Java Console I/O and Code Examples

Console Output

• System.out -- out is a PrintStream object, a static data member of class System. This represents standard output
• Use this object to call functions print, println, and even printf
  • print() -- converts parameter to a string (if not already one) and prints it out
  • println() -- prints parameter, and also prints a newline after
  • printf -- works like in C programming. Formatted string, followed by parameters to "fill in the blanks"
• Sample calls:

    System.out.print("Hello, World");	// no newline
    System.out.println("Hello\n\nWorld"); // adds newline at end
  
    int feet = 6, inches = 3;
    System.out.printf("I am %d feet and %d inches tall\n", feet, inches);
    // just like printf in C
  

• If the + operator is used with at least one string operand, then the operation is string concatenation. Other types will be auto-converted to type string if needed:

    System.out.print("The number of states in the U.S. is " + 50);
  
    int sides = 8;
    System.out.println("Number of sides on a stop sign = " + sides);
  

Example links

• Example1.java - Illustrates basic console output
• Sample.java - Our sample program from the intro page
• Cat.java - Basic string concatenation
• Cat2.java - Be careful about string concatenation vs. regular use of + operator. The order of operations matters!

Special: Formatting with printf

• When printing values with decimal precision (type float or type double), it is often useful to be able to specify how many decimal places should be printed
• Since Java 1.5.0, the C-style printf function has been available, and it provides an easy way to format decimal precision
• Format of printf calls:

    System.out.printf(format string, list of parameters);
  

• The format string is a string in quotes, with special format symbols inserted:
  • %d specifies an integer
  • %c specifies a character
  • %s specifies a String
  • %f specifies a floating point type
• Consider the format symbols to be "fill-in-the-blanks" spots in the format string. These are filled in with the list of parameters
• Example:

    int numStudents = 25;
    char letterGrade = 'A';
    double gpa = 3.95;
  
    System.out.printf("There are %d students\n", numStudents);
    System.out.printf("Bobby's course grade was %c, and his GPA is %f\n", 
                      letterGrade, gpa);
  
    // The output from this example is:
    //   There are 25 students
    //   Bobby's course grade was A, and his GPA is 3.950000
  

• To specify how many decimal places for the output of a floating point value, modify the %f symbol in this format:

    %.Nf  // where N is the number of decimal places to print
  

• Example:

    double gpa = 3.275;
    double PI = 3.1415;
  
    System.out.printf("gpa = %.2f", gpa);
    System.out.printf("PI = %.3f", PI);
  
    // Output is:
    //    gpa = 3.28
    //    PI = 3.142
  

Console Input

• Before Java version 1.5.0, console input was harder. Since 1.5.0, we have the Scanner class
• class Scanner is a text parser. Contains easy methods for grabbing different types of input
• System.in is an InputStream object that represents standard input
• To use Scanner to read from standard input...
  1. Put the appropriate import statement at the top of the file:

     import java.util.Scanner;

  2. Create a Scanner object
  3. Pass in System.in into the Scanner constructor, when creating the object
• Example:

    import java.util.Scanner;
    // yadda yadda
  
    Scanner input = new Scanner(System.in);
  
    // now we can use the object to read data from the keyboard.
    // Some sample calls:
    int x = input.nextInt();
    double y = input.nextDouble();
    String s = input.next();
  

Examples:

• ConsoleInput1.java - A program demonstrating simple I/O with integers and strings
• Arithmetic.java - Illustrates some input and the arithmetic operations