Create Objects Dynamically in Java

Question

Is there a (better) way to dynamically create Objects?

Right now I'm using a simple 'factory pattern' solution as following:

   String classType = generalObject.getClass().toString();

    if(classType.equals("class be.testApp.UserObject")) {
        return UserObject.fromByteArray(data);
//return new UserObject();
    }
    else if(classType.equals("class.be.testApp.NewsObject"))    {
        return NewsObject.fromByteArray(data);
//return new NewsObject();      
    }

Please do not use toString to get your class as a String... use getPackage and getName instead - or use the class attribute. — Jean Hominal
– Jean Hominal, Commented Feb 3, 2011 at 10:44
@Jean no need for getPackage(). getName() returns the fully qualified name — Sean Patrick Floyd
– Sean Patrick Floyd, Commented Feb 3, 2011 at 10:45
Try using getClass().getName() instead of getClass().toString(). At least you'll get rid of the unreadable "class " part of the string. Better: use Class type = generalObject.getClass(); and if (type.equals(UserObject.class)). — Olivier Grégoire
– Olivier Grégoire, Commented Feb 3, 2011 at 10:46
this.getClassType() is also a posibility. Which is best and why? — Vincent
– Vincent, Commented Feb 3, 2011 at 10:47

Andreas Dolk · Accepted Answer · 2011-02-03 10:47:52Z

6

This code is not a factory pattern and no object is created. You evaluate the class name and call a static method on a class.

Now it looks like you have an object (generalObject) and want to create a new instance of the very same type. If all possible types have a public default constructor (convention!), then you can use this to create a new instance based on the given object:

Object newObject = generalObject.getClass().newInstance();

(but maybe I still didn't get your idea...)

edited Feb 3, 2011 at 10:47

answered Feb 3, 2011 at 10:38

Andreas Dolk

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1 Comment

Vincent Over a year ago

I'll try to briefly explain it. I've created a general DataObject class which is used by a few different Databases so I can generelise the use of it (e.g. addData(DataObject data)) in the database superclass. But I have several more explicit Classes (UserObject) that inherit basic db stuff from DataObject, but add specifik properties. I could use the method you suggest, only there are no objects stored in the database, but bytes are (that's why I use the fromByteArray method).

maaartinus · Accepted Answer · 2011-02-03 10:47:12Z

1

You could use reflection here, something like

final Class<?> clazz = generalObject.getClass();
final Method method = clazz.getMethod("fromByteArray", data.getClass());
return method.invoke(null, data);

should do.

answered Feb 3, 2011 at 10:47

maaartinus

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Comments

Sean Patrick Floyd · Accepted Answer · 2011-02-03 10:44:06Z

0

String classType = generalObject.getClass().toString();
if(classType.equals("class be.testApp.UserObject")) {
    return UserObject.fromByteArray(data);
}else if(classType.equals("class.be.testApp.NewsObject"))    {
    return NewsObject.fromByteArray(data);
}

This is very complicated. Since apparently both classes are on your compile classpath, just use the class objects, not their string representations:

Class<?> classType = generalObject.getClass();
if(UserObject.class.equals(classType)) {
    return UserObject.fromByteArray(data);
}else if(NewsObject.class.equals(classType)) {
    return NewsObject.fromByteArray(data);
}

answered Feb 3, 2011 at 10:44

Sean Patrick Floyd

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