Introduction to Programming in Java Part III: Exception Handling and I/O Marko Boon September 2007
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Exception Handling public class ExceptionExample1 { public static void main(String[] arg) { int number1 = Integer.parseInt(arg[0]); int number2 = Integer.parseInt(arg[1]); int result = number1 - number2; System.out.print(number1+" - "+number2+" = "); System.out.println(result); } }
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>java ExceptionExample1 2 4 2 - 4 = -2
>java ExceptionExample1 Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 0 at ExceptionExample1.main(ExceptionExample1.java:4)
>java ExceptionExample1 2 5.6 Exception in thread "main" java.lang.NumberFormatException: For input string: "5 .6" at java.lang.NumberFormatException.forInputString(Unknown Source) at java.lang.Integer.parseInt(Unknown Source) at java.lang.Integer.parseInt(Unknown Source) at ExceptionExample1.main(ExceptionExample1.java:5)
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public class ExceptionExample2 { public static void main(String[] arg) {
} }
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public class ExceptionExample2 { public static void main(String[] arg) { if (arg.length != 2) System.err.println("Specify 2 arguments."); else {
} } }
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public class ExceptionExample2 { public static void main(String[] arg) { if (arg.length != 2) System.err.println("Specify 2 arguments."); else { boolean ok = true;
} } }
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public class ExceptionExample2 { public static void main(String[] arg) { if (arg.length != 2) System.err.println("Specify 2 arguments."); else { boolean ok = true; for (int i = 0; i < 2; i++) {
}
} } }
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public class ExceptionExample2 { public static void main(String[] arg) { if (arg.length != 2) System.err.println("Specify 2 arguments."); else { boolean ok = true; for (int i = 0; i < 2; i++) { char[] chars = arg[i].toCharArray();
}
} } }
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public class ExceptionExample2 { public static void main(String[] arg) { if (arg.length != 2) System.err.println("Specify 2 arguments."); else { boolean ok = true; for (int i = 0; i < 2; i++) { char[] chars = arg[i].toCharArray(); for (int j = 0; j < chars.length; j++) if (chars[j] < ’0’ || chars[j] > ’9’) ok = false; }
} } }
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public class ExceptionExample2 { public static void main(String[] arg) { if (arg.length != 2) System.err.println("Specify 2 arguments."); else { boolean ok = true; for (int i = 0; i < 2; i++) { char[] chars = arg[i].toCharArray(); for (int j = 0; j < chars.length; j++) if (chars[j] < ’0’ || chars[j] > ’9’) ok = false; } if (ok) // parse strings else System.err.println("No integer!"); } } }
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Exception Handling In Java some methods may ’throw’ an Exception. This means that you have to ’try’ to execute this code. If an exception is thown, you can ’catch’ it and handle it. In the Java documentation you can see whether a certain method throws an Exception or not. One method can throw several kinds of exceptions. You can catch them seperately: try { // method that throws Exceptions } catch (SomeKindOfException e) { // deal with it } catch (OtherKindOfException e) { // deal with it }
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public class ExceptionExample3 { public static void main(String[] arg) {
} }
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public class ExceptionExample3 { public static void main(String[] arg) { int number1 = Integer.parseInt(arg[0]); int number2 = Integer.parseInt(arg[1]); int result = number1 - number2; System.out.print(number1+" - "+number2+" = "); System.out.println(result);
} }
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public class ExceptionExample3 { public static void main(String[] arg) { try { int number1 = Integer.parseInt(arg[0]); int number2 = Integer.parseInt(arg[1]); int result = number1 - number2; System.out.print(number1+" - "+number2+" = "); System.out.println(result); }
} }
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public class ExceptionExample3 { public static void main(String[] arg) { try { int number1 = Integer.parseInt(arg[0]); int number2 = Integer.parseInt(arg[1]); int result = number1 - number2; System.out.print(number1+" - "+number2+" = "); System.out.println(result); } catch (ArrayIndexOutOfBoundsException e) { System.err.println("Specify 2 arguments!"); }
} }
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public class ExceptionExample3 { public static void main(String[] arg) { try { int number1 = Integer.parseInt(arg[0]); int number2 = Integer.parseInt(arg[1]); int result = number1 - number2; System.out.print(number1+" - "+number2+" = "); System.out.println(result); } catch (ArrayIndexOutOfBoundsException e) { System.err.println("Specify 2 arguments!"); } catch (NumberFormatException e) { System.err.println("No integer!"); } } }
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You can also create and throw your own Exceptions: public class TooYoungException extends Exception { public TooYoungException(String msg) { super(msg); } } Declare that your method might throw an exception: public void show(int age) throws TooYoungException { String err = "Too young to watch Finding Nemo!"; if (age < 18) throw new TooYoungException(err); // show movie } You can use the method getMessage( ) to get the error message.
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File I/O Package java.io Important classes: 1. File (also for directories) 2. InputStream, Stream
BufferedInputStream,
FileInputStream,
ObjectInput-
3. OutputStream, BufferedOutputStream, FileOutputStream, ObjectOutputStream, PrintStream 4. Reader, BufferedReader, FileReader 5. Writer, BufferedWriter, FileWriter, PrintWriter
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Reading from plain text files
F ile
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Reading from plain text files
F ile R e a d e r F ile
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Reading from plain text files
B u ffe re d R e a d e r F ile R e a d e r F ile
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Reading from plain text files S trin g [] B u ffe re d R e a d e r F ile R e a d e r F ile
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Java Implementation public static String[] readFile(String fileName) {
}
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Java Implementation public static String[] readFile(String fileName) throws IOException {
}
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Java Implementation public static String[] readFile(String fileName) throws IOException { FileReader fr = new FileReader(fileName);
}
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Java Implementation public static String[] readFile(String fileName) throws IOException { FileReader fr = new FileReader(fileName); BufferedReader reader = new BufferedReader(fr);
}
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Java Implementation public static String[] readFile(String fileName) throws IOException { FileReader fr = new FileReader(fileName); BufferedReader reader = new BufferedReader(fr); Vector lines = new Vector();
}
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Java Implementation public static String[] readFile(String fileName) throws IOException { FileReader fr = new FileReader(fileName); BufferedReader reader = new BufferedReader(fr); Vector lines = new Vector(); while (reader.ready()) {
}
}
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Java Implementation public static String[] readFile(String fileName) throws IOException { FileReader fr = new FileReader(fileName); BufferedReader reader = new BufferedReader(fr); Vector lines = new Vector(); while (reader.ready()) { String s = reader.readLine(); }
}
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Java Implementation public static String[] readFile(String fileName) throws IOException { FileReader fr = new FileReader(fileName); BufferedReader reader = new BufferedReader(fr); Vector lines = new Vector(); while (reader.ready()) { String s = reader.readLine(); lines.addElement(s); }
}
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Java Implementation public static String[] readFile(String fileName) throws IOException { FileReader fr = new FileReader(fileName); BufferedReader reader = new BufferedReader(fr); Vector lines = new Vector(); while (reader.ready()) { String s = reader.readLine(); lines.addElement(s); } reader.close();
}
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Java Implementation public static String[] readFile(String fileName) throws IOException { FileReader fr = new FileReader(fileName); BufferedReader reader = new BufferedReader(fr); Vector lines = new Vector(); while (reader.ready()) { String s = reader.readLine(); lines.addElement(s); } reader.close(); String[] strLines = new String[lines.size()];
}
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Java Implementation public static String[] readFile(String fileName) throws IOException { FileReader fr = new FileReader(fileName); BufferedReader reader = new BufferedReader(fr); Vector lines = new Vector(); while (reader.ready()) { String s = reader.readLine(); lines.addElement(s); } reader.close(); String[] strLines = new String[lines.size()]; lines.copyInto(strLines); }
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Java Implementation public static String[] readFile(String fileName) throws IOException { FileReader fr = new FileReader(fileName); BufferedReader reader = new BufferedReader(fr); Vector lines = new Vector(); while (reader.ready()) { String s = reader.readLine(); lines.addElement(s); } reader.close(); String[] strLines = new String[lines.size()]; lines.copyInto(strLines); return strLines; }
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Writing to plain text files S trin g
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Writing to plain text files S trin g P rin tW rite r
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Writing to plain text files S trin g P rin tW rite r F ile W rite r
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Writing to plain text files S trin g P rin tW rite r F ile W rite r F ile
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Java Implementation public static void writeFile(String fileName, String[] lines) {
}
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Java Implementation public static void writeFile(String fileName, String[] lines) throws IOException {
}
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Java Implementation public static void writeFile(String fileName, String[] lines) throws IOException { FileWriter fw = new FileWriter(fileName);
}
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Java Implementation public static void writeFile(String fileName, String[] lines) throws IOException { FileWriter fw = new FileWriter(fileName); PrintWriter writer = new PrintWriter(fw);
}
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Java Implementation public static void writeFile(String fileName, String[] lines) throws IOException { FileWriter fw = new FileWriter(fileName); PrintWriter writer = new PrintWriter(fw); for (int i = 0; i < lines.length; i++) writer.println(lines[i]);
}
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Java Implementation public static void writeFile(String fileName, String[] lines) throws IOException { FileWriter fw = new FileWriter(fileName); PrintWriter writer = new PrintWriter(fw); for (int i = 0; i < lines.length; i++) writer.println(lines[i]); writer.flush(); }
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Java Implementation public static void writeFile(String fileName, String[] lines) throws IOException { FileWriter fw = new FileWriter(fileName); PrintWriter writer = new PrintWriter(fw); for (int i = 0; i < lines.length; i++) writer.println(lines[i]); writer.flush(); writer.close(); }
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I/O Exception Handling public static void main(String[] arg) {
}
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I/O Exception Handling public static void main(String[] arg) { String[] lines = readFile(arg[0]);
}
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I/O Exception Handling public static void main(String[] arg) { String[] lines = readFile(arg[0]); writeFile(arg[1], lines);
}
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I/O Exception Handling public static void main(String[] arg) { try { String[] lines = readFile(arg[0]); writeFile(arg[1], lines); }
}
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I/O Exception Handling public static void main(String[] arg) { try { String[] lines = readFile(arg[0]); writeFile(arg[1], lines); } catch (ArrayIndexOutOfBoundsException e) { System.err.println("Specify two filenames."); }
}
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I/O Exception Handling public static void main(String[] arg) { try { String[] lines = readFile(arg[0]); writeFile(arg[1], lines); } catch (ArrayIndexOutOfBoundsException e) { System.err.println("Specify two filenames."); } catch (FileNotFoundException e) { System.err.println("File not found."); }
}
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I/O Exception Handling public static void main(String[] arg) { try { String[] lines = readFile(arg[0]); writeFile(arg[1], lines); } catch (ArrayIndexOutOfBoundsException e) { System.err.println("Specify two filenames."); } catch (FileNotFoundException e) { System.err.println("File not found."); } catch (IOException e) { System.err.println("Unknown error."); } }
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1.
Packages and CLASSPATH
CLASSPATH A system environment variable that contains one or more directories. The Java compiler will only find source files located in directories that are part of the CLASSPATH. −→ make sure that the current directory (.) is in your CLASSPATH! Is is also recommended to add the root directory of your project to the CLASSPATH. Packages
Collection of classes that somehow belong together. The sources of these classes should be stored in a directory with the same name as the package. Importing classes from another package is possible using the full name (e.g. java.io.File) or importing the package (e.g. import java.io.* or import java.io.File).
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Compiling and running Java files Not part of package javac MainFile.java java MainFile Part of package javac package\MainFile.java java package.MainFile
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2.
Creating an Executable Program
1. Create a JAR file. This file can be executed on any system with the Java Runtime Environment installed by double-clicking on it. 2. Create an installation CD-ROM.
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Step 1: create a JAR file Create a file in project root directory, called manifest.mf: Manifest-Version: 1.0 Main-Class: GUITest Created-By: Your name If your project loads classes from an external jar file (e.g. from the JExcel jar file jxl.jar), you need to add another line that adds this jar file to the CLASSPATH (multiple jar files should be delimited by a space character): Class-Path: jxl.jar
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Step 1: create a JAR file Usage: -c -t -x -u -v -f -m
jar ctxu[vfm] [jar-file] [manifest-file] files create new archive list table of contents for archive extract named (or all) files from archive update existing archive generate verbose output on standard output specify archive file name include manifest information from specified manifest file
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Random Number Generator Main class: GUITest.java Packages: data, gui, math, statistics, tools Nested packages (like tools.chart) do not have to be called separately. The command line becomes: jar -cfm rng.jar manifest.mf data gui math statistics tools images GUITest.* jar -i rng.jar
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Step 2: create the directory structure
• download self-extracting compressed zip file installcd.exe. • Unzip this file to a suitable location (e.g. in your project directory). It will create a directory called installcd, containing the files setup.exe, autorun.inf and settings.ini (and a required DLL file). It also contains a subdirectory called program which contains the files program.exe and the same DLL file.
• Copy the JAR file to this program directory. • Copy all non-class files that belong to the project (external program, JPG or GIF images) used in your program. Use the same directory structure!
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Step 3: set the program name • Open installcd\settings.ini in a text editor
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Step 3: set the program name • Open installcd\settings.ini in a text editor
[Main] ProgName= ProgDir= JarExeName=
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Step 3: set the program name • Open installcd\settings.ini in a text editor • add a program name to the second line (spaces allowed, no quotes)
[Main] ProgName= ProgDir= JarExeName=
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Step 3: set the program name • Open installcd\settings.ini in a text editor • add a program name to the second line (spaces allowed, no quotes)
[Main] ProgName=Random Number Generator ProgDir= JarExeName=
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Step 3: set the program name • Open installcd\settings.ini in a text editor • add a program name to the second line (spaces allowed, no quotes) • add a default installation directory name
[Main] ProgName=Random Number Generator ProgDir= JarExeName=
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Step 3: set the program name • Open installcd\settings.ini in a text editor • add a program name to the second line (spaces allowed, no quotes) • add a default installation directory name
[Main] ProgName=Random Number Generator ProgDir=rng JarExeName=
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Step 3: set the program name • Open installcd\settings.ini in a text editor • add a program name to the second line (spaces allowed, no quotes) • add a default installation directory name • add the name of the main Jar file
[Main] ProgName=Random Number Generator ProgDir=rng JarExeName=
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Step 3: set the program name • Open installcd\settings.ini in a text editor • add a program name to the second line (spaces allowed, no quotes) • add a default installation directory name • add the name of the main Jar file
[Main] ProgName=Random Number Generator ProgDir=rng JarExeName=rng
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Step 4: add an icon (optional)
• create an icon file • give it the same name as your JAR file (of course replace .jar by .ico)
• put it into the installcd\program directory. Shareware program to create icons:
• IconXP (www.aha-soft.com) • MicroAngelo (www.impactsoftware.com)
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Step 5: add the Java Runtime Environment Add the installation program for the Java Runtime Environment to the installcd directory.
• download the latest version from http://java.sun.com/j2se • place the downloaded file in the installcd directory • rename it to jre.exe.
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Step 6: test and burn the CD-ROM
• run setup.exe to test the installation from your harddisk • if possible, test it on other computers without Java in your network by mapping your drive
• burn the CD-ROM. The installcd directory should be the root of the CDROM, otherwise the autorun program will not work.
• test the installation program again
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