Reading input from the console input

1. Reading input from the console input

2. 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

3. Java's console input • 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: which represents the console input The variable System.in is included in every Java program (you don't need to define it). System.in

4. Java's console input • A Java program can obtains inputs from the console through the keyboard • In other words: • The Java system variable System.in represents the keyboard

5. 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: • It is too early in the course to explain the notation System.in • We will explain this after we have covered classes • The variable named System.inrepresents the keyboard

6. Java's Scanner library functions • Fact: • 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) • There is a lot of work that the computer must do to read in a floating point number

7. Java's Scanner library functions (cont.) • Webpage of the Java documentation on Scanner class: http://download.oracle.com/javase/6/docs/api/java/util/Scanner.html

8. Java's Scanner library functions (cont.) • We will now learn how to use the methods in the Scanner class to read in floating point numbers

9. Importing the Scanner class definition • Recall the Rule of usage of methods in the Java library: (See: http://mathcs.emory.edu/~cheung/Courses/170/Syllabus/04/java-lib.html) • If a Java program wants to use a method in the Java library, the Java program must firstimport the containing class • All classes in the java.lang package have already been imported into a Java program • (You can use methods in these classes without the import clause)

10. Importing the Scanner class definition (cont.) • We can use the following import clause to import the Scanner class: import java.util.Scanner;

11. 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

12. Preparation before we can read input from the keyboard (cont.) • A Scanner object is constructed using the following statement: The name varName is an identifier Example: constructing a Scanner object named in Scanner varName = new Scanner(System.in); Scanner in = new Scanner(System.in);

13. Reading in a floating point number from the keyboard • Afterhaving constructed the Scanner object named in, you can use the following expression to read a floating point number from the keyboard: You mustsave (store) the number read in by "in.nextDouble()" in a variable with an assignment statement in.nextDouble()

14. Reading in a floating point number from the keyboard (cont.) • 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:

15. Reading in a floating point number from the keyboard (cont.) If you type in "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 !!!

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

17. Example: reading input for the a,b,c-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 • Programming Example: ABC formula

18. Reading in a floating point number from the keyboard (cont.) 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); } }

19. Reading in a floating point number from the keyboard (cont.) • Example Program: (Demo above code)     • Prog file: http://mathcs.emory.edu/~cheung/Courses/170/Syllabus/04/Progs/Abc2.java  • How to run the program: • Right click on link and save in a scratch directory • To compile:   javac Abc2.java • To run:          java Abc2

20. 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.

21. Good programming practice: Prompting user for input (cont.) • Good programming courtesy: • When the program needs the user to enter input from the keyboard, it must print out a (short) prompt message

22. Good programming practice: Prompting user for input (cont.) • 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

23. Good programming practice: Prompting user for input (cont.) 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); } }

24. 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:

25. Reading other types of input from the keyboard (cont.) • The only different is that we need to use a different method in the Scanner class that read the correct type of data.

26. Reading other types of input from the keyboard (cont.) • Reading an integer number from the keyboard: use nextInt()

27. Reading other types of input from the keyboard (cont.) • Note: you also need to use an int typed variable to store an integer value !!!