Question

I would like to instantiate an object from its Class object, using the constructor that accepts a single String argument.

Here is some code that approaches what I want:

Object object = null;
Class classDefinition = Class.forName("javax.swing.JLabel");
object = classDefinition.newInstance();

However, it instantiates the JLabel object with no text. I would like to use the JLabel constructor that accepts a string as the initial text. Is there a way to select a specific constructor from a Class object?

Answer 1

Class.newInstance invokes the no-arg constructor (the one that doesn’t take any parameters). In order to invoke a different constructor, you need to use the reflection package (java.lang.reflect).

Get a Constructor instance like this:

Class<?> cl = Class.forName("javax.swing.JLabel");
Constructor<?> cons = cl.getConstructor(String.class);

The call to getConstructor specifies that you want the constructor that takes a single String parameter. Now to create an instance:

Object o = cons.newInstance("JLabel");

And you’re done.

P.S. Only use reflection as a last resort!