How to Instantiate Inner Classes in Java Using Reflection and Avoid InstantiationException?

Instantiation of Inner Classes via Reflection in Java

When attempting to instantiate an inner class, developers may encounter the InstantiationException. This exception occurs when an extra "hidden" parameter is not accounted for. This parameter is the instance of the encasing class and must be provided when using Class.getDeclaredConstructor(). Additionally, if the nested class does not require the encasing instance, it can be declared as a nested static class.

Solution:

To instantiate an inner class using reflection:

1. Obtain the class object of the encasing class:

   <code class="java">Class<?> enclosingClass = Class.forName("com.mycompany.Mother");</code>

2. Create an instance of the encasing class:

   <code class="java">Object enclosingInstance = enclosingClass.newInstance();</code>

3. Obtain the class object of the inner class:

   <code class="java">Class<?> innerClass = Class.forName("com.mycompany.Mother$Child");</code>

4. Use Class.getDeclaredConstructor() to obtain the constructor of the inner class and provide the enclosing class as an argument:

   <code class="java">Constructor<?> ctor = innerClass.getDeclaredConstructor(enclosingClass);</code>

5. Create an instance of the inner class by invoking the constructor and passing the enclosing instance as an argument:

   <code class="java">Object innerInstance = ctor.newInstance(enclosingInstance);</code>

Alternative Solution:

If the inner class does not require the encasing instance, consider declaring it as a nested static class:

<code class="java">public class Mother {
     public static class Child {
          public void doStuff() {
              // ...
          }
     }
}</code>

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