Instantiate a class from the value of a string

[Larry Homes]

Hello,

I was wondering if there was a way to instantiate a class from the value of a string. Here's an example:

There is a main class that prompts the user for a string. Say the user enters "Hello". Assuming Hello is an instantiable class, is there a way for the main class to take that string and create a Hello object?

Also, assume that there are too many classes defined that the user might type in for a switch case statement to be feasable or reliable.

If anything is unclear, please ask. Thanks for the help!

[Campbell Ritchie]

Welcome to JavaRanch

Go through the Class class, and you find you can load a Class object with the forName method. When you have got your hands on that Class object, you can call the newInstance() method on it.

Foo fff = Class.forName("Foo").newInstance();

Beware:

The tiniest spelling error and you will get Exceptions

the newInstance method probably declares a checked Exception.

You usually need to provide the fully-qualified name of the class.

I think you can only instantiate classes like that if they have a no-arguments constructor.

You might need to cast the resultant object, but I am not sure on this last point. It probably depends on whether you have got a formal type parameter anywhere.

[Ernest Friedman-Hill]

Campbell Ritchie wrote:I think you can only instantiate classes like that if they have a no-arguments constructor.

If you need to create an instance of a class that doesn't have a no-argument constructor, you can use the several "getConstructors()" methods of java.lang.Class to allow you to call any other constructor.

[Campbell Ritchie]

That's useful information, Ernest, thank you.
Now, can I remember it until I next need it???

[Vadim Vararu]

Thanks a lot guys. It's, really, very useful. I have 6 months JAVA programming experience, but there are always things that we miss.

[Vadim Vararu]

Constructor[] allConstructors = xxx.getConstructors();
            for (int i = 0; i < allConstructors.length; i++) {
                System.out.println(allConstructors[i].getModifiers());
            }

This throws an exception like this, when i have a constructor without modifiers at all. As i know we CAN have constructors with default modifier access. Then, what's the problem?

java.lang.IllegalAccessException: Class test.Main can not access a member of class test1.Woman with modifiers ""
        at sun.reflect.Reflection.ensureMemberAccess(Reflection.java:65)
        at java.lang.Class.newInstance0(Class.java:349)
        at java.lang.Class.newInstance(Class.java:308)

Probably, it's because i try to access a default modifier from another package? Is this the reason? Is there a way to work with any modifiers from anywhere?