java – Instantiate nested static class using Class.forName-ThrowExceptions

Exception or error:

I have a nested static class like:

package a.b
public class TopClass {

    public static class InnerClass {
    }
}

I want to instantiate with Class.forName() but it raises a ClassNotFoundException .

Class.forName("a.b.TopClass"); // Works fine.
Class.forName("a.b.TopClass.InnerClass"); // raises exception

TopClass.InnerClass instance = new TopClass.InnerClass(); // works fine

What is wrong in my code?

Udo.

How to solve:

Nested classes use “$” as the separator:

Class.forName("a.b.TopClass$InnerClass");

That way the JRE can use dots to determine packages, without worrying about nested classes. You’ll spot this if you look at the generated class file, which will be TopClass$InnerClass.class.

(EDIT: Apologies for the original inaccuracy. Head was stuck in .NET land until I thought about the filenames…)

Answer:

try

Class.forName("a.b.TopClass$InnerClass");

Answer:

Inner classes are accessed via dollar sign:

Class.forName("a.b.TopClass"); 
Class.forName("a.b.TopClass$InnerClass"); 

Answer:

Inner class is always accessed via dollar sign because when java compiler compile the java source code file it generates .class file(byte code).

if there is only one class for example Hello.java and this class is an outer class then java compiler on compilation generate Hello.class file but if this class has an inner class HelloInner then java compiler generates d Hello$HelloInner.class(byte code).

so bytecode always looks like for following snippet with name Outer.java:

   public class   Outer
   {
     public  var;//member variable
       Outer()//constructor
       {
        }
       class  Inner1
        {
          class Inner2
             {  
              }
         }
       }

so byte code is:Outer$Inner1$Inner2.class

that’s why we use $ sign to access inner class .:)

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