and Inheritance Interfaces
Single inheritance in Java
Inheritance models an "is-a" relationship
A class can extend only one class in Java
A dog is a mammal, an ArrayList is a List, a square is a shape,
Write general programs to understand the abstraction, advantages?
void doShape(Shape s) { System.out.println(s.area()); System.out.println(s.perimeter()); s.expand(2.0); }
All classes extend Object --- it's the root of the inheritance hierarchy tree Can extend something else (which extends Object), why?
Why do we use inheritance in designing programs/systems?
But a dog is also a quadruped, how can we deal with this?
Facilitate code-reuse (what does that mean?) Ability to specialize and change behavior o If I could change how method foo() works, bar() is ok Design methods to call ours, even before we implement o Hollywood principle: don't call us,
6.2
CompSci 100E
6.1
CompSci 100E
Multiple Interfaces
Inheritance Example
public class UtilRecord extends Record { // assumes Records fields are protected, not private private String kind; public UtilRecord(){ this("", "", 0, 0, "", ""); } public UtilRecord(String a, String f, int s, int o, String e, String k) { super(a, f, s, o, e); kind = k; } /* etc */
6.4
Classes (and interfaces) can implement multiple interfaces
A dog is a mammal, a quadruped, a pet How come canine is different? What behavior do quadrupeds have? Pets have?
An interface specifies the name (and signature) of methods
No implementation, no state/fields Yes for this constants
In course, by convention, we'll often use interfaces
Emphasize design before implementation Use abstract/default classes for code reuse, state
CompSci 100E
6.3
CompSci 100E
Comparable and Comparator
Comparable Example
public class Record implements Comparable { public Record(String a, String f, int s, int o, String e) { . . . } public Record(Record rec) { . . . } int compareTo(Record r) { // comparision code goes here // set n to neg # if <, 0 if ==, and pos # if > return n }
CompSci 100E 6.6
Both are interfaces, there is no default implementation
Contrast with .equals(), default implementation? Contrast with .toString(), default? In its own .java file, nothing wrong with that Private, used for implementation and not public behavior o Use a nested class, then decide on static or non-static o Non-static is part of an object, access inner fields Sort, Sets, Maps (in the future)
Where do we define a...