Is there a way to instantiate a class by name in Java?

java string object instantiation
ict1991 picture ict1991 · Mar 27, 2012 · Viewed 125.7k times · Source

I was looking as the question : Instantiate a class from its string name which describes how to instantiate a class when having its name. Is there a way to do it in Java? I will have the package name and class name and I need to be able to create an object having that particular name.

Answer

Simon Nickerson picture Simon Nickerson · Mar 27, 2012

Two ways:

Method 1 - only for classes having a no-arg constructor

If your class has a no-arg constructor, you can get a Class object using Class.forName() and use the newInstance() method to create an instance (though beware that this method is often considered evil because it can defeat Java's checked exceptions).

For example:

Class<?> clazz = Class.forName("java.util.Date");
Object date = clazz.newInstance();

Method 2

An alternative safer approach which also works if the class doesn't have any no-arg constructors is to query your class object to get its Constructor object and call a newInstance() method on this object:

Class<?> clazz = Class.forName("com.foo.MyClass");
Constructor<?> constructor = clazz.getConstructor(String.class, Integer.class);
Object instance = constructor.newInstance("stringparam", 42);

Both methods are known as reflection. You will typically have to catch the various exceptions which can occur, including things like:

  • the JVM can't find or can't load your class
  • the class you're trying to instantiate doesn't have the right sort of constructors
  • the constructor itself threw an exception
  • the constructor you're trying to invoke isn't public
  • a security manager has been installed and is preventing reflection from occurring

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