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Classes, objects, inheritance, and polymorphism
1. Classes, objects, inheritance, and polymorphism
2. Object
In Java, the root base class
Object, or
java.lang.Object, can store any object but if not handled properly will case a
ClassCastException to be raised.
3. Liskov Substitution Principle
An object is an instance of a class.
The Liskov Substitution Principle (LSP, from Barbara Liskov, 1987, refined in 1994) can be stated (more simply) as follows.
Anywhere in a program an instance of a superclass may be used, an instance of a subclass may be used.
What does this mean? Let us look at it graphically rather than textually
.
Note: Rather than using a tree diagram, the classes will be compared side by side, much as one might compare photographs of generations of the same family for resemblance.
4. Class A
Anywhere in a program an instance of a superclass may be used, an instance of a subclass may be used.
For class A (variables/fields are omitted) there are two methods,
method1 and
method2. With one class, there are no overrides and no need/way to invoke
super.
5. Class B extends class A
Anywhere in a program an instance of a superclass may be used, an instance of a subclass may be used.
To share the similarities between class A and class B, class B overrides
method2 from class A, but can still invoke it via
super. Class B appears as if it were of class A.
6. Class C extends class B
Anywhere in a program an instance of a superclass may be used, an instance of a subclass may be used.
To share similarities between class A, B, and C, Class C adds
method4 and overrides
method2 from class B (but can still invoke it via
super). Class C appears as if it were of class B and also appears as if it were of class A.
7. End of page
