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lessons/Polymorphism.md

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+# Polymorphism
+
+Polymorphism is the ability to treat subclasses like their base classes, for
+example passing cat, which extends animal to a function that expects an animal
+as parameter. or storing objects of different types that all extend a common
+class in an array of that common type.
+
+You lose acess to the properties of the subclass, but you can still access the
+base class like usual. When the subclass overrides behaviour of the base class,
+the overridden functionality in the subclass is still called.
+
+You can regain access to properties of the subclass with casting, but you have
+to make sure that the object is actually an instance of that class or you might
+run into runtime errors.
+
+- [Java](../java/Polymorphism.md)
+- [Rust](../rust/Inheritance.md)
+- [C++](../cpp/Polymorphism.md)
+- [JavaScript (using TypeScript)](../typescript/Polymorphism.md)