Simple Programs

1. Simple Programs Simple Strings Writing some programs Only in Parameters? I/O Gadget

2. Comments • As noted Java accepts comments which look like those we used in pseudocode // This is a comment // Each line must start with the double // slashes • Java also understands multi-line comments • They start with /* and end with */ /* So this would be just fine */

3. Jumping Ahead • To make life easier we’re going to start using a type of object that comes with Java: • A String • We’ll give you a simplified version for now and simply say that it will be a little more complicated than Pseudocode Strings

4. Simple Strings • Creating a String variable* String s1; s1 = "My first String"; String s2 = "Hi there!"; *Warning: If you have experience with Basic, C or many other languages, Java Strings don’t work the same!

5. String Stuff • Strings can be concatenated "Hello " + "World" + "!" • Java will make a lot of simple conversions for us: System.out.println("Value of x is " + x); • More later!

6. Average some numbers class Average { public static void main(String args[]) { double x = 27.33; double y = 37.98; double z = 12.00; System.out.println((x + y + z)/3.); } // main } // Average Recall: Typing java Average will cause Java to search in class Average for method main

7. Average using a Function class Average { public static double average (double a, double b, double c) { return (a + b + c)/3.; } // average public static void main(String args[]) { double x = 27.33; double y = 37.98; double z = 12.00; System.out.println(average(x, y, z)); } // main } // Average

8. Java only has in parameters! class SwapDemo { public static void swap(int x, int y) { int t; t = x; x = y; y = t; System.out.println("Swap x = "+ x +" y = " + y); } // swap public static void main(String args[]) { int a = 33; int b = 56; swap(a, b); System.out.println("Swap a = "+ a +" b = " + b); } // main } // SwapDemo Swap x = 56 y = 33 Swap a = 33 b = 56 !!!!!!

9. Print some text class PrintDemo { public static void main(String args[]) { System.out.println("If lines are too long " + "They can be broken up with the + sign " + "In addition special characters can be\n" + "inserted. They are always prefaced by " + "the backslash character \\"); } } If lines are too long They can be broken up with the + sign In addition special characters can be inserted. They are always prefaced by the backslash character \

10. Data Input • Java has very poor console (DOS Screen) input facilities • Why? • Most real Java applications use Graphical User Interfaces (GUIs) • Thus Input typically occurs in some kind of text box or other Widget!

11. Instructions • Cut and paste the following slides into files called: Stub.java IOGadget.java • The file names must be exactly as shown • Put both of files in a separate directory • i.e. Both files in the same separate directory! • Compile with javac *.java • Test with java Stub • Now you have a gadget that will let you read in strings, doubles and ints with built-in prompting!

12. Tester class Stub { public static void main(String args[]) { String s; double d; int i; s = IOGadget.readLine("Enter a string"); d = IOGadget.readDouble("Enter a double"); i = IOGadget.readInt("Enter an int"); System.out.println ("Just read in string: " + s + " double: " + d + " int: " + i); } // main } // Stub

13. IOGadget import java.io.*; public class IOGadget { private IOGadget(){} private static InputStreamReader isr = new InputStreamReader(System.in); private static BufferedReader br = new BufferedReader(isr); public static final String readLine(String p) { String retVal = ""; System.out.print(p+"> "); try {retVal = br.readLine(); } catch (Exception e) {System.err.println("IOGadget: " + e.getMessage());} return retVal; } // readLine

14. IOGadget public static int readInt(String prompt) { try { return Integer.parseInt(readLine(prompt)); } catch(Exception e) { System.out.println("Error reading int"); return 0; } } public static double readDouble(String prompt) { try { return Double.parseDouble(readLine(prompt)); } catch(Exception e) { System.out.println ("Error reading double"); return 0.0; } } } // IOGadget

15. Read in a number and make a calculation class NumReader { public static void main(String args[]) { double x, y; x = IOGadget.readDouble ("Enter a number"); y = 2. * x; System.out.println("x = " + x + " 2x = " + y); } // main } // NumReader

16. Calculate a square root import java.math.*; class SqrtTest { public final static double epsilon = .0000001; public static double sqrt(double d) { double answer = 2.; while(Math.abs(answer*answer - d) > epsilon) { double guess = d/answer; answer = (guess + answer)/2.; } // while return answer; } // sqrt public static void main(String args[]) { double number; number = IOGadget.readDouble("Enter a double"); System.out.println("Sqrt is " + sqrt(number)); } // main } // Sqrttest

17. And factorial??? class FactDemo { public static int fact(int n) { if(n == 0) return 1; else return n * fact(n - 1); } // fact public static void main(String args[]) { int number; number = IOGadget.readInt ("Enter a non-negative int"); System.out.println("Factorial " + fact(number)); } // main } // FactDemo