Using reflection in Java to create a new instance with the reference variable type set to the new instance class name

Question

All the examples I look at for reflection show creating a new instance of an unknown implementation, and casting that implementation to it's interface. The issue with this is that now you can't call any new methods (only overrides) on the implementing class, as your object reference variable has the interface type. Here is what I have:

Class c = null;
try {
    c = Class.forName("com.path.to.ImplementationType");
} catch (ClassNotFoundException e) {
    e.printStackTrace();
}
InterfaceType interfaceType = null;
try {
    interfaceType = (InterfaceType)c.newInstance();
} catch (InstantiationException e) {
    e.printStackTrace();
} catch (IllegalAccessException e) {
    e.printStackTrace();
}

If I only have a reference to "com.path.to.ImplementationType", and I don't know what that type might be (it is coming from a config file), then how can I use the class name to cast it to ImplementationType? Is this even possible?

User · Answer

If you knew the Class of ImplementationType you could create an instance of it  So what you are trying to do is not possible

User · Answer

I m not absolutely sure I got your question correctly  but it seems you want something like this       Class c   null      try           c   Class forName  com path to ImplementationType          catch  ClassNotFoundException e            e printStackTrace              T interfaceType   null      try           interfaceType    T  c newInstance          catch  InstantiationException e            e printStackTrace          catch  IllegalAccessException e            e printStackTrace            Where T can be defined in method level or in class level  i e   lt T extends InterfaceType gt

User · Answer

This line seems to sum up the crux of your problem      The issue with this is that now you can t call any new methods  only overrides  on the implementing class  as your object reference variable has the interface type    You are pretty stuck in your current implementation  as not only do you have to attempt a cast  you also need the definition of the method s  that you want to call on this subclass   I see two options   1  As stated elsewhere  you cannot use the String representation of the Class name to cast your reflected instance to a known type   You can  however  use a String equals   test to determine whether your class is of the type that you want  and then perform a hard-coded cast   try      String className    com path to ImplementationType     really passed in from config    Class c   Class forName className      InterfaceType interfaceType    InterfaceType c newInstance       if  className equals  com path to ImplementationType             ImplementationType interfaceType  doSomethingOnlyICanDo            catch  Exception e       e printStackTrace        This looks pretty ugly  and it ruins the nice config-driven process that you have   I dont suggest you do this  it is just an example    2  Another option you have is to extend your reflection from just Class Object creation to include Method reflection   If you can create the Class from a String passed in from a config file  you can also pass in a method name from that config file and  via reflection  get an instance of the Method itself from your Class object   You can then call invoke http   java sun com javase 6 docs api java lang reflect Method html invoke java lang Object  java lang Object      on the Method  passing in the instance of your class that you created   I think this will help you get what you are after   Here is some code to serve as an example   Note that I have taken the liberty of hard coding the params for the methods   You could specify them in a config as well  and would need to reflect on their class names to define their Class obejcts and instances   public class Foo        public void printAMessage         System out println toString     a message              public void printAnotherMessage String theString            System out println toString     another message     theString              public static void main String   args            Class c   null          try               c   Class forName  Foo                Method method1   c getDeclaredMethod  printAMessage   new Class                   Method method2   c getDeclaredMethod  printAnotherMessage   new Class   String class                Object o   c newInstance                System out println  this is my instance     o toString                 method1 invoke o               method2 invoke o   this is my message  from a config file  of course              catch  ClassNotFoundException e                e printStackTrace              catch  NoSuchMethodException nsme               nsme printStackTrace              catch  IllegalAccessException iae                iae printStackTrace              catch  InstantiationException ie                ie printStackTrace              catch  InvocationTargetException ite                ite printStackTrace                        and my output   this is my instance Foo e0cf70 Foo e0cf70 a message Foo e0cf70 another message this is my message  from a config file  of course

User · Answer

As an addendum to akf s answer you could use instanceof checks instead of String equals   calls   String cname  com some vendor Impl   try     Class c this getClass   getClassLoader   loadClass cname     Object o  c newInstance      if o instanceof Spam        Spam spam  Spam  o      process spam         else if o instanceof Ham        Ham ham    Ham  o      process ham            etcetera      catch SecurityException se      System err printf  Someone trying to game the system  nOr a rename is in order because this JVM doesn t feel comfortable with      s      cname     se printStackTrace      catch LinkageError le      System err printf  Seems like a bad class to this JVM      s       cname     le printStackTrace      catch RuntimeException re          runtime exceptions I might have forgotten  Classloaders are wont to produce those    re printStackTrace      catch Exception e       e printStackTrace        Note the liberal hardcoding of some values  Anyways the main points are    Use instanceof rather than equals    If anything  it will co-operate better when refactoring  Be sure to catch these runtime errors and security ones too

User · Answer

Single Class Reference used to retrieve object for fields and initial values  Performance enhancing only               Class lt   gt     reference               vector get 0  getClass    Object      obj                     reference newInstance    Field       objFields               obj getClass   getFields

User · Answer

You want to be able to pass in a Class and get a type-safe instance of that class  Try the following   public static void main String    args  throws Exception       String s   instanceOf String class      public static  lt T gt  T instanceOf  Class lt T gt  clazz  throws Exception       return clazz newInstance

[java] Using reflection in Java to create a new instance with the reference variable type set to the new instance class name?

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