CS 121 Intro to Programming:Java - Lecture 11 Announcements

CS 121 Intro to Programming:Java - Lecture 11 Announcements Next Owl assignment up, due Friday (it s short!) Programming assignment due next Monday morning Preregistration advice: More computing? Take 191B, or - if you re really thriving in this class - 187 (come see me about this) Final exam: 12/19, 8 AM

Topics for today: File IO - How to read from, write to external files Exceptions will also get some attention

import java.util.scanner; import java.io.*; public class DisplayFile{ public static void main(string[] args) throws IOException { String filename; Scanner namereader = new Scanner(System.in); System.out.println("Enter a file name"); filename = namereader.nextline(); Scanner scan = new Scanner(new FileReader(fileName)); while(scan.hasnext()){ System.out.println(scan.nextLine()); scan.close();

import java.util.scanner; import java.io.*; public class Echo{ String filename; // external file name Scanner scan; // reads from external file public Echo(String f) throws IOException { filename = f; scan = new Scanner(new FileReader(fileName));

public void readlines(){ while(scan.hasnext()){ processline(scan.nextline()); scan.close(); public void processline(string line){ System.out.println(line);

import java.util.scanner; import java.io.*; public class ReadDriver{ public static void main(string[] args) { throws IOException String filename; Scanner namereader = new Scanner(System.in); System.out.println("Enter a file name"); filename = namereader.nextline(); Echo e = new Echo(fileName); e.readlines();

import java.io.*; public class LineCount extends Echo{ private int count = 0; public LineCount(String f) throws IOException { super(f); public void processline(string line){count++; public void reportlinecount(){ System.out.println("Line count: " + count);

import java.io.*; import java.util.*; public class WriteFile{ public static void main(string[] args) throws IOException { String filename; System.out.println("Enter a file name. It will hold output"); Scanner namereader = new Scanner(System.in); filename = namereader.nextline(); PrintWriter writer = new PrintWriter(fileName); Scanner scan = new Scanner(System.in); String s = " "; // a String of length 1 System.out.println("Enter text, end with 2 returns"); while(s.length() > 0){ s = scan.nextline(); writer.println(s);

writer.close(); // now echo the file back to the console Echo e = new Echo(fileName); System.out.println("Here comes the echo"); System.out.println(); e.readlines();

Our next example ilatlstrues a bizzare anodecte that ciurctaled on the web seevral years ago, and casued wiesrpdead giglges among all who saw it. The story corecnns an exereipmnt by Brtiish reehrsaecrs that put forawrd the foiwlolng arguemnt: it turns out that if you take a seibsnle body of text, exartct each word, leave the first two and last two letters of every word unoctuhed, but then raondmly rernaarge the rest - that is, the mildde, then the enuisng text is colpmteely unbdasdearnlte.

import java.util.*; import java.io.*; public class ScrambleDriver{ public static void main(string[] args) throws IOException { Scanner scan = new Scanner(System.in); System.out.println("enter file name"); String filename = scan.next(); TextScramble scram = new TextScramble(fileName); scram.readlines();

import java.io.*; import java.util.*; public class TextScramble extends Echo{ Random r = new Random(); String blank = " "; public TextScramble(String f) throws IOException { super(f);

public void processline(string line){ String s = blank; String answer = ""; char[] mid; StringTokenizer str = new StringTokenizer(line); while(str.hasmoretokens()){ s = str.nexttoken(); if (s.length() < 6) answer = answer+ s + blank; else { // carve up the token.. String front = s.substring(0,2); String back = s.substring(s.length() - 2, s.length()); String middle = s.substring(2,s.length() -2); mid = middle.tochararray(); // make middle an array shuffle(mid); // shuffle middle = new String(mid); // transform back to a String answer = answer +front + middle + back + blank; // end loop processing all tokens in line System.out.println(answer);

Java has a shadow system for handling errors: Exceptions This is a system of classes and objects that employ special machinery for detectings errors and shifting responsibilites for handling these problems Java recognized two kinds of errors: Unchecked exceptions: errors that occur because of flaws in logical thinking on the part of the programmer (e.g. division by 0) Checked exceptions: errors that can be expected to occur, and may occur despite your best efforts to avoid them (trying to read from a file that doesn t exist)

public class Except0{ public static void main(string[] args){ int k; int a = 3; ; int b = 0; k = a/b; Java reports: Exception in thread "main" java.lang.arithmeticexception: / by zero ÏÏ Ï at Except0.main(Except0.java:7)

Java has special machinery, the try/catch construction, for dealing with exceptions (handling exceptions) An exception is handled if your code detects the error and takes some action in response -- as in the next example.

public static void main(string[] args){ int k; int a = 3; int b = 0; try{ k = a/b; catch(arithmeticexception e) { System.out.println( e); Java reports: Ïjava.lang.ArithmeticException: / by zero ÏÏ Ï

public class Except3{ public static void main(string[] args){ String s = "98.6"; int n; try{ n = Integer.parseInt(s); System.out.println(n*n); catch(exception e) { e.printstacktrace();

Ïjava.lang.NumberFormatException: For input string: "98.6" ÏÏ Ï at java.lang.numberformatexception.forinput String(NumberFormatException.java:48) ÏÏ Ï at java.lang.integer.parseint(integer.java:456) ÏÏ Ï at java.lang.integer.parseint(integer.java:497) ÏÏ Ï at Except3.main(Except3.java:8) ÏÏ Ï

A more realistic example - you are supposed to enter an integer, representing your name (the program then reports your age for next year..) Some interaction: Ïenter your age ¼¼ Ï39e ÏÏ ÏBad input. 39e is not an integer. You must input an integer Ïenter your age ¼¼ Ï39 ÏÏ Ïnext year you will be 40 ÏÏ Ï

import java.io.*; import java.util.*; public class IntegerInput{ public static void main(string[] args){ int n = -1; Scanner scan = new Scanner(System.in); while (n < 0) { System.out.println("enter your age"); try { if (scan.hasnextint()) n = scan.nextint(); else { // non integer submitted String userinput = scan.next(); throw new Exception("Bad input. "+ userinput + " is not an integer. You must input an integer");

catch (Exception e) { System.out.println(e.getMessage()); System.out.println("next year you will be " + (n + 1)); What does throw do: it presents an exception object of the indicated kind at a new location in the program..

What if you enter a negative number? Not handled very well (No message).. enter your age ¼¼ Ï-39 enter your age

One way to handle this: make a new kind of Exception.. public class NegativeException extends Exception{ public NegativeException() { ; public NegativeException(String msg){ super(msg); And then:

while (n < 0) { System.out.println("enter your age"); try { if (scan.hasnextint()) n = scan.nextint(); else { String userinput = scan.next(); throw new Exception("Bad input. "+ userinput + "is not an integer. You must input an integer"); if (n < 0) throw new NegativeException();

catch (NegativeException e) { System.out.println("age must be >= 0"); catch (Exception e) { System.out.println(e.getMessage()); // end while System.out.println("next year you will be " + (n + 1)); Notice: two kinds of exceptions, more specific one first

One more example: You have a data file of numerical data, integers, one per line It may contain errors (deep space information?) You want to compute the average value of the entries - but you re happy just to skip over the bad data

Here is nums.txt 50 60 3e 10 Here s the output: ÏBad Entry: 3e Ï40.0

import java.io.*; import java.util.*; public class NumberReader { String filename; int sum = 0; int count = 0; Scanner scan; Handle IOEX when I m called public NumberReader(String f) throws IOException { filename = f; scan = new Scanner(new FileReader(fileName)); public void readlines(){while(scan.hasnext()){ processline(scan.nextline()); scan.close();

public void processline(string line){ try{ int num = Integer.parseInt(line); sum += num; count++; catch(exception e) {System.out.println("Bad Entry: " + line); public void printresult(){ System.out.println((double)sum/count);

public class NumDriver{ public static void main(string[] args){ try{ NumberReader num = new NumberReader("nums.txt"); num.readlines(); num.printresult(); catch(exception e) {e.printstacktrace();

This is tacky: public class ReadDriver{ public static void main(string[] args) { throws IOException String filename; Scanner namereader = new Scanner(System.in); System.out.println("Enter a file name"); filename = namereader.nextline(); Echo e = new Echo(fileName); e.readlines();

public class ReadDriver{ public static void main(string[] args) { try{ String filename; Scanner namereader = new Scanner(System.in); System.out.println("Enter a file name"); filename = namereader.nextline(); Echo e = new Echo(fileName); e.readlines(); Catch(IOException e){system.out.println(e);