Create new instances of a class from an array of classes with Java

Question

I'm trying to create an array of classes in Java. Not objects, but classes. Currently I have a class MyBaseClass and I extend three classes MyClass1, MyClass2, and MyClass3 from it. I store these classes to a static array like this:

private static MyBaseClass[] classes = {
        new MyClass1(),
        new MyClass2(),
        new MyClass3()
};

public static MyBaseClass getInstanceOfClass(int index) {
    return classes[index];
}

and then I use those methods like this:

try {
    MyBaseClass obj = getInstanceOfClass(index).getClass().newInstance();
} catch (InstantiationException | IllegalAccessException e) {
    e.printStackTrace();
}

Now I'm wondering if I could do this in an easier way. If I could create an array of classes I might be able to escape the getInstanceOfClass() method and some possible exceptions. I tried doing this:

private static Class<MyBaseClass>[] classes = {
        MyClass1.class,
        MyClass2.class,
        MyClass3.class
};

But this gives me an error "Incompatible types" as MyClass1 is not equal to MyBaseClass. Interestingly enough, this seemingly works:

private static Class<?>[] classes = {
        new MyClass1().getClass(),
        new MyClass2().getClass(),
        new MyClass3().getClass()
};

But the idea of that is horrible and it's even marked by my debugger. So, any better ways of doing this?

Edit:

This works:

private static Class<?>[] classes = {
        MyClass1.class,
        MyClass2.class,
        MyClass3.class
};

But then the result of getInstanceOfClass(index).newInstance(); is an Object so I have to do typecasting. I'm not really sure if that's safe in this case...

I dont think you can create arrays of generic type,due to type erasure — Kumar Abhinav
– Kumar Abhinav, Commented Aug 11, 2014 at 16:50
The most unnerving thing in this is that at least Android Studio doesn't mark Class<?>[] or Class<MyBaseClass>[] with an error. But Class<? extends MyBaseClass>[] is marked. — Finnboy11
– Finnboy11, Commented Aug 12, 2014 at 17:45

Thirler · Accepted Answer · 2014-08-11 14:36:23Z

5

You have to indicated that you are looking for possibly subclasses of MyBaseClass. For this you can add ? extends to the generic type description. Note that this will also accept MyBaseClass itself.

Added to this, you can use .class to get the Class object of a class, instead of making an isntance and calling getClass().

Unfortunately this doesn't work with arrays because they don't allow generic types. So you have to add them to a list (which is almost always a good thing):

private static ArrayList<Class<? extends MyBaseClass>> classes = new ArrayList<Class<? extends MyBaseClass>>();

...

classes.add(MyClass1.class),
classes.add(MyClass2.class),
classes.add(MyClass3.class),

edited Aug 11, 2014 at 14:36

answered Aug 11, 2014 at 14:28

Thirler

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8 Comments

Finnboy11 Over a year ago

This gets marked as an error "Generic array creation".

Thirler Over a year ago

Ah you are right, arrays don't allow subclasses, you can store them in a List.

Finnboy11 Over a year ago

I don't have a List. The code you posted above gets marked with an error. Funny enough that Class<?>[] or Class<MyBaseClass>[] doesn't get marked but Class<? extends MyBaseClass>[]gets.

Eran Over a year ago

You cannot create arrays of parameterized types.

Kumar Abhinav Over a year ago

From java 7 ,you can use diamond operator too

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