Filter on null property

[vst...@gmail.com]

Hi,

Am I missing something obvious?

Want to filter the list when one of the properties is null.

Let's say there is a class named File with Boolean property shouldCopy, and a list of those objects: files

Expression:

val excludeFiles = files.filter [ shouldCopy == null ]

reports error in syntax 'The operator '==' is undefined for the argument types boolean and null"

filterNull method does not accept parameters, meaning it can only filter if object File is null.

Yes, I can implement a workaround by changing Boolean to boolean and tweak the logic, but seems 

somehow basic to have this feature, so I still believe that I am missing something obvious :-(

Thanks,

Strahinja

[Sebastian Zarnekow]

Hi,

there seems to be a problem with primitive boolean and null, though the check will always return false anyway.

Why don't you just use files.filter [ !shouldCopy ]

Best regards,

Sebastian

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[Strahinja]

Thanks, Sebastian for a quick reply.

files.filter [ !shouldCopy ]  

throws NPE in runtime.

Any ideas?

Regards,

Strahinja

[Sebastian Zarnekow]

How does the implementation of shouldCopy look like?

Regards,

Sebastian

[Pavel Tavoda]

I guess your problem can be that collection contains null or Boolean -> boolean conversion raise NPE. You have to check both conditions. Maybe this can help you:

class BooleanTest {

def public static void main (String[] arg) {

val f = newHashSet(new MyFile(true), new MyFile(null), null, new MyFile(false))

println("f=" + f)

val q1 = f.filter[it != null && bool != null && bool]

println("q1=" + q1)

val q2 = f.filter[it.bool != null]

println("q2=" + q2)

}

}

class MyFile {

public Boolean bool

new(Boolean b) {

bool = b

}

override toString() {

"Tst@bool="+bool

}

}

class MyFile {

public Boolean bool

new(Boolean b) {

bool = b

}

override toString() {

"Tst@bool="+bool

}

}

Output:

f=[null, Tst@bool=null, Tst@bool=true, Tst@bool=false]

q1=[Tst@bool=true]

Exception in thread "main" java.lang.NullPointerException

Pavel

[Strahinja]

@Pavel  thanks for the example, but in concrete example there are no null values in list itself, only a property of the object in the list can be null

@Sebastian  following is the example to reproduce the NPE:

class File {

var Boolean shouldCopy

new(Boolean sc) {

shouldCopy = sc

}

def public static void main(String[] args) {

val files = newArrayList(new File(Boolean.FALSE), new File(Boolean.TRUE), new File(null) )

println(files.filter [ !shouldCopy ])

}

}

Strahinja

[Sebastian Zarnekow]

That's a NPE which is caused by the unboxing.

In this case, the following code will work:

class File {

var Boolean shouldCopy

new(Boolean sc) {

shouldCopy = sc

}

def public static void main(String[] args) {

val files = newArrayList(new File(Boolean.FALSE), new File(Boolean.TRUE), new File(null) )

println(files.filter [ shouldCopy != null ])

}

}

The method / attribute shouldCopy from your first example was defined with a lowercase boolean type, wasn't it?

[Strahinja]

Well, this is pretty much embarrassing ... 

I spent 2 hours yesterday trying to figure out what is the cause of the syntax error on shouldCopy == null

(It was Boolean from the start not lowercased boolean)

Then I created this post and implemented the workaround.

Now, I can not reproduce it anymore, which is a good news for Xtend and embarrassing one for me ...

Maybe I just forgot to clean the project yesterday ...

Sorry guys for taking your time