Announcements

1. Announcements • Homework W1 is due today • Regrades: Ok, but we will regrade the entire assignment. Must be w/in 72 hours of when returned • Consultants start on Sunday. Alan is often in the lab as well. Lecture 4

2. Today’s Topics • Review • The Class String • Printing output: System.out.print, System.out.println, and System.out.flush() • Accessing modifiers public and private • Functions versus procedures, void • Constructors Lecture 4

3. Review • Classes are conceptually similar to types, like int, char, double. • Classes are programmer-defined • Objects are instances of classes. Objects of a particular class type get all the fields and methods associated with that class. Lecture 4

4. Review example Coordinate c1; Coordinate d1; c1 = new Coordinate(); d1 = new Coordinate(); c1.setX(3); d1.setX(4); d1.setY(2); c1.setY(d1.sumSquares()); d1.setY(c1.y); Lecture 4

5. Today’s example // An instance of Employee contains a person's name, // salary, and year hired. It has a constructor and // methods for raising the salary, printing the data, and // retrieving the person's name and the year hired. public class Employee { public String name; // The person's name publicdouble pay; // The person's yearly salary publicint hireDate; // The year hired Lecture 4

6. // Set the name to n publicvoid setName(String n) {name= n;} // Set the salary to s publicvoid setSalary(double s) {pay= s;} // Set the year to y publicvoid setHireDate(int y) {hireDate= y;} // Yield the year the person was hired publicint hireYear() {return hireDate;} Lecture 4

7. // Raise the pay by p percent publicvoid raiseSalary(double p) {pay= pay * (1 + p/100.0);} // Yield the person's name public String getName() {return name;} // Yield a String containing person’s data public String toString() { String s= name; s= s + " " + pay + " " + hireDate; return s;} } Lecture 4

8. Class String • An instance of class string is a “string” of characters: abcde, oo145^^!2l, . . . • Declare asString d; • Assigned as: d = new String(“L. Millett”) // just like other classesord = “L. Millett” // only because strings are common • Lots of methods available in the String class Lecture 4

9. Some String Methods • charAt(int index) // return char at index • endsWith(String suffix) // return true if . . . • toLowerCase() // return string w/no caps • length() // return length of the string String str1 = “Strings are cool.”; int n; if (str1.endsWith(“ool.”)) System.out.println(“Yes”); n = str1.length(); String str2 = str1.toLowerCase(str1); Lecture 4

10. Catenating Strings • Use the infix operator ‘+’ -- here it means something different than addition: • d + “xyz” + “ ” + “qed”evaluates to“Lyn Millettxyz qed” • Note that you need to explicitly put in spaces “ ” where you want them Lecture 4

11. millett // s1 millett // s2 String equality • Suppose: • if (s1 == s2)returns false! • Use if (s1.equals(s2)) instead • equals is another method of class String Lecture 4

12. Printing output • To print string s in the output window, use System.out.print(s) or System.out.println(s); • The latter prints an “end of line” after s • The following are equivalent:(1) System.out.print(s1); System.out.println(s2);(2) System.out.println(s1 + s2); Lecture 4

13. Flush • // Flush the output buffer// Make sure everything is sent to the screen • System.out.flush(); • Use this when you want to be sure the output is seen immediately • println automatically flushes, print does not • See pp.86-87 of text. Lecture 4

14. Now, back to our example • Accessing fields of an Employee objectEmployee c;c= new Employee();c.name= “G”;c.pay= 1000000;c.hireDate= 1998;c.pay= 220000; • Anyone see any problems here? Logically? Lecture 4

15. Security Issue • Allowing access to the pay field means that any program that can reference c can change pay. • This could be bad . . . • Changing a ‘sensitive’ field like pay should be limited to methods of the class • How to do this in Java? Lecture 4

16. Public/Private • // Anyone can access salarypublic int salary; • // Only methods w/in Employee can // reference payprivate int pay; • The following is now illegal:Employee c;c = new Employee();c.pay = 1000000; Lecture 4

17. So, how to change pay? • Now to change pay it’s necessary to call the method setSalary or raiseSalary • If the methods were declared private, then this would not be possible except from within the Class Lecture 4

18. Procedures vs. Functions • Method raiseSalary has prefix voidpublic void raiseSalary(double p); • void indicates that it does not return a result -- a procedure • Method getName has prefix Stringpublic String getName(); • A non-void prefix indicates that a result is returned -- a function Lecture 4

19. A Function • // Yield the person’s namepublic String getName() { return Name; } • Recall: Executing return <expression> terminates execution of the method (function) and returns the value of <expression> to the place of the call • String s = e1.getName(); • Think of functions in math: f(g(h(x))) • Can do similar things in Java Lecture 4

20. Constructors • A regular method except: • its name is the same as the name of the Class • it has no type class prefix of void • Use to initialize fields//Constructor: name n, year d hired, salary spublic Employee(String n, double s, int d) {name= n; pay= s; hireDate= d; } • c = new Employee(“millett”, 50000, 1999) Suuuuuuuure... Lecture 4

21. Another class (p. 142 in text) Class Rectangle { int length, width; public Rectangle (int side1, int side2) { length = side1; width = side2; } public int area () { return (length * width); } Lecture 4

22. Use of class Rectangle int userLength = Integer.parseInt(stdin.readLine()); int userWidth = Integer.parseInt(stdin.readLine()); Rectangle r1; r1 = new Rectangle(userLength, userWidth); System.out.println(“The area of your rectangle is: ”); System.out.println(r1.area()); Rectangle r2 = new Rectangle(5, 10); if (r2.area() > r1.area()) { System.out.println(“Your rectangle is smaller.”); } Lecture 4

23. What Have We Learned? • How to define a class -- fields, methods • Definition of a class variable • Creation of a new instance of a class • Referencing and assigning fields • Assigning class instance to class variable to create an alias • Class methods • Class constructors Lecture 4