📓9.6: Polymorphism

Table of Contents

📖 This page is a condensed version of CSAwesome Topic 9.6

Polymorphism

Polymorphism is a big word that you can break down into “poly” which means many and “morphism” which means form. So, it just means many forms. In Java it means that the method that gets called at run-time (when the code is run) depends on the type of the object at run-time.

This is similar to the See N-Say toddler toy that has pictures of animals and when a handle is pulled an arrow spins. When the arrow stops the toy plays the sound associated with that animal:

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If you were simulating this toy in software you could create an Animal class that had a makeNoise method. Each subclass of Animal would override the makeNoise method to make the correct noise for that type. This type of polymorphism is called inheritance-based polymorphism. You have a common parent class, but the behavior is specified in the child class.

In Java an object variable has both a declared (compile-time) type and an actual (run-time) type. The declared (compile-time) type of a variable is the type that is used in the declaration. The actual (run-time) type is the class that actually creates the object using new.

The variable nameList declared below has a declared type of List and an actual or run-time type of ArrayList.

  • The compiler will check if the declared type has the methods or inherits the methods being used in the code, and give an error if it doesn’t find the method(s).
  • At run-time the execution environment will first look for the add method in the ArrayList class since that is the actual or run-time type. If it doesn’t find it there it will look in the parent class and keep looking up the inheritance tree until it finds the method. It may go up all the way to the Object class. The method will be found, since otherwise the code would not have compiled.
ArrayList<String> nameList = new ArrayList<String>();
nameList.add("Hi");

The variable message declared below has a declared type of Object and an actual or run-time type of String. Since the declared type of message is Object the code message.indexOf("h"); will cause a compiler error since the Object class does not have an indexOf method.

Object message = new String("hi");
message.indexOf("h"); // ERROR!! Objects don't have indexOf!

At compile time, the compiler uses the declared type to check that the methods you are trying to use are available to an object of that type. The code won’t compile if the methods don’t exist in that class or some parent class of that class. At run-time, the actual method that is called depends on the actual type of the object.

In the last lesson on inheritance hierarchies, we were actually seeing polymorphic behavior at run-time in the following ways:

  1. Polymorphic assignment statements such as Shape s = new Rectangle();
  2. Polymorphic parameters such as print(Shape) being called with different subclass types.
  3. Polymorphic array and ArrayList types such as Shape[] shapeArray = { new Rectangle(), new Square() };

In all of these cases, there are no errors at compile-time because the compiler checks that the “subclass is-a superclass” relationship is true. But at run-time, the Java interpreter will use the object’s actual subclass type and call the subclass methods for any overridden methods. This is why they are polymorphic – the same code can have different results depending on the object’s actual type at run-time.

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⭐️ Summary

  • At compile time, methods in or inherited by the declared type determine the correctness of a non-static method call.

  • At run-time, the method in the actual object type is executed for a non-static method call. This is called POLYMORPHISM.

  • For static methods, only the declared type is used to determine what method to execute.

Acknowledgement

Content on this page is adapted from Runestone Academy - Barb Ericson, Beryl Hoffman, Peter Seibel.