Can I Instantiate a class using the class object? What about Constructors?

Question

I am storing a list of classes through (Classname.class) and would like to instantiate one? Is this possible?

newInstance seems to the method I am after but it doesn't support a constructor?

Answer 1

You can use Class.getConstructors (or Class.getConstructor) to get a list of available constructors, and invoke any of them with Constructor.newInstance, which does accept parameters.

Answer 2

The Java tutorial on reflection covers this well. But yeah, basically Class.getConstructors, then Constructor.newInstance is where it's at.

Answer 3

If you have a list of Class objects obtained through class literals, you might as well statically reference the constructors rather than slipping into reflection evilness.

Answer 4

Hello Eddie,

Java is designed so you can never "trick" it as long as you use the java.lang/java. classes or other standard libraries. One of the most important things of OOP is that objects should be in a defined state, thus you can be safe that the constructor is always run. Even if you're using some strange-looking reflection libraries to get your work done.

So, using Class.forName("me.Test").newInstance(); (or similar) will under-the-hood invoke the Test() constructor for you.

If you want to invoke another constructor the code is something like:

Test test = (Test)Class.forName("Test").getConstructor(String.class).newInstance("Hello World");

Here the getConstructor asks what the constructor looks like (it wants a string) and then you call it with a string.

Answer 5

Just to add one point I see missing:

You can invoke newInstance directly on the Class object if it has a public null constructor. (Null constructor is the constructor with no arguments.)

Otherwise, you can find constructors via Class.getConstructors() as others have said.

Answer 6

You cannot construct new classes this way.

If you have the name of a class you can use Class.forName(className) to load/reference a class.

If you have the byte code for a class you want to create you can have a class loader load the byte code and give you the class. This is likely to be more advanced than you intended.