How to test if one java class extends another at runtime?

java

reflection

subclass-checking

inheritance-testing

byNikita Barsukov·Jan 7, 2025

A quick way to check class inheritance at runtime is by using the Class.isAssignableFrom method. In the context of SuperClass and SubClass, execute SuperClass.class.isAssignableFrom(SubClass.class) to verify if SubClass is an extension of SuperClass.

Example:

public class SuperClass {}

public class SubClass extends SuperClass {} // SubClass is the "rebellious teenager" class.

boolean isSubclass = SuperClass.class.isAssignableFrom(SubClass.class);  // Returns true, because, yes, SubClass still "lives at home".

Reflection and checking subclass relationships at runtime

Reflection is your friend when it comes to checking if one class extends another at runtime. By using it, you can dynamically inspect classes and their relationships.

Undressing complex hierarchies

Good news! isAssignableFrom works even if classes are not direct offspring of each other. It inspects the whole family tree, including grandparents, great-grandparents, and on it goes.

Handling interfaces

isAssignableFrom can also be used to check if a class implements an interface, deeming any interface as a superclass.

public interface Lovable {}

public class Puppy implements Lovable {} // All puppies are lovable, prove me wrong!

boolean isInLove = Lovable.class.isAssignableFrom(Puppy.class); // Returns true, because, look at that fluffy goodness!

The importance of stress testing

Be like a typical parent: test every scenario for your code. Make sure your code delivers accurate results in every imaginable scenario.

Advanced strategies for subclass checking

isAssignableFrom() is great, but there are other tools in the toolkit that can add nuance to your subclass checks.

The `instanceof` operator

instanceof can be used to check if an object is born from a specific class or subclass:

SuperClass oldMan = new SubClass(); // It's like Benjamin Button!
boolean isBabyBoomer = oldMan instanceof SuperClass; // Returns true. What a plot twist!

Direct descent

The parent ain't always the grandparent. To know for certain if a class is directly down the line, use Class#getSuperclass.

Class<?> momOrDad = SubClass.class.getSuperclass();
boolean isChild = momOrDad.equals(SuperClass.class); // Returns true if SuperClass is the immediate parent and not some distant relative.

Keep in mind, instanceof and Class#getSuperclass() cannot skip the birth. They must work with actual objects, not class literals!

Traps and how to avoid them

Unexpected results? Consider the following pitfalls:

Class Loaders: Different class loaders can create class identity issues. They can lead to ClassCastException or a false isAssignableFrom result.
Proxy Classes: Dynamically created classes might slip through your checks.
Type Erasure: During compilation, generic types are erased. This affects the outcome of checks involving generic classes.

The role of inheritance testing in unit tests

Unit testing is useful in identifying issues early on. Testing for subclass relationships can ensure your classes adhere to their anticipated blueprints.

explain-codes / Java / How to test if one java class extends another at runtime?

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