Input and Output

Chapter 19
Input and Output

   19.1 Dialog Boxes
   19.2 Standard Output
   19.3 Standard Input
   19.4 Writing into Files
   19.5 Reading from Files
   19.6 Assignment #24: I/O
   19.7 Assignment #25: I/O + arrays

19.1 Dialog Boxes

The ‘javax.swing.JOptionPane’ class offers dialog box methods.
Output messages
JOptionPane.showMessageDialog (null, <message> )

import javax.swing.JOptionPane;
class prg{
  public static void main(String[] args){
     JOptionPane.showMessageDialog (null, "My message");
     System.exit(0);
} }
Input messages
<String variable> = JOptionPane.showInputDialog (<message> )

import javax.swing.JOptionPane;
class prg{
  public static void main(String[] args){
    String input;
    input = JOptionPane.showInputDialog
                ("Please enter a value ");
    System.exit(0);
} }
The ‘System.exit(0);’ method is required for terminating the execution.

19.2 Standard Output

- System.out.println
- System.out.print
The ‘System’ class introduces a class field named ‘out’, set to direct text from the program to an external destination. The destination is standard for the computing environment in use. Typically, it is the screen of a monitor.
The class field ‘out’ references an object of class type ‘PrintStream’.
The ‘PrintStream’ class introduces ‘print’ and ‘println’ instance methods
The ‘print’ and ‘println’ methods provide to ‘System.out’ the text for the output.

System.out.println("The class of " + System.out +
" is " + (System.out).getClass().getName());
// The class of java.io.PrintStream@bf786fde is java.io.PrintStream

19.3 Standard Input

The ‘System’ class introduces a class field named ‘in’. The field is set to direct text into the program from an external source that is standard for the computing environment in use. Typically, this source is a keyboard.
The ‘read’ method provides the next byte from the input, returned as a value of type ‘int’.
The ‘read’ method throws a ‘java.io.IOException’ exception, when the method encounters communication problems. The exception must be caught.

class prg {
  public static void main(String args[])
                 throws java.io.IOException {
     int c;
    c = System.in.read();
    System.out.print( c + " : " + (char)c );
    c = System.in.read();
    System.out.print( c + " : " + (char)c );
} }

19.4 Writing into Files

Open an output file stream
FileWriter <id> =
new FileWriter( <String file name> );
Associate the ‘print’, ‘println’, and ‘close’ methods of class ‘PrintWriter’ with the stream
PrintWriter <id> =
new PrintWriter( <FileWriter id> );
‘close’
Must handle ‘java.io.IOException’ exceptions.

try{
    FileWriter file = new FileWriter( "try.txt" );
    PrintWriter myout = new PrintWriter( file );
    myout.println("Hello, file!");
    myout.close();
} catch(java.io.IOException e){
    // do something
}

java.io: FileWriter, PrintWriter

19.5 Reading from Files

Open an input file stream, and associate parameterless methods ‘read’ and ‘close’ with the stream.
FileReader <id> = new FileReader( <file name> );
The ‘read’ method returns a character casted into an ‘int’ representation. When the end of the file is reached, the value of -1 is returned.
Must handle ‘java.io.IOException’ exceptions

try{
   int c;
   FileReader myIn  = new FileReader( "try.txt" );
   while( (c=myIn.read()) != -1 ){
     System.out.print( (char) c );
   }
   System.out.println();
} catch(java.io.IOException e){
   // do something
}

The ‘BufferedReader’ class can serve as a wrapper to the ‘FileReader’ class.
BufferedReader <id> = new BufferedReader( <FileReader> );
It offers a parameter-less method ‘readLine’, for returning an input line as a ‘String’.

try{
   String s;
   FileReader fr  = new FileReader( "try.txt" );
   BufferedReader myIn = new BufferedReader( fr );
   while( (s=myIn.readLine()) != null ){
     System.out.println( s );
   }
} catch(java.io.IOException e){
   // do something
}

java.io: FileReader, BufferedReader

19.6 Assignment #24: I/O

Tentative assignment; subject to change

Write a program that does the following.

Prompts the user for an input file name through a dialog box.
Prompts the user for an output file name through a dialog box.
Copies the input file into the output file, subject to the removal of the space characters listed below from each line.
1. The leading space characters
2. The trailing space characters
3. The space characters that are preceded by space characters

Example. An input

This is a
fragment of
text.

should result in the following output.

This is a
fragment of
text.

Note: Files that fail to compile, and execute when applicable, will not be examined. They will be awarded a grade of 0 points.

Q&A

Assume ‘lab24’ for the submit program.

19.7 Assignment #25: I/O + arrays

Tentative assignment; subject to change

Write a program that reads 50 integer values from a file and prints out the following information

The list of input values
The list of input values in reverse order
The largest value in the input
The smallest value in the input
A message stating whether the list of input values is equal to the list of input values in reverse order.

Assume ‘lab25’ for the submit program.

Q&A

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Chapter 19Input and Output