• A Common Need:
  • To accept input from the user and provide output to the user
• Input/Output in Java:
  • Input and output in Java are object-oriented
• The Issue:
  • You don't know about objects

• Now:
  • We'll use a class that is not part of the Java library named JMUConsole (you'll have to download the .class file into the directory containing your source code)
• Later:
  • We'll use standard library classes/methods

• The Issue:
  • The " character is used to denote a String literal. So, how can one include a " character in a String?
• The Resolution:
  • Use the escape sequence \" (which is a single char)
• Example:
  • "She said \"None for me, thanks.\" and left."

• A Resulting Issue:
  • If you want to output a \ character it too must be escaped (i.e., \\)
• A Related Issue:
  • Some characters (like tab, carriage return, and line feed) that can be printed can't be included in output strings in the obvious way because pressing the corresponding key causes an action (e.g., Tab) or because there isn't a corresponding key (e.g., the end-of-file character)
• Examples:
  • \n for newline
  • \r for carriage return
  • \t for tab

• What We've Already Discussed:
  • Declaring String variables
  • String literals
  • Assigining a String literal to a String variable
• A Convenient Operator:
  • The binary + can be used to concatenate its two operands (one of which must be a String) and evaluates to a String
• An Example:

  • int     number;
    String  course, department;
    
    number     = 149;
    department = "CS";
    course     = department + number;
    

• Setup:
  • You must call JMUConsole.open() exactly once, before you can use the JMUConsole for input or output
• Shutdown:
  • You should call JMUConsole.close() after you are done (i.e., just before the program terminates)

• Unformatted Output:
  • JMUConsole.print(value) prints the value (whatever type it is)
  • JMUConsole.println(value) prints the value followed by a line separator
• Formatted Output:
  • JMUConsole.printf(Format, value[, value]...) prints one or more values formatted according to the Format string

• Components:
  • String literals
  • Format specifiers
• Format Specifiers:
  • Syntax: %[flags][width][.precision]conversion
• Conversions:
  • 'b' for a boolean
  • 'c' for a char
  • 'd' for an integer
  • 'e' for a real number in scientific notation
  • 'f' for a real number
  • 's' for a String
• Flags:
  • '-' for left-justified
  • '+' to always include the sign
  • ' ' to use a space for the sign of a positive number
  • '0' to pad with zeros
  • ',' to use grouping separators
  • '(' to put negative numbers in parentheses

javaexamples/jmuconsole/PrintfExample.java

/**
 * An example that illustrates the use of the printf()
 * method in the JMUConsole class
 *
 * @author  Prof. David Bernstein, James Madison University
 * @version 1.0
 */
public class PrintfExample
{
    /**
     * The entry point of the application
     *
     * @param args   The command-line arguments
     */
    public static void main(String[] args)
    {
       int     age, weight;
       double  monday, tuesday, wednesday;
       
       age = 27;

       // Prepare the JMUConsole for input/output
       JMUConsole.open();       
       
       JMUConsole.printf("%d\n", age);
       JMUConsole.printf("%10d\n", age);
       
       weight = 155;
       
       JMUConsole.printf("Prof. B. is %d years old\n", age);
       JMUConsole.printf("Age: %d\tWeight: %d\n", age, weight);
       

       monday    =  35.8;
       tuesday   =   0.0;
       wednesday = -10.2;

       JMUConsole.println("\nWith No Flags, Width or Precision");       
       JMUConsole.printf("%f\n", monday);
       JMUConsole.printf("%f\n", tuesday);
       JMUConsole.printf("%f\n", wednesday);

       JMUConsole.println("\nWith No Flags");       
       JMUConsole.printf("%5.1f\n", monday);
       JMUConsole.printf("%5.1f\n", tuesday);
       JMUConsole.printf("%5.1f\n", wednesday);

       JMUConsole.println("\nWith + Flags");       
       JMUConsole.printf("%+5.1f\n", monday);
       JMUConsole.printf("%+5.1f\n", tuesday);
       JMUConsole.printf("%+5.1f\n", wednesday);

       // Close the JMUConsole
       JMUConsole.close();
    }
}
        

• nextDouble(), nextInt()
  • Return the next double value or int value
• nextLine()
  • Returns the next line as a String (i.e., what the user has typed up to the next line delimiter)
• next()
  • Finds and returns the next token as a String (i.e., what the user has typed up to the next token delimiter)

javaexamples/jmuconsole/InputExample.java

/**
 * An example that illustrates the use of the JMUConsole class
 * for (output and) input.
 *
 * @author  Prof. David Bernstein, James Madison University
 * @version 1.0
 */
public class InputExample
{
    /**
     * The entry point of the application
     *
     * @param args   The command-line arguments
     */
    public static void main(String[] args)
    {
        double  weight;
        
        int     age;

        // Prepare the JMUConsole for input/output
        JMUConsole.open();       
       
        // Prompt the user for her/his age
        JMUConsole.print("Enter your age (in years): ");
       
        // Read the response
        age = JMUConsole.nextInt();
       
        // Prompt the user for her/his weight and read the response
        JMUConsole.print("Enter your weight (in pounds): ");
        weight = JMUConsole.nextDouble();        

        // Echo the responses
        JMUConsole.printf("You are %d years old and weight %5.1f pounds.\n", 
                          age, weight);

        // Close the JMUConsole
        JMUConsole.close();
    }
}
        

• Output:
  • The textbook talks about System.out.print(), System.out.printf(), etc...
  • JMUConsole.print(), JMUConsole.printf(), etc... work the same way but don't use the out object which you don't yet understand
• Input:
  • The textbook talks about a Scanner class and methods like nextInt(), nextDouble(), etc...
  • JMUConsole.nextInt(), JMUConsole.nextDouble(), etc... work the same way but don't require the use of new and an assignment to an object that you don't yet understand (but does require that you call open() instead)

javaexamples/jmuconsole/InconvenienceExample.java

/**
 * An example that illustrates an important consideration when
 * using the next___() methods.
 *
 * @author  Prof. David Bernstein, James Madison University
 * @version 1.0
 */
public class InconvenienceExample
{
    public static void main(String[] args)
    {
        int    age;
        String name;
        
        // If you run this program you won't be able to enter your
        // name because the return you enter after your age will
        // be considered the "next line"
        JMUConsole.open();        
        JMUConsole.print("Enter your age: ");
        age = JMUConsole.nextInt();
        JMUConsole.print("Enter your name: ");
        name = JMUConsole.nextLine();
        JMUConsole.close();
    }
}

        

• readDouble(), readInt()
  • Return the next double value or int value and "consume" the newline character
• readLine()
  • Works just like nextLine()

javaexamples/jmuconsole/ConvenienceExample.java

/**
 * An example that illustrates the convenience of the
 * JMUConsole class (as compared to the Scanner class).
 *
 * @author  Prof. David Bernstein, James Madison University
 * @version 1.0
 */
public class ConvenienceExample
{
    public static void main(String[] args)
    {
        double weight;        
        int    age;
        String address, name;
        
        JMUConsole.open();        
        JMUConsole.print("Enter your age: ");
        age = JMUConsole.readInt();
        JMUConsole.print("Enter your name: ");
        name = JMUConsole.readLine();
        JMUConsole.print("Enter your weight: ");
        weight = JMUConsole.readDouble();
        JMUConsole.print("Enter your address: ");
        address = JMUConsole.readLine();

        JMUConsole.printf("%s lives at %s is %d years old and weighs %5.1f",
                          name, address, age, weight);
        
        JMUConsole.close();        
    }
}