CS 121 Intro to Programming:Java - Lecture 11 Announcements

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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

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Topics for today:

File IO - How to read from, write to

external files

Exceptions will also get some attention

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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();

} }

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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));

}

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public void readLines(){

while(scan.hasNext()){

processLine(scan.nextLine());

}

scan.close();

}

public void processLine(String line){

System.out.println(line);

}

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import java.util.Scanner;

import java.io.*;

public class ReadDriver{

public static void main(String[] args)

throws IOException {

String fileName;

Echo e = new Echo(fileName);

e.readLines();

} }

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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);

} }

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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");

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);

}

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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();

} }

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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.

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import java.util.*;

import java.io.*;

public class ScrambleDriver{

public static void main(String[] args) throws

IOException {

System.out.println("enter file name");

String fileName = scan.next();

TextScramble scram = new

TextScramble(fileName);

scram.readLines();

} }

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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);

}

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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);

}

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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)

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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 zeroÏÏ§Ï at Except0.main(Except0.java:7)

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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.

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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

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public class Except3{

String s = "98.6";

int n;

try{

n = Integer.parseInt(s);

System.out.println(n*n); } catch(Exception e)

{

e.printStackTrace();

} } }

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Ï java.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

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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:

Ï enter your age

¼¼§Ï 39e

ÏÏ§Ï Bad input. 39e is not an integer. You must

input an integer

Ï enter your age

¼¼§Ï 39

ÏÏ§Ï next year you will be 40

ÏÏ§Ï

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import java.io.*;

import java.util.*;

public class IntegerInput{

int n = -1;

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");

}

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}

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..

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What if you enter a negative number? Not handled

very well

(No message)..

enter your age

¼¼§Ï -39

enter your age

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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:

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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();

}

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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

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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

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Here is nums.txt

50

60 3e

10 Here’s the output:

Ï Bad Entry: 3e

Ï 40.0

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import java.io.*;

import java.util.*;

public class NumberReader { String fileName;

int sum = 0;

int count = 0;

Scanner scan;

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();

}

Handle

IOEX when I’m called

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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);

} }

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public class NumDriver{

try{

NumberReader num

= new NumberReader("nums.txt");

num.readLines();

num.printResult();

}

catch(Exception e) {e.printStackTrace();}

} }

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This is tacky:

public static void main(String[] args)

throws IOException {

String fileName;

e.readLines();

} }

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public static void main(String[] args) { try{

String fileName;

e.readLines();

}

Catch(IOException e){System.out.println(e);}

} }