PolymorphismΒΆ

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 simliar to a 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.

Picture of a See N Say toy

If you were simulating this toy in software you could create an Animal class that had an abstract 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.

Note

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

The variable nameList declared below has a declared type of List and an actual or run-time type of ArrayList. The complier 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). The List interface does have a add method so this code will compile. 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 till it finds the method. The method will be found, since otherwise the code would not have compiled.

List<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 thie 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");

Note

Any object variable can refer to an object of the declared type or any descendant (subclass) of the declared type at run-time. The class String inherits from the class Object so an Object variable can hold a reference to a String object. But, you can only call methods that are available in the Object class unless you cast it back to the String class.

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. Remember that an object keeps a reference to the class that created it (an object of the class called Class). When a method is called at run-time the first place that is checked for that method is the class that created the object. If the method is found there it will be executed. If not, the parent of that class will be checked and so on until the method is found.

Check your understanding

You can step through this code using the Java Visualizer by clicking on the following link Shape Example.

Check your understanding

You can step through this code using the Java Visualizer by clicking on the following link Student Example.

You can step through the code using the Java Visualizer by clicking on the following link: Car Example.

You can step through this code using the Java Visulaizer by clicking on the following link: Base Example.

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