DSA 103 Object Oriented Programming :: Week 4

Object – Oriented Programming
Week 4 – do - while and switch
Ferdin Joe John Joseph, PhD
Faculty of Information Technology
Thai-Nichi Institute of Technology

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
Faculty of Information Technology,

• 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

• 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:
• 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.in represents the
keyboard

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

Java's Scanner library functions (cont.)

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

Importing the Scanner class definition
• Recall the Rule of usage of methods in the Java library:
• If a Java program wants to use a method in the Java library, the Java
program must first import 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)

Importing the Scanner class definition
(cont.)
• 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

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);

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

• 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 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 !!!

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);
}
}

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

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);
}
}

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:

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.

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

Loops – While, Do, For
• Repetition Statements
– Do – While
• Control Statements
- Switch

While Vs Do While

The do-while Repetition
Structure
do
{
statement(s)
} while ( condition ) ;
• The body of a do-while is ALWAYS
executed at least once. Is this true of a
while loop? What about a for loop?Faculty of Information Technology,

Example
do
{
num = prompt("Enter a positive number: ");
num = parseInt(num);
if (num <= 0)
{
alert("That is not positive. Try again.");
}
}while (num <= 0);

An Equivalent while Loop
while ( num <= 0 )
{
alert("That is not positive. Try again.");
}
• Notice that using a while loop in this case requires a priming
read.

Switch Case

The Switch Statement
• The switch statement
provides another way to
decide which statement to
execute next
• The switch statement
evaluates an expression,
then attempts to match the
result to one of several
possible cases
• The match must be an exact
match.
switch ( expression ){
case value1 :
statement-list1
case value2 :
statement-list2
case value3 :
statement-list3
case ...
}

• Each case
contains a value
and a list of
statements
• The flow of control
transfers to
statement
associated with
the first case value
that matches
case value1 :
statement-list1
case value2 :
statement-list2
case value3 :
statement-list3
case ...
}

Switch - syntax
• The general syntax of a switch statement is:
case value1 :
statement-list1
case value2 :
statement-list2
case value3 :
statement-list3
case ...
}
switch
and
case
are
reserved
words
If expression
matches value3,
control jumps
to hereFaculty of Information Technology,

• The break statement can
be used as the last
statement in each case's
statement list
• A break statement
causes control to transfer
to the end of the switch
statement
• If a break statement is
not used, the flow of
control will continue into
the next case
case value1 :
statement-list1
break;
case value2 :
statement-list2
break;
case value3 :
statement-list3
break;
case ...
}

Switch Example
switch (option){
case 'A':
aCount++;
break;
case 'B':
bCount++;
break;
case 'C':
cCount++;
break;
}
• Examples of the switch statement:

Switch – no breaks!!!
switch (option){
case 'A':
aCount++;
case 'B':
bCount++;
case 'C':
cCount++;
}
• Another Example:
switch (option){
case 'A':
aCount++;
break;
case 'B':
bCount++;
break;
case 'C':
cCount++;
break;
}

Switch - default
• A switch statement can have an optional
default case
• The default case has no associated value and
simply uses the reserved word default
• If the default case is present, control will transfer
to it if no other case value matches
• If there is no default case, and no other value
matches, control falls through to the statement
after the switch

The switch Statement
switch (option){
case 'A':
aCount++;
break;
case 'B':
bCount++;
break;
case 'C':
cCount++;
break;
default:
otherCount++;
break;
}
• Switch
with
default
case:

To Switch or not to Switch
• The expression of a switch statement must result in an
integral type, meaning an integer (byte, short, int,
long) or a char
• It cannot be a boolean value or a floating point value
(float or double)
• The implicit boolean condition in a switch statement is
equality
• You cannot perform relational checks with a switch
statement

DSA 103 Object Oriented Programming :: Week 4

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