Errors driving me crazy! But, it all compiles fine

Question

Errors driving me crazy! although compiles fine
I am working on a project for an online class - I am teaching myself really! My last assignment I cannot get to work. I had a friend who "knows" what he is doing help me. Well that didn't work out too well, my class is a beginner and he put stuff in that I never used yet. I am using Jgrasp and Eclipse. I really am trying but, there really is no teacher with this online class. I can't get questions answered in time and stuff goes past due. I am getting this error:
Exception in thread "main" java.lang.NullPointerException
at java.io.Reader.<init>(Reader.java:61)
at java.io.InputStreamReader.<init>(InputStreamReader .java:55)
at java.util.Scanner.<init>(Scanner.java:590)
at ttest.main(ttest.java:54)
----jGRASP wedge2: exit code for process is 1.
----jGRASP: operation complete.
This is my code:
import java.util.*;
import java.io.*;
public class ttest
static Scanner console = new Scanner(System.in);
public static void main(String[] args)throws IOException
FileInputStream fin = null; // input file reference
PrintStream floser = null; // output file references
PrintStream fwinner = null;
Scanner rs; // record scanner
Scanner ls; // line scanner
String inputrec; // full record buffer
int wins; // data read from each record
int losses;
double pctg;
String team;
String best = null; // track best/worst team(s)
String worst = null;
double worst_pctg = 2.0; // track best/worst pctgs
double best_pctg = -1.0;
int winner_count = 0; // counters for winning/losing records
int loser_count = 0;
// should check args.length and if not == 1 generate error
try
Scanner inFile = new Scanner(new FileReader("football.txt"));
catch( FileNotFoundException e )
System.exit( 1 );
try
floser = new PrintStream( new FileOutputStream( "loser.txt" ) );
fwinner = new PrintStream( new FileOutputStream( "winner.txt" ) );
catch( FileNotFoundException e )
System.out.printf( "unable to open an output file: %s\n", e.toString() );
System.exit( 1 );
try
rs = new Scanner( fin );
while( rs.hasNext( ) )
inputrec = rs.nextLine( ); /* read next line */
ls = new Scanner( inputrec ); /* prevents stumble if record has more than expected */
team = ls.next( );
wins = ls.nextInt();
losses = ls.nextInt();
if( wins + losses > 0 )
pctg = ((double) wins)/(wins + losses);
else
pctg = 0.0;
if( pctg > .5 )
if( pctg > best_pctg )
best_pctg = pctg;
best = team;
else
if( pctg == best_pctg )
best += ", " + team;
fwinner.printf( "%10s %2d %2d %5.3f\n", team, wins, losses, pctg );
winner_count++;
else
if( pctg < worst_pctg )
worst_pctg = pctg;
worst = team;
else
if( pctg == worst_pctg )
worst += ", " + team;
floser.printf( "%10s %2d %2d %5.3f\n", team, wins, losses, pctg );
loser_count++;
fin.close( );
floser.close( );
fwinner.close( );
catch( IOException e ) {
System.out.printf( "I/O error: %s\n", e.toString() );
System.exit( 1 );
System.out.printf( "%d teams have winning records; %d teams have losing records\n", winner_count, loser_count );
System.out.printf( "Team(s) with best percentage: %5.3f %s\n", best_pctg, best );
System.out.printf( "Team(s) with worst percentage: %5.3f %s\n", worst_pctg, worst );
The assignment is:
Create a Java program to read in an unknown number of lines from a data file. You will need to create the data file. The contents of the file can be found at the bottom of this document. This file contains a football team's name, the number of games they have won, and the number of games they have lost.
Your program should accomplish the following tasks:
1. Process all data until it reaches the end-of-file. Calculate the win percentage for each team.
2. Output to a file ("top.txt") a listing of all teams with a win percentage greater than .500. This file should contain the team name and the win percentage.
3. Output to a file ("bottom.txt") a listing of all teams with a win percentage of .500 or lower. This file should contain the team name and the win percentage.
4. Count and print to the screen the number of teams with a record greater then .500 and the number of teams with a record of .500 and below, each appropriately labeled.
5. Output in a message box: the team with the highest win percentage and the team with the lowest win percentage, each appropriately labeled. If there is a tie for the highest win percentage or a tie for the lowest win percentage, you must output all of the teams.
Dallas 5 2
Philadelphia 4 3
Washington 3 4
NY_Giants 3 4
Minnesota 6 1
Green_Bay 3 4

Answer 1

If you are working in Eclipse IDE, you can set a breakpoint and single step through your code to determe what line crashes. You can also examine variable values before the crash. Locate a line before the crash and set a breakpoint by double clicking on the vertical blue line that appears immediately to the left of the line of code. A blue 'dot' will appear on the line. Then,from the menu, select 'debug as application'. This will run your main() function up to that break point. In the menu, there are icons to allow you to single step through lines of code from that point, and another icon to step over lines of code if you want.
Alternatively to setting break points, you can sprinkle your code with piles of System.out.println statements. Example: you have System.out.println("1"), and in another part of your code System.out.println("2"). If you code runs and prints out "1", but not "2", then you code crashed between them. I use breakpoints and System.out.println to debug.
In your trace, you have "at java.util.Scanner.<init>(Scanner.java:590)" which means it crashed at line 590 (note in all the tracings, Scanner.java is a name of one of your custom classes and not a vendor class, thats how you can quickly find what part of the trace is important to you).
NullPointerException means you tried to call a function on an object that is not instansiated (its null).
Example: person.getName() will not work if person is null. Its the call to getName() that crashes.
Debugging code is a very important skill set that you have to develop in which you have to logically track down and isolate problems.