CSE 21 Intro to Computing II. Inheritance

Transcription

1 CSE 21 Intro to Computing II Inheritance 1

2 Administrative Business Lab11 (this week) is due on November 27 th extension Extra lab section on Tuesday November 20 th from 3-6pm (SE138) No lab November 20/23 (next week) Project #2 opens on November 16 Due on December 3 rd (11:55pm) Lecture next week (November 21): Post-midterm review Last Lecture (with new content) on November 28 Lab 12 on November 27/30 due December 4 Review/Lecture on December 5 Lab 13 (last lab!) on December 4/7 due December 11 Final Exam: December 12 th from 3:00 to 6:00pm Understanding Lecture Concepts Code samples: UCMCROPS->Resources->CodeSamples 2

3 Inheritance: Motivation Different objects often share similar states and behaviors Imagine you need an Object that is slightly different from one that you already implemented Instead of re-designing and re-implementing an entire new object from scratch, you can inherit (or derive) the existing object and just add the necessary modifications. 3

4 Inheritance: Bicycle Example All bicycles have a similar state (variables) Color, Height, Speed, Gear, Cadence, etc All bicycles have a similar behavior (functions) changegear(), changecadence(), calcspeed(), etc Specific bicycles, however, have different states/behaviors Tandem Bike Number of Seats, Number of Handlebars, calcspeed() Mountain Bike Tire Width, Suspension Stiffness Road Bike Tire Width, Tire Pressure, Aerodynamics 4

5 Inheritance: Bicycle Example If we have already designed/implemented a bicycle class, we do not want to implement the MountainBike, RoadBike, and TandemBike classes from scratch This is where inheritance comes in The class being inherited by another is called a superclass Bicycle is a superclass A class can be derived from a superclass It is then called a subclass MountainBike is a subclass of Bicycle RoadBike is a subclass of Bicycle RandemBike is a subclass of Bicycle 5

6 Inheritance in JAVA Assuming we already have a superclass Bicycle in this example A subclass is defined as follows class MountainBike extends Bicycle { // Your class definition goes here Benefits of inheritance (why do we care?) A subclass (MountainBike) can access any public or protected functions or variables defined in the superclass (Bicycle) A subclass (MountainBike) cannot access private functions or variables defined in the superclass (Bicycle). 6

7 Brief Example: Class Skeletons public class Bicycle { public int cadence; public int gear; private int speed; public Bicycle() { public void inccad(){ public void incgear(){ private void calcspd(){ public void printspd(){ public class RoadBike extends Bicycle { public int tirewidth; public int tirepressure; public RoadBike() { public void winrace(){ cadence = 100; gear = 21; printspd(); The new RoadBike class (subclass) inherits the Bicycle class (superclass) Inside the main function: RoadBike myroadbike = new RoadBike(); myroadbike.cadence = 5; myroadbike.gear = 15; myroadbike.printspd(); myroadbike.winrace(); Bicycle mybicycle = new Bicycle(); mybicycle.winrace(); // winrace does not exist inside Bicycle class myroadbike.speed = 8; myroadbike.calcspd(); // Private! 7

8 Count Class Example public class Counter { private int mycount; public Counter() { mycount = 0; public void increment(){ mycount++; public void reset() { mycount = 0; public int value() { return mycount; public class Cycler extends Counter { 8

9 Count Class Example public class Counter { private int mycount; public Counter() { mycount = 0; public void increment(){ mycount++; public void reset() { mycount = 0; public int value() { return mycount; public class Cycler extends Counter { Cycler = new Cycler; c.increment(); // THIS IS CORRECT 9

10 public class Counter { private int mycount; public Counter() { mycount = 0; public void increment(){ mycount++; public void reset() { mycount = 0; public int value() { return mycount; Count Class Example public class Cycler extends Counter { private int maxcycle; Additional instance variable Provides the maximum number that the Cycler can go to before cycling back to 0 10

11 public class Counter { private int mycount; public Counter() { mycount = 0; public void increment(){ mycount++; public void reset() { mycount = 0; public int value() { return mycount; Count Class Example public class Cycler extends Counter { private int maxcycle; public Cycler(int n) { maxcycle = n; Subclasses need their own constructors 11

12 public class Counter { private int mycount; public Counter() { mycount = 0; public void increment(){ mycount++; public void reset() { mycount = 0; public int value() { return mycount; Count Class Example public class Cycler extends Counter { private int maxcycle; public Cycler(int n) { maxcycle = n; public void increment ( ){ mycount++; if (mycount > maxcycle) mycount=0; Override the function increment (function overloading) 12

13 public class Counter { private int mycount; public Counter() { mycount = 0; public void increment(){ mycount++; public void reset() { mycount = 0; public int value() { return mycount; Count Class Example public class Cycler extends Counter { private int maxcycle; public Cycler(int n) { maxcycle = n; public void increment ( ){ mycount++; if (mycount > maxcycle) mycount=0; public int max ( ){ return maxcycle; New Function 13

14 Protected Access Qualifier As written, Cycler will not compile! The mycount variable is private (only accessible in the Count class) We can fix this by making it protected: Only classes that extend Counter can access its protected variables/methods Inside the main, protected variables cannot be accessed Three different access types: public: any class can read/modify protected: only this class and subclass descendents can read/modify private: only this class can read/modify 14

15 public class Counter { private int mycount; public Counter() { mycount = 0; public void increment(){ mycount++; public void reset() { mycount = 0; public int value() { return mycount; Count Class Example public class Cycler extends Counter { private int maxcycle; public Cycler(int n) { maxcycle = n; public void increment ( ){ mycount++; if (mycount > maxcycle) mycount=0; public int max ( ){ return maxcycle; 15

16 public class Counter { protected int mycount; public Counter() { mycount = 0; public void increment(){ mycount++; public void reset() { mycount = 0; public int value() { return mycount; Count Class Example - Final mycount 0 public class Cycler extends Counter { private int maxcycle; public Cycler(int n) { maxcycle = n; public void increment ( ){ mycount++; if (mycount > maxcycle) mycount=0; public int max ( ){ return maxcycle; mycount 0 maxcycle 0 16

17 Inheritance - Diagram increment() class Counter Superclass reset() mycount value() Subclass inherits members Subclass increment() reset() maxcycle mycount max() class Cycler value() 17

18 Inheritance Terminology Suppose class B inherits class A The classes form a part of a class hierarchy. B is a subclass of A, B derives A. A is a superclass of B, A is a base class of B The class immediately above a given class is known as its immediate superclass. A subclass inherits all members of the base class Includes functions/inherited by that class It can add additional variables and functions. It can override (change) the inherited functions. Function overloading Can refer to superclass using keyword super. 18

19 public class Counter { protected int mycount; public Counter() { mycount = 0; public void increment(){ mycount++; public void reset() { mycount = 0; public int value() { return mycount; Example of Using super public class Cycler extends Counter { private int maxcycle; public Cycler(int n) { maxcycle = n; public void increment ( ){ mycount++; if (mycount > maxcycle) mycount=0; public int max ( ){ return maxcycle; 19

20 public class Counter { protected int mycount; public Counter() { mycount = 0; public void increment(){ mycount++; public void reset() { mycount = 0; public int value() { return mycount; Example of Using super public class Cycler extends Counter { private int maxcycle; public Cycler(int n) { maxcycle = n; public void increment ( ){ super.increment(); if (mycount > maxcycle) mycount=0; public int max ( ){ return maxcycle; 20

21 Relations Two common relationships are: Is-a: All objects in one class also in another Cycler is a Counter, MountainBike is a Bicycle Has-a: One class contains one or more reference(s) to another class object Shop has a Swiss Cheese Is-a refers to inheritance Class MountainBike extends Bicycle { MountainBike is a Bicycle Has-a refers to an instance variable that is an object class Brake { Class Bike { Brake frontbrake; Brake rearbrake; Class Bike has a Brake 21

22 Inheritance Can be Multiple Levels Panel Window Button Square Button Round Button Complex class hierarchies can be created No limit on the number of levels 22

23 Object is at the Top Object Panel Window Button Square Button Round Button A the top of Java s inheritance hierarchy is the special type Object It comes with a few predefined methods such as tostring Any class/object is, ultimately, a subclass of Object 23

24 Using Equals (==) on Objects With primitive data types, we can check if two values are equal if(i==0), if (j==0), if(d==0.56), What if we want to compare two objects? if(mybike==yourbike), if(counter1==counter2), Implement the equals function in your class Returns a boolean Takes in an object as a parameter To check whether two Counters are equal: public boolean equals (Counter c) { return mycount == c.mycount; 24

25 Reading/Processing Files import java.io.*; // Top of your file String filename = nums.txt ; Scanner input = new Scanner (new FileReader(filename)); input.usedelimiter( [\r] ); // use tab and carriage return while (input.hasnext()) { System.out.println(input.next()); input.close(); This code segment reads a file and outputs its contents line by line Exceptions must be handled when reading files: FileNotFoundException NoSuchElementException 25

26 Reading a CSV file import java.io.*; System.out.print("Enter the file name: "); Scanner kdb = new Scanner(System.in); String filename = kdb.next(); try { // TRY it out Scanner input = new Scanner (new FileReader(filename)); while (input.hasnextline()) { Scanner line = new Scanner(input.nextLine()); line.usedelimiter( [,] ); // Tab delimited file while (line.hasnext()){ System.out.print(line.next()); // Read each token System.out.println(); // Done reading one line input.close(); catch (FileNotFoundException e){ // Catch error System.out.println( File could not be found ); catch (NoSuchElementException e) { // Catch error System.out.println( Tried to use next, with nothing to get ); 26

27 Different Scanner Functions while (input.hasnextline()) { Scanner line = new Scanner(input.nextLine()); line.setdelimiter( [\t\r] ); short s = line.nextshort(); int i = line.nextint(); double d = line.nextdouble(); float f = line.nextfloat(); String str = line.next(); char c = line.next().charat(0); String rest = line.nextline(); 27

28 Filenames and Paths String s; s = myfile.txt ; // in current folder s =../myfile.txt ; // previous folder // (relative path) s = C:/tmp/myfile.txt ; // full path specified s = C:\temp-file.txt ; // Error! (\t is a tab!) s = C:\\tmp\\myfile.txt ; // Ok in windows Scanner input = new Scanner (new FileReader(s)); 28