How to instantiate an inner class with reflection in Java?

java reflection instantiationexception
Stephan picture Stephan · Jul 5, 2013 · Viewed 38.2k times · Source

I try to instantiate the inner class defined in the following Java code:

 public class Mother {
      public class Child {
          public void doStuff() {
              // ...
          }
      }
 }

When I try to get an instance of Child like this

 Class<?> clazz= Class.forName("com.mycompany.Mother$Child");
 Child c = clazz.newInstance();

I get this exception:

 java.lang.InstantiationException: com.mycompany.Mother$Child
    at java.lang.Class.newInstance0(Class.java:340)
    at java.lang.Class.newInstance(Class.java:308)
    ...

What am I missing ?

Answer

Jon Skeet picture Jon Skeet · Jul 5, 2013

There's an extra "hidden" parameter, which is the instance of the enclosing class. You'll need to get at the constructor using Class.getDeclaredConstructor and then supply an instance of the enclosing class as an argument. For example:

// All exception handling omitted!
Class<?> enclosingClass = Class.forName("com.mycompany.Mother");
Object enclosingInstance = enclosingClass.newInstance();

Class<?> innerClass = Class.forName("com.mycompany.Mother$Child");
Constructor<?> ctor = innerClass.getDeclaredConstructor(enclosingClass);

Object innerInstance = ctor.newInstance(enclosingInstance);

EDIT: Alternatively, if the nested class doesn't actually need to refer to an enclosing instance, make it a nested static class instead:

public class Mother {
     public static class Child {
          public void doStuff() {
              // ...
          }
     }
}

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