RE : Java.lang.NoSuchMethodException

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ramakrishna - 04 May 2006 13:36 GMT

HI,
can any one tell me, why NoSuchMethodException will come in
Class.getConstructor( Class[] parameterTypes) method.
In my program Iam using the one constructor and iam calling the
constructor by passing array of class object

i created the same object using "new" operator with constructor
arguments and its works fine.

my code is :
ActionHelper class:

public ActionHelper(BaseAction tbAction) {
iAction = tbAction;
}

GetAuthorisedUnauthorisedRecordsAction is the subclass of the
BaseAction.

with new :
GetAuthorisedUnauthorisedRecordsAction gauUnau = new
GetAuthorisedUnauthorisedRecordsAction();

ActionHelper actionHelper = new ActionHelper( gauUnau );

it is creating the ActionHelper object.

using reflection package iam creating the ActionHelper object.

Class[] tParameterClasses = new Class[1];
tParameterClasses[0] = gauUnau.getClass();

Class tClass = Class.forName( ActionHelper );
Constructor tConstructor =
tClass.getConstructor(tParameterClasses);

Object[] tParameterObjects = new Object[1];
tParameterObjects[0] = (Object) gauUnau;
ActionHelper actionHelper = (ActionHelper)
tConstructor..newInstance(tParameterObjects);

in the above line it is showing exception at
Constructor tConstructor = tClass.getConstructor(tParameterClasses);
exception is java.lang.NoSuchMethodException..

As iam feeling that it does not accept the SubClass object.

it is accepting if i replace the ActionHelper code with below one.

public ActionHelper(GetAuthorisedUnauthorisedRecordsAction tbAction) {
iAction = tbAction;
}

1. Is there any difference b/w "new" and reflection package creation
object.
2. why this is happing.

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Bjorn Abelli - 04 May 2006 14:23 GMT

"ramakrishna" wrote...

> can any one tell me, why NoSuchMethodException will come in
> Class.getConstructor( Class[] parameterTypes) method.
> In my program Iam using the one constructor and iam calling the
> constructor by passing array of class object

I believe I have already answered your question once...

[snipped a lot of code]

> As iam feeling that it does not accept the SubClass object.

> 1. Is there any difference b/w "new" and reflection package
> creation object.

When using "new" the JVM looks for the "best case" constructor, i.e. if
there is no constructor with *exactly* the type of the argument, it looks
for constructors where polymorphism can be used, i.e. if there are any
constructors that can take a supertype of the parameters.

When using reflection to get a constructor, you need to provide the
*declared* parametertypes. There you provide/get instances of "Class", but a
Class-instance of a subtype is not polymorph to a Class-instance of a
supertype.

You need to search through the constructors yourself, to see which of them
that has the "best fit".

Here's a quick fix you could use to get such a Constructor:

--------------------------------------------

public static Constructor getPossibleConstructor
(Class motherClass, Class[] pars) {

Constructor[] ctors = motherClass.getConstructors();

for (Constructor c : ctors) {
Class[] cpars = c.getParameterTypes();

if (cpars.length != pars.length) continue;

boolean found = true;

for (int i = 0; i < cpars.length; ++i) {

if (!cpars[i].isAssignableFrom(pars[i])) {
found = false;
break;
}
}
if (found) return c;
}
return null;
}

--------------------------------------------

Now, using the names you've used in previous posts, you could:

--------------------------------------------

...

Class[] tParameterClasses = new Class[1];
tParameterClasses[0] = gauUnau.getClass();

Class tClass = Class.forName( ActionHelper );

// using the method provided above...
// though it also should have a check for null
// in case no constructors can fit with the
// provided parameter classes...

Constructor tConstructor =
getPossibleConstructor(tClass, tParameterClasses);

Object[] tParameterObjects = new Object[1];
tParameterObjects[0] = (Object) gauUnau;

ActionHelper actionHelper =
(ActionHelper)
tConstructor.newInstance(tParameterObjects);

// Bjorn A

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ramakrishna - 04 May 2006 15:40 GMT

Thank y Bjorn its woring........................

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RE : Java.lang.NoSuchMethodException

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