How to Resolve InstantiationException When Using newInstance with Reflection in Java?

What causes InstantiationException when calling newInstance on a class via reflection in Java?

Class<?> clazz = Class.forName("com.example.MyClass");
MyClass myClassInstance = (MyClass) clazz.newInstance();

The InstantiationException in Java occurs when attempting to create an instance of a class using reflection, but the class cannot be instantiated. This can happen for several reasons, including the class being abstract, an interface, or lacking a no-argument constructor.

// Example with a public no-argument constructor
Class<?> clazz = Class.forName("com.example.MyClass");
Constructor<?> constructor = clazz.getConstructor();
MyClass myClassInstance = (MyClass) constructor.newInstance();

Causes

• The class is abstract and cannot be instantiated directly.
• The class is an interface and does not provide a concrete implementation.
• The class does not have a public no-argument constructor, making it inaccessible for instantiation.
• The class is final and has not been properly loaded or initialized.

Solutions

• Ensure that the class you are trying to instantiate is not abstract or an interface.
• Check that the class has a public no-argument constructor. Consider creating the constructor if it's missing.
• Make sure that the class is accessible in the context where you are trying to instantiate it (i.e., proper access modifiers).
• Use `Constructor.newInstance()` for more complex instantiation if the class has parameters.

Mistake: Assuming the class has a default constructor when it doesn't.

Solution: Always verify that the class has a public no-argument constructor before using newInstance.

Mistake: Calling newInstance on an abstract class or an interface.

Solution: Check if the class is an abstract class or interface and instantiate a concrete implementation instead.

Mistake: Ignoring the access level of the class constructor.

Solution: If the constructor is not public, use `setAccessible(true)` on the constructor to bypass access checks.