Java Text IO

Handling Command Line Arguments

• Possible to pass an array of Strings into main method of java program

public class ArgsTest {
  public static void main(String[] args) {
    for(String arg : args) {
      System.out.println(arg);
    }
  }
}

• Then we can run the above with:

javac ArgsTest.java
java ArgsTest Each one of these words will become an argument

Each
one
of
these
words
will
become
an
argument

• More often then not, we use these command line arguments to change how a program runs at runtime

public class ArgsTest {
  public static void main(String[] args) {
    if(args.length != 1) {
      System.out.println("Invalid number of parameters");
      System.out.println("Usage: java ArgsTest file_path");
      System.exit(1);
    }
    
    String filePath = args[0];
    ...
  }
}

Java Input/Output

• Read/write text files
• Read/write binary files
• Read/write Java objects
• Java provides many classes for handling IO, though we're only going to look at a few

Reading Text Files

• Easiest to accomplish using Scanner class for small files

Scanner in = null;
File textFile = new File("/path/to/file");
try {
  in = new Scanner(textFile);
  while(in.hasNextLine()) {
    System.out.println(in.nextLine());
  }
}
catch (FileNotFoundException e) {
  System.err.format("File %s not found\n", textFile);
  e.printStackTrace();
}

• For large files, use BufferedReader in combination with FileReader
• Buffering the data means less accesses to disk (read or write in bulk)

File textFile = new File("/path/to/file");
BufferedReader bufferedReader = null;
String line;

try {
  bufferedReader = new BufferedReader(new FileReader(textFile));
  line = bufferedReader.readLine();
  while(line != null) {
    System.out.println(line);
    line = bufferedReader.readLine();
  }
  bufferedReader.close();
}
catch (FileNotFoundException e) {
  System.err.format("File %s not found\n", textFile);
  e.printStackTrace();
}
catch (IOException e) {
  System.err.format("Problem reading file %s\n", textFile);
  e.printStackTrace();
}

Writing Text Files

• Use FileWriter and BufferedWriter
• Take special care if intention is to append to file rather than overwrite

File textFile = new File("/home/anthony/scrap/test2.txt");
BufferedWriter bufferedWriter = null;

try {
  bufferedWriter = new BufferedWriter(new FileWriter(textFile));
  for(int i = 0; i < 10; i++) {
    bufferedWriter.write(String.format("Hello, World! %d\n", i));
  }
  bufferedWriter.close();
}
catch (IOException e) {
  System.err.format("Problem writing to file %s\n", textFile);
  e.printStackTrace(); 
}

In Lab Exercise

• Download the files located at https://github.com/anthonyjchriste/ics211f13/tree/master/src/io
• Complete readFile which should return a list of lines of the text file
• Complete writeFile which writes a list of lines to a text file
• Complete the main method so that the read in file is the first command line parameter and the file being written to comes in as the second command line parameter