Java Reference
In-Depth Information
Methods in Abstract Classes
Abstract classes can include abstract methods. Any class that extends a class with an abstract method
must implement that method. For example, our
Mammal
class includes an abstract
speak
() method. Any
class that extends
Mammal
must implement the
speak
method, and that implementation must have the
same signature. So, in this case, the implementations must return void and accept no arguments.
Abstract classes can also include regular methods that their descendant classes can use without
needing to implement them. Listing 6-1 shows both kinds of methods within the Mammal class.
Listing 6-1. Methods within an abstract class
package com.apress.java7forabsolutebeginners.examples.animalKingdom;
abstract class Mammal {
// And here's a method for making the sound.
// Each child class must implement it.
abstract void speak();
// All descendant classes can call this and do
// not need to implement their own versions of it
protected void sayWhatIAm() {
System.out.println("I am a mammal");
}
}
That's a trivial method, but it raises an interesting point. Classes that extend the
Mammal
class might
also have methods with the same signature. Those methods are said to
override
this method. For
example, the
Cat
class might have a
sayWhatIAm
method of its own. Let's suppose it was identical but
printed “I am a cat” to
System.out
. Then, to use the
Mammal
class's method, we'd have to use the
super
keyword, as shown in Listing 6-2.
■
Note
I made the
sayWhatIAm()
method protected so that only descendants of the
Mammal
class can say
they're mammals. If it were public, we might have
Snake
objects saying they're mammals, and that would be
wrong. Being protected forces any overriding methods in descendant classes to also be protected. Consequently,
the
sayWhatIAm()
method in the
Cat
class has to be protected. That makes perfect sense, though. If we were to
add
Lion
and
Tiger
classes, we would want them to say they are mammals and cats.
Listing 6-2. The Cat class with an overridden sayWhatIAm method
package com.apress.java7forabsolutebeginners.examples.animalKingdom;
class Cat extends Mammal {
// implement the super class's abstract methods
