---

Reading input from the console input

---

• Java's console input
  • The console is the terminal window that is running the Java program

    I.e., that's the terminal window where you type in the command java ProgramName

    ---

  • When a Java program starts running, the Java runtime system will initialize many variables in support for the running program.

    One of these variables is the Java system variable:

    System.in

    which represents the console input

    The variable System.in is included in every Java program (you don't need to define it).

    ---

  • A Java program can obtains inputs from the console through the keyboard

    In other words:

    The Java system variable System.in represents the keyboard

  ---

  ---

• A note on the notation "System.in"
  • At this moment in the course, we want to learn how to read input from the keyboard

    All you need to know is:

    The variable named System.in represents the keyboard

  • It is too early in the course to explain the notation System.in

  • We will explain this after we have covered classes

  ---

  ---

  ---

  ---

  ---

  ---

• Java's Scanner library class (of helpful programs)
  • Fact:

    There is a lot of work that the computer must do to read in a floating point number

    ---

  • The details of what the computer must do to read in a number will be discussed in CS255

  • The Java programming language provides a collection of methods stored in the Scanner class that perform read operations

          (Remember that a class is a container for methods)

    ---

  • Webpage of the Java documentation on Scanner class: click here

    ---

  • We will now learn how to use the methods in the Scanner class to read in floating point numbers

  ---

  ---

  ---

  ---

• Identifying the Scanner class inside Java's library
  • The class path of the Scanner class inside the Java library is:

    java.util.Scanner

    (see: click here)

  ---

  ---

  ---

  ---

  ---

  ---

• Importing the Scanner class definition
  • Recall the Rule of usage of methods in the Java library: (See: click here)

    If you want to make a class inside the Java library accessible without using the class path, a Java program must first import the class (with an import clause)

    ---

  • We can use the following import clause to import the Scanner class:

    import java.util.Scanner;

  ---

  ---

  ---

  ---

  ---

  ---

• Preparation before we can read input from the keyboard
  • Before a Java program can read input from the keyboard, the program must " construct a Scanner object

    It is too early to explain what this means... I will only tell you how to do it

    ---

  • A Scanner object is constructed using the following statement:

    Scanner varName = new Scanner(System.in);

    The name varName is an identifier

    Example: constructing a Scanner object named in

    Scanner in = new Scanner(System.in);

  ---

  ---

• Reading in a floating point number from the keyboard
  • After having constructed the Scanner object named in, you can use the following expression to read a floating point number from the keyboard:

    a = in.nextDouble() ;

    You must save (store) the number read in by "in.nextDouble()" in a double typed variable with an assignment statement

    ---

  • What happens inside the computer:

    Just like Math.sqrt(..), the method call in.nextDouble() will invoke (run) a method in Java's library. The task performed by in.nextDouble() is to read a floating point number from the keyboard: If you type ing "3.5" on the keyboard at the time that in.nextDouble() is running, then the call will return the value 3.5 The return value will replace the method call: The input value 3.5 is then stored in the variable a !!!

  ---

  ---

  ---

  ---

  ---

  ---

• Summary: steps to read in a floating point number
  • This figure summarizes the programming steps to read in a floating point number:

  ---

  ---

  ---

  ---

• Example: reading input for the a,b,c-formula

  • Programming Example: ABC formula

    import java.util.Scanner; // Import Scanner class (contains methods // for reading keyboard input) public class Abc2 { public static void main(String[] args) { double a, b, c, x1, x2; // Define 5 variable Scanner in = new Scanner(System.in); // Construct a Scanner object a = in.nextDouble(); // Read in next number and store in a b = in.nextDouble(); // Read in next number and store in b c = in.nextDouble(); // Read in next number and store in c x1 = ( -b - Math.sqrt( b*b - 4*a*c ) ) / (2*a); x2 = ( -b + Math.sqrt( b*b - 4*a*c ) ) / (2*a); System.out.print("a = "); System.out.println(a); System.out.print("b = "); System.out.println(b); System.out.print("c = "); System.out.println(c); System.out.print("x1 = "); System.out.println(x1); System.out.print("x2 = "); System.out.println(x2); } }

    ---

  • Example Program: (Demo above code)                                                
    • Prog file: click here

    How to run the program:

    Right click on link and save in a scratch directory To compile:   javac Abc2.java To run:          java Abc2

  ---

  ---

  ---

  ---

  ---

  ---

  ---

  ---

• Good programming practice: Prompting user for input
  • The previous program works, but requires the users to know exactly what to do

    In other words:

    An unaware user may not know that he/she needs to enter some input before the program can perform its task.

    ---

  • Good programming courtesy:

    When the program needs the user to enter input from the keyboard, it must print out a (short) prompt message

    ---

  • Example:

    import java.util.Scanner; // Import Scanner class (contains methods // for reading keyboard input) public class Abc2 { public static void main(String[] args) { double a, b, c, x1, x2; // Define 5 variable Scanner in = new Scanner(System.in); // Construct a Scanner object System.out.print("Enter a = "); // ******* Prompt message a = in.nextDouble(); // Read in next number and store in a System.out.print("Enter b = "); b = in.nextDouble(); // Read in next number and store in b System.out.print("Enter c = "); c = in.nextDouble(); // Read in next number and store in c x1 = ( -b - Math.sqrt( b*b - 4*a*c ) ) / (2*a); x2 = ( -b + Math.sqrt( b*b - 4*a*c ) ) / (2*a); System.out.print("a = "); System.out.println(a); System.out.print("b = "); System.out.println(b); System.out.print("c = "); System.out.println(c); System.out.print("x1 = "); System.out.println(x1); System.out.print("x2 = "); System.out.println(x2); } }

  ---

  ---

  ---

  ---

  ---

  ---

• Reading other types of input from the keyboard
  • The procedure to read other types of inputs from the keyboard is similar to the one above.

    This one discussed is used to read in a floating point number:

    To read in a different kind of item, we need to use a different method in the Scanner class that read the correct type of data.

    ---

  • Reading an integer number from the keyboard: use nextInt()

    Note: you also need to use an int typed variable to store an integer value !!!