Specifically, you could use
public <T> T getInstance(Class<T> clazz)
throws InstantiationException, IllegalAccessException{
return clazz.newInstance();
}
The Class<T> object has a method newInstance() that creates an instance of a
class with the default constructor. The exceptions it throws represent cases
where you don't have visibility permissions for the constructor, there is no
default constructor, the class is abstract, or the constructor call throws an
error (which is then wrapped and rethrown). The method outlined above is just a
wrapper - if you already have the class object you can just instantiate it. If
you need to not use the default constructor, try something like:
public <T> T getInstance(Class<T> clazz)
throws IllegalArgumentException,
SecurityException,
InstantiationException,
IllegalAccessException,
InvocationTargetException,
NoSuchMethodException {
return clazz
.getConstructor(
Parameter1Type.class,
Parameter2Type.class)
.newInstance(
parameter1,
parameter2);
}
The getConstructor methods takes all they types for it's parameters in
declaration order. This is to resolve the method signature. In this class your
class would have a constructor:
MyClass(Parameter1Type parameter1, Parameter2Type parameter2){
// constructor stuff here
}
The newInstance method takes the actual parameters to pass to the constructor.
In this example, they are parameter1 (which is a Parameter1Type) and parameter2
(which is a Parameter2Type). The errors occur if the constructor doesn't exist,
the arguments are the wrong type, the caller doesn't have visibility
permissions, the class is abstract, or the constructor throws an error (which
is then wrapped and rethrown).
You could also pass the parameters into the getInstance method and pick out the
constructer dynamically like so:
public <T> T getInstance(Class<T> clazz, Object... args)
throws InvocationTargetException {
T newObject = null;
for (java.lang.reflect.Constructor<T> c :
clazz.getConstructors()) {
// try creating objects with the passed
// args until one works.
try {
newObject = c.newInstance(args);
break;
} catch (IllegalArgumentException e) {
} catch (InstantiationException e) {
} catch (IllegalAccessException e) {
}
}
return newObject;
}
This method returns an instance of the class passed created with the
constructor parameters passed. If the constructor throws an error it is wrapped
in an InvocationTargetException and rethrown. If no constructor matches the
method returns null.
-----Original Message-----
From: Giovanni Azua [mailto:[EMAIL PROTECTED]
Sent: September 27, 2007 11:56 AM
To: Struts Users Mailing List
Subject: Re: [ot] help on generics...
how about:
public static <T> T
getInstance(Class<T> aClass)
{
// TODO:
}
regards,
Giovanni
nicolas de loof wrote:
> Hello,
>
> my question is fully off topic, but Struts2 is the only java5 project I know.
>
> I'd like a method to return an instance of a class passed as parameter :
>
> public Object getInstance( Class clazz )
>
> I'd like to use generics to make the return type in sync with the
> class type. Is this possible ???
>
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