CSE 21 Intro to Computing II. Post-Midterm Review

Transcription

1 CSE 21 Intro to Computing II Post-Midterm Review 1

2 Post-Midterm Review Topics Object Oriented Programming Classes Inheritance String/File Manipulation String functions (substr, concat, +) Scanner functions Multi-dimensional arrays (next/last lecture) 2

3 OOP: Setting up an object Software objects are modeled after real-world Consequently, they have a state and behaviors A software object maintains its state using one or more variables A software object implements its behaviors using one or more functions Put differently: An object is a software bundle of variables (what it knows) and related functions (what it can do) Classes allow programmers to generate objects They are factories for generating objects 3

4 Designing a Class To design a class, think about what the objects in that class should do 1. Determine functions (your API, or behavior ) Constructors (these initialize an instance) Accessors (these query your state) Mutators (these change your state) tostring (this prints your state) equals (this compares two objects are they same?) 2. Determine the set of variables (your state) Those specific/different for each object (instance variables) Those shared by all objects in a class (class variables) Once the structure is set fill in the blanks 4

5 Creating Classes: Skeleton /** A Counter remembers the number of times it has * been asked to increment itself. */ public class Counter { /** Instance variable */ private int mycount = 0; /** Modify the counter by incrementing itself. */ public void increment () { } /** Return the current counter reading. */ public int getcount () { } } These are called method Signatures. This is a design step! 5

6 Creating Classes: Functions Constructors Called when you create a new object Java provides a default constructor (with no arguments) Bike mybike = new Bike(); Accessors Get functions, or getters read the values of instance variables mybike.getspeed(); mybike.getgear(); Mutators Set functions, or setters set the values of instance variables mybike.setspeed(15); mybike.setgear(5); tostring Creates a String representation of the contents of the object System.out.println(mybike) calls object s tostring equals Returns true when the objects are the same, false otherwise if(mybike.equals(yourbike)) System.out.println( Cool, we have the same bike ); 6

7 Constructors Constructors run when you create a new object They are used to initialize class variables Function Declaration public <Class> (args ) { } Examples public Bike(double s) { speed=s;} public Bike() { speed=0;} Calling the constructor <Class> var = new Class(args ) Examples Bike mybike = new Bike(15); Bike mybike = new Bike(); Java provides a default constructor (with no arguments) 7

8 Accessor Example public class Date { private int month; private int day; private int year; public void getmonth() { return month; } } 8

9 Mutator Example public class Date { private int month; private int day; private int year; public void setmonth(int month) { if (month > 0 && month <= 12) this.month = month; else System.out.println( Invalid month ); } } 9

10 Use of Accessors and Mutators Date johnny = new Date(); johnny.setmonth(7); System.out.println( This person was born + in month # + johnny.getmonth()); 10

11 Symmetry public int getmonth( ) { return month; } get has no params public void setmonth(int month) { if (month > 0 && month <= 12) this.month = month; else System.out.println( Invalid month ); } set has no return value 11

12 Symmetry public int getmonth( ) { return month; } get returns a value of the data member s type public void setmonth(int month) { if (month > 0 && month <= 12) this.month = month; else System.out.println( Invalid month ); } set takes in a value of the data member s type 12

13 Variables: Instance vs. Class (static) A class variable (aka static variable) is shared by all instances of a class. An instance variable can be different for each instance Example Suppose all bikes had the same number of gears. If we made this a class variable, and we wanted to change it, it would change for ALL bikes. static int numberofgears = 18; int currentspeed = 15; 13

14 Creating Objects Use the new keyword Bike mybike = new Bike(); Bike yourbike = new Bike(); Bike herbike = newbike(); The Member Access Operator (.) The members of a class (instance variables and functions) are accessed using the member access operator, or dot operator (.) Example Bike mybike = new Bike(); mybike.speed // Instance variable mybike.changegears() // Instance function 14

15 Array of Objects Date johnny = new Date(); Creates an object pointed to by variable johnny Date[] birthdays = new Date[MAX]; Creates MAX # of Date pointers Does not have objects yet Not valid to use birthdays[0].setmonth(12) yet It created MAX # of entries birthdays[0] = new Date(); // Now we can access birthdays[0].setmonth(12); Need to instantiate two things for arrays (new) Pointers using Square brackets Objects using parenthesis 15

16 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). 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 Access-Level Three access level modifiers Changes where variables/functions can be accessed Three different access types: public: any class can read/modify protected: only the class and subclass descendents can read/modify private: only the class can read/modify 18

19 Brief Example: Class Skeletons public class Bicycle { protected 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 tirepressure; public RoadBike() {} public void winrace(){ cadence = 100; gear = 21; printspd(); calcspd(); } } The new RoadBike class (subclass) inherits the Bicycle class (superclass) Inside the main function: RoadBike rbike = new RoadBike(); Bicycle mybicycle = new Bicycle(); rbike.cadence = 5; rbike.gear = 15; rbike.printspd(); rbike.winrace(); mybicycle.winrace(); // winrace does not exist inside Bicycle class rbike.speed = 8; rbike.calcspd(); // Private! 19

20 Creating Strings To create a String: String s1, s2; // Create two String objects s1 = "This is a test."; // Assign to String Literal s2 = new String(); // Create a blank string These steps can be combined on a single line: String s3 = "String 3."; // All together Two simple/useful functions charat(index): System.out.println(s1.charAt(0) + + s1.charat(2)); Output: Ti length(): System.out.println(s1.charAt(s1.length()-1)) Output:. 20

21 String Function: Substring A substring is a portion of a String. The String function substring creates a new String object containing a portion of another String. The function can be called two different ways: s.substring(int startindex); // From <start> to s.length() s.substring(int startindex, int endindex);// <start> to <end> NOTE: IT DOES NOT INCLUDE endindex! This method returns another String object containing the characters from start to end (or the end of the string). 21

22 Substring Example Example: String s1 = this is an example ; System.out.println(s1.substring(0)); System.out.println(s1.substring(0,1); System.out.println(s1.substring(5)); System.out.println(s1.substring(8)); System.out.println(s1.substring(11)); System.out.println(s1.substring(5,7)); System.out.println(s1.substring(5,s1.length())); Output: this is an example t is an example an example example is is an example 22

23 String Function: Concat The String function concat creates a new String object containing the contents of two strings. The form of this function is: s1.concat(string s2); // Combine s1 and s2 This method returns a String object containing the contents of s1 followed by the contents of s2. 23

24 Concatenating Strings: Example Example (concat): String s1 = "abc"; String s2 = def ; System.out.println(s1); System.out.println(s2); String s3 = s1.concat(s2); String s4 = s2.concat(s1); System.out.println(s1); System.out.println(s2); System.out.println(s3); System.out.println(s4); Output: abc def abc def abcdef defabc Example (+): String s1 = "abc"; String s2 = def ; System.out.println(s1); System.out.println(s2); String s3 = s1 + s2; String s4 = s2 + s1; System.out.println(s1); System.out.println(s2); System.out.println(s3); System.out.println(s4); Output: abc def abc def abcdef defabc 24

25 Scanners To read data from user: Scanner kdb = new Scanner (System.in); To parse a String (stored inside a String variable) String example = This is an example ; Scanner line = new Scanner (example); To parse a file on the computer String filename = file1.txt ; Scanner file = new Scanner(new FileReader(filename)); Scanner Functions: next() kdb.next(); line.next(); file.next(); // get next input word from keyboard // get next input word from string example Scanner Functions: hasnext() line.hasnext() ; check if there is another word in string example Returns a Boolean value (true or false) 25

26 Example 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