Input and Output
(Example code and figures from Computing Concepts with Java Essentials, 3rd edition, by Cay Horstmann.
Visit www.wiley.com/college/horstmann for information on the current edition of this excellent text.)

• from the AP CS Java Subset
  • User input is not part of the AP Java subset.
    • There are many possible ways for supplying user input; e.g., by reading from a BufferedReader that is wrapped around System.in, reading from a stream (such as a file or an URL), or from a dialog box.
    • There are advantages and disadvantages to the various approaches.
    • In particular, reading from System.in is both fraught with complexities (two nested readers and the handling of checked exceptions) and considered old fashioned by some instructors.
    • The exam does not prescribe any one approach.
    • Instead, if reading input is necessary, it will be indicated in a way similar to the following:

      double x = call to a method that reads a floating-point number;

      or

      double x = IO.readDouble(); // read user input

  • Testing of output is restricted to System.out.print and System.out.println.
    • As with user input, there are many possible ways for directing the output of a program, for example to System.out, to a file, or to a text area in a graphical user interface.
    • The AP Java subset includes the ability to print output to System.out, because it makes it easy to formulate questions.
    • Since most graphical environments allow printing of debug messages to System.out (with output being collected in a special window, e.g. the "Java console" in a browser), students are usually familiar with this method of producing output.
    • Formatted output (e.g., with NumberFormat) is not in the subset.
      
      
• Converting between Strings and Numbers
  
  

  • Convert to number:

    String str = "25"; int n = Integer.parseInt(str); double x = Double.parseDouble(str);

    
    
  • Integer and Double are called wrapper classes because they "wrap" a fundamental data type
    
    
  • Convert to string:

    String str = "" + n; str = Integer.toString(n);

   

• To get input from the user by way of a dialog box
  • JOptionPane is a class in the Java API that provides a variety of dialog boxes

    String prompt = "How many nickles do you have?"; String input = JOptionPane.showInputDialog(prompt);

    
    




• The program will pause until the user enters something and clicks OK
• Convert strings to numbers if necessary:

  int count = Integer.parseInt(input);

  
  
• Conversion throws an exception (run-time error) if user doesn't supply a number
  
  
• Must add

  System.exit(0);

  to the end of the main method of any program that uses JOptionPane
  
  
• Getting input from System.in
  • we must handle any possible exceptions
    • the easiest way is to just have main( ) throw an exception 

      public static void main(String[] args) throws IOException

  • then we can construct a BufferedReader

    BufferedReader console = new BufferedReader(new InputStreamReader(System.in));

  • then we can use the readLine method

    String input = console.readLine();