign a value of undefined to variables that are untyped. Untyped variables are variables that either lack any type annotation, or use the asterisk (*) symbol for type annotation. You can use void only as a return type annotation.
Some people will argue that we should be rid of null altogether, which seems fairly valid. After all, why stop at two nulls? Why not three or four and so on, each representing a "no value" state?
Imagine this, with refused, null, invalid:
var apple;
while (apple is refused)
{
askForApple();
}
if (apple is null)
{
sulk();
}
else if(apple is invalid)
{
discard();
}
else
{
eatApple(apple);
}
It's been tried: Visual Basic 6 had Nothing, Null, and Empty. And it led to such poor code, it featured at #12 in the legendary Thirteen Ways to Loathe VB article in Dr Dobbs.
Use the Null Object pattern instead, as others have suggested.
The problem is that in a strongly typed language these extra nulls are expected to hold specific information about the type.
Basically your extra null is meta-information of a sort, meta-information that can depend on type.
Some value types have this extra information, for instance many numeric types have a NaN constant.
In a dynamically typed language you have to account for the difference between a reference without a value (null) and a variable where the type could be anything (unknown or undefined)
So, for instance, in statically typed C# a variable of type String can be null, because it is a reference type. A variable of type Int32 cannot, because it is a value type it cannot be null. We always know the type.
In dynamically typed Javascript a variable's type can be left undefined, in which case the distinction between a null reference and an undefined value is needed.
Some people are one step ahead of you already. ;)
boolean getAnswer() throws Mu
Given how long it took Western philosophy to figure out how it was possible to talk about the concept of "nothing"... Yeah, I'm not too surprised the distinction got overlooked for a while.
I think having one NULL is a lower-common denominator to deal with the basic pattern
if thing is not NULL
work with it
else
do something else
In the "do something else" part, there are a wide range of possibilities from "okay, forget it" to trying to get "thing" somewhere else. If you don't simply ignore something that's NULL, you probably need to know why "thing" was NULL. Having multiple types of NULL, would help you answering that question, but the possible answers are numerous as hinted in the other answers here. The missing thing could be simply a bug, it could be an error when trying to get it, it may not be available right now, and so on. To decide which cases apply to your code -- which means you have to handle them -- it domain specific. So it's better to use an application defined mechanism to encode these reasons instead of finding a language feature that tries to deal with all of them.
It's because Null is an artifact of the language you're using, not a programmer convenience. It describes a naturally occurring state of the object in the context in which it is being used.
If you are using .NET 3.0+ and need something else, you might try the Maybe Monad. You could create whatever "Maybe" types you need and, using LINQ syntax, process accordingly.
AppleInformation appleInfo;
while (appleInfo is null)
{
askForAppleInfo();
}
Apple apple = appleInfo.apple;
if (apple is null)
{
sulk();
}
else
{
eatApple(apple);
}
First you check if you have the apple info, later you check if there is an apple or not. You don't need different native language support for that, just use the right classes.
For me null represents lack of value, and I try to use it only to represent that. Of course you can give null as many meanings as you like, just like you can use 0 or -1 to represent errors instead of their numerical values. Giving various meanings to one representation could be ambiguous though, so I wouldn't recommend it.
Your examples can be coded like apple.isRefused() or !apple.isValid() with little work; you should define beforehand what is an invalid apple anyway, so I don't see the gain of having more keywords.
You can always create an object and assign it to same static field to get a 2nd null.
For example, this is used in collections that allow elements to be null. Internally they use a private static final Object UNSET = new Object which is used as unset value and thus allows you to store nulls in the collection. (As I recall, Java's collection framework calls this object TOMBSTONE instead of UNSET. Or was this Smalltalk's collection framework?)
VB6
Nothing => "There is no value."
Null = > "I don't know what the value is" - Same as DBNull.Value in .NET
Two nulls would be the wrongest answer around. If one null is not enough, you need infinity nulls.
Null Could mean:
'Uninitialized'
'User didn't specify'
'Not Applicable here, The color of a car before it has been painted'
'Unity: This domain has zero bits of information.'
'Empty: this correctly holds no data in this case, for example the last time the tires were rotated on a new car'
'Multiple, Cascading nulls: for instance, the extension of a quantity price when no quantity may be specified times a quantity which wasn't specified by the user anyway'
And your particular domain may need many other kinds of 'out of band' values. Really, these values are in the domain, and need to have a well defined meaning in each case. (ergo, infinity really is zero)