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Create an Object from an Array Class

 
 
Z
Guest
Posts: n/a
 
      10-30-2007
I am using reflection to getmethod(methodName, Class parameters) and
invoke(object, obj[] parameters) methods from some Objects.

My problem is:

Say: I want to invoke a method that takes String[] object as parameter
(I was able to getmethod correctly). When I try to create an object
(to pass as args to invoke):

Object someObject= (Object) String[].class.newInstance();

(Note that the object is not always an instance of String[]... it
could be an instance of SomeOtherClass[])

I do get the following exception:

java.lang.InstantiationException: [Ljava.lang.String;

My question is:

Does the "[L" at the beginning and the ";" at the end have a meaning?
How can I solve this problem?

I am using IntelliJ 6.0.5.

Thanks in advance.

 
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Joshua Cranmer
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      10-30-2007
Z wrote:
> I am using reflection to getmethod(methodName, Class parameters) and
> invoke(object, obj[] parameters) methods from some Objects.
>
> My problem is:
>
> Say: I want to invoke a method that takes String[] object as parameter
> (I was able to getmethod correctly). When I try to create an object
> (to pass as args to invoke):
>
> Object someObject= (Object) String[].class.newInstance();
>
> (Note that the object is not always an instance of String[]... it
> could be an instance of SomeOtherClass[])


RTFM:
[ From Class.newInstance(): ]
Throws:
[ ... ]
InstantiationException - if this Class represents an abstract
class, an interface, an array class, a primitive type, or void; or if
the class has no nullary constructor; or if the instantiation fails for
some other reason.
[ ... ]

Arrays cannot be created with a new instance, because they are a
special-case class.

> My question is:
>
> Does the "[L" at the beginning and the ";" at the end have a meaning?


Yes, this is the internal representation of the class (it means that
this is an array of java.lang.String's).

> How can I solve this problem?


What you are probable intending is something like this:

Method m = Class.forName("some.pkg.Type").getMethod("main",
String[].class);
m.invoke(null, new String[0]);

> I am using IntelliJ 6.0.5.


Java version matters, not IDE version.

--
Beware of bugs in the above code; I have only proved it correct, not
tried it. -- Donald E. Knuth
 
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Joshua Cranmer
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Posts: n/a
 
      10-30-2007
Joshua Cranmer wrote:
> What you are probable intending is something like this:
>
> Method m = Class.forName("some.pkg.Type").getMethod("main",
> String[].class);
> m.invoke(null, new String[0]);


If the type of array must be determined at runtime, the following idiom
can also be used:

// arrayType is a class representing the type we want.
Object array = Array.newInstance(arrayType, 0); // Empty array

Array has another overloaded version that handles multiple array dimensions.

--
Beware of bugs in the above code; I have only proved it correct, not
tried it. -- Donald E. Knuth
 
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Z
Guest
Posts: n/a
 
      10-30-2007
Thanks for the clarifications. I hope that my phrasing of the question
shows where I am stuck.

What I am intending to do is:

//here is how my program is flowing

String className = namesOfAllClasses[j]; //I have it coming through a
loop

Class class1 = Class.forName(className);

Fields fields = class1.getDeclaredFields(); //assuming that all fields
are Class type (not primitive)

for (int i=0; i < fields.length; i++) {
Class newClass = Class.forName(fields[i].getType().getName()); //
create a class of the field type
Object newObject = newClass.newInstance(); //create an object of
the new field type
Method m = newClass.getMethod(methodName, newClass); /*get method;
we can assume it's "set"+fields[i].getName()*/
Object arguments[] = {"dummy"};
m.invoke(newObject, arguments);
}

/*
When a field is someClass[], I am facing troubles since now the
fields[i].getType().getName() = L[some.pkg.name.someClass;
In this case:
Class newClass = Class.forName(fields[i].getType().getName()) is
returning with a result (no exceptions)

But using the newInstance is generating exceptions since it's a
special-case class as you explained.

You have used
m.invoke(null, new String[0]); //it gave me back a
nullPointerRxception even though it's a static method

My problem is that the second parameter in invoke does not have to be
a String type for the parameter object but is an object instance of
the L[some.pkg.name.someClass; (object array)

Example: This method looks like this in ClassName.java

public void setsomeObjectProperty(some.pkg.name.someClass[]
someObjectProperty) {
this.someObjectProperty= someObjectProperty;
}

Any tips?
*/

Thanks again for your time


On Oct 30, 5:53 pm, Joshua Cranmer <(E-Mail Removed)> wrote:
> Z wrote:
> > I am using reflection to getmethod(methodName, Class parameters) and
> > invoke(object, obj[] parameters) methods from some Objects.

>
> > My problem is:

>
> > Say: I want to invoke a method that takes String[] object as parameter
> > (I was able to getmethod correctly). When I try to create an object
> > (to pass as args to invoke):

>
> > Object someObject= (Object) String[].class.newInstance();

>
> > (Note that the object is not always an instance of String[]... it
> > could be an instance of SomeOtherClass[])

>
> RTFM:
> [ From Class.newInstance(): ]
> Throws:
> [ ... ]
> InstantiationException - if this Class represents an abstract
> class, an interface, an array class, a primitive type, or void; or if
> the class has no nullary constructor; or if the instantiation fails for
> some other reason.
> [ ... ]
>
> Arrays cannot be created with a new instance, because they are a
> special-case class.
>
> > My question is:

>
> > Does the "[L" at the beginning and the ";" at the end have a meaning?

>
> Yes, this is the internal representation of the class (it means that
> this is an array of java.lang.String's).
>
> > How can I solve this problem?

>
> What you are probable intending is something like this:
>
> Method m = Class.forName("some.pkg.Type").getMethod("main",
> String[].class);
> m.invoke(null, new String[0]);
>
> > I am using IntelliJ 6.0.5.

>
> Java version matters, not IDE version.
>
> --
> Beware of bugs in the above code; I have only proved it correct, not
> tried it. -- Donald E. Knuth


 
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Lasse Reichstein Nielsen
Guest
Posts: n/a
 
      10-31-2007
Z <(E-Mail Removed)> writes:

> I am using reflection to getmethod(methodName, Class parameters) and
> invoke(object, obj[] parameters) methods from some Objects.
>
> My problem is:
>
> Say: I want to invoke a method that takes String[] object as parameter
> (I was able to getmethod correctly). When I try to create an object
> (to pass as args to invoke):
>
> Object someObject= (Object) String[].class.newInstance();


That fails, because the array class does not have a normal
constructor. Here you try to invoke a default constructor.

> (Note that the object is not always an instance of String[]... it
> could be an instance of SomeOtherClass[])
>
> I do get the following exception:
>


> java.lang.InstantiationException: [Ljava.lang.String;


> My question is:
>
> Does the "[L" at the beginning and the ";" at the end have a meaning?


Yes. Java writes the name of and array of object type as "[L" followed
by the name of the class and ended by ";". I.e., the type
my.foo.Foo[]
has the name "[Lmy.foo.Foo;". Arrays of base types have other abbreviations,
e.g., "[I" for int[].

> How can I solve this problem?


Array.newInstance(String.class, 0); // equivalent to: new String[0]

/L
--
Lasse Reichstein Nielsen - http://www.velocityreviews.com/forums/(E-Mail Removed)
DHTML Death Colors: <URL:http://www.infimum.dk/HTML/rasterTriangleDOM.html>
'Faith without judgement merely degrades the spirit divine.'
 
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Daniel Pitts
Guest
Posts: n/a
 
      10-31-2007
Z wrote:
> On Oct 30, 5:53 pm, Joshua Cranmer <(E-Mail Removed)> wrote:
>> Z wrote:
>>> I am using reflection to getmethod(methodName, Class parameters) and
>>> invoke(object, obj[] parameters) methods from some Objects.
>>> My problem is:
>>> Say: I want to invoke a method that takes String[] object as parameter
>>> (I was able to getmethod correctly). When I try to create an object
>>> (to pass as args to invoke):
>>> Object someObject= (Object) String[].class.newInstance();
>>> (Note that the object is not always an instance of String[]... it
>>> could be an instance of SomeOtherClass[])

>> RTFM:
>> [ From Class.newInstance(): ]
>> Throws:
>> [ ... ]
>> InstantiationException - if this Class represents an abstract
>> class, an interface, an array class, a primitive type, or void; or if
>> the class has no nullary constructor; or if the instantiation fails for
>> some other reason.
>> [ ... ]
>>
>> Arrays cannot be created with a new instance, because they are a
>> special-case class.
>>
>>> My question is:
>>> Does the "[L" at the beginning and the ";" at the end have a meaning?

>> Yes, this is the internal representation of the class (it means that
>> this is an array of java.lang.String's).
>>
>>> How can I solve this problem?

>> What you are probable intending is something like this:
>>
>> Method m = Class.forName("some.pkg.Type").getMethod("main",
>> String[].class);
>> m.invoke(null, new String[0]);
>>
>>> I am using IntelliJ 6.0.5.

>> Java version matters, not IDE version.
>>
>> --
>> Beware of bugs in the above code; I have only proved it correct, not
>> tried it. -- Donald E. Knuth

> Thanks for the clarifications. I hope that my phrasing of the question
> shows where I am stuck.
>
> What I am intending to do is:
>
> //here is how my program is flowing
>
> String className = namesOfAllClasses[j]; //I have it coming through a
> loop
>
> Class class1 = Class.forName(className);
>
> Fields fields = class1.getDeclaredFields(); //assuming that all fields
> are Class type (not primitive)
>
> for (int i=0; i < fields.length; i++) {
> Class newClass = Class.forName(fields[i].getType().getName()); //
> create a class of the field type
> Object newObject = newClass.newInstance(); //create an object of
> the new field type
> Method m = newClass.getMethod(methodName, newClass); /*get method;
> we can assume it's "set"+fields[i].getName()*/
> Object arguments[] = {"dummy"};
> m.invoke(newObject, arguments);
> }
>
> /*
> When a field is someClass[], I am facing troubles since now the
> fields[i].getType().getName() = L[some.pkg.name.someClass;
> In this case:
> Class newClass = Class.forName(fields[i].getType().getName()) is
> returning with a result (no exceptions)
>
> But using the newInstance is generating exceptions since it's a
> special-case class as you explained.
>
> You have used
> m.invoke(null, new String[0]); //it gave me back a
> nullPointerRxception even though it's a static method
>
> My problem is that the second parameter in invoke does not have to be
> a String type for the parameter object but is an object instance of
> the L[some.pkg.name.someClass; (object array)
>
> Example: This method looks like this in ClassName.java
>
> public void setsomeObjectProperty(some.pkg.name.someClass[]
> someObjectProperty) {
> this.someObjectProperty= someObjectProperty;
> }
>
> Any tips?
> */
>
> Thanks again for your time
>
>

First, please don't top-post. Reply after the bottom or interleaved.

Second, Don't look for set+name or get+name methods. Use the
introspection API. (java.beans.*). There is more to JavaBeans than just
a naming convention.

Also, ask yourself if you REALLY need to use reflection/introspection at
all. I wrote an article about it on my blog sometime last year
<http://virtualinfinity.net/wordpress/>

I'll post the exact link when I have a chance to look it up.

--
Daniel Pitts' Tech Blog: <http://virtualinfinity.net/wordpress/>
 
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