Casting a class using reflection in java

Question

I have the following classes.

abstract class A{}

class B extends A{}

class C extends A{}

I need to create an object like this

A a = new B();

I get the class name of the subclass at runtime. So, I need to use reflection to create an object.

I have done this.

Class<?> klass = Class.forName(className);

Now I am not able to cast this class to the subclass.

I want to achieve

A a = klass.newInstance(); // returns Object

I want the object to be casted into the subclass (either B or C, decided on runtime)

How do I do this?

What's wrong with switch (name) { case "B": return new B(); case "C": return new C(); default: throw new SomeException(); }? / What's wrong with Class.newInstance? Plenty. If you're going through Constructor.newInstance, better to use asSubclass up front. — Tom Hawtin - tackline
– Tom Hawtin - tackline, Commented Jun 26, 2013 at 13:39
Since you are using reflection anyway, you don't really have to cast them to use the functions in the subclass... — acdcjunior
– acdcjunior, Commented Jun 26, 2013 at 13:41

Marko Topolnik · Accepted Answer · 2013-06-26 14:41:27Z

3

You can use Generics to declare

Class<? extends A> klass;

then you'll be allowed to write

A a = klass.newInstance();

However, since Class.forName has no type information on your class, and has no type inference declared, you'll be getting "unchecked cast" warnings. This approach is no more typesafe than just downcasting the result of the second line to A.

edited Jun 26, 2013 at 14:41

answered Jun 26, 2013 at 13:37

Marko Topolnik

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3 Comments

rgksugan Over a year ago

Incompatible types. found: Class<capture<?>>

Marko Topolnik Over a year ago

Yes. You need an explicit cast on the right-hand side. Anyway, it doesn't give you much mileage: you can't make an inherently type-unsafe piece of code type-safe.

Holger Over a year ago

A a = Class.forName( … ).asSubclass(A.class).newInstance(); does even work without unchecked type casts and is type safe.

Jon Skeet · Accepted Answer · 2013-06-26 13:36:25Z

2

I want the object to be casted into the subclass (either B or C, decided on runtime)

That makes no sense. Casting is something which is primarily a compile-time operation, to end up with an expression of the appropriate type. It's then verified at execution time.

If you're really trying to achieve a variable of type A, then you only need to cast to A:

A a = (A) klass.newInstance();

answered Jun 26, 2013 at 13:36

Jon Skeet

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Comments

Grzegorz Żur · Accepted Answer · 2013-06-26 13:39:14Z

0

Having klass object of type Class<T> you can call Class.cast() method to cast to type T using

klass.cast(object)

For example, you have a of type B, you cast a to type B loaded at runtime.

Class<?> klass = Class.forName("B");
klass.cast(a);

answered Jun 26, 2013 at 13:39

Grzegorz Żur

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Comments

Aleks G · Accepted Answer · 2013-06-26 13:50:28Z

0

To follow up on the comments, you can create your instance of the class with either

Class klass = Class.forName(className);
A a = (A)klass.newInstance();

or

Class<? extends A> klass = Class.forName(className);
A a = klass.newInstance();

Then you can invoke a method on it with:

klass.getMethod(methodName, null).invoke();

Or, if the method takes arguments (e.g. int and String)

int param1 = ...;
String param2 = ...;
klass.getMethod(a, new Class[] { Integer.class, String.class }).invoke(param1, param2);

Of course, you'll need to be catching appropriate exceptions.

edited Jun 26, 2013 at 13:50

answered Jun 26, 2013 at 13:45

Aleks G

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