3a	Advanced Placement Computer Science Lab

Control Structures - Conditional Statements

Introduction

Computer programs are executed sequentially (i.e., statement by statement) unless programmed to do otherwise. To get a program to execute in a non-sequential manner, programmers insert control structures which move the point of execution to another part of the program. Control structures include conditional statements, loops, and the execution and return statement of methods. We will go into greater detail with respect to loops and methods in future lab activities.

Conditional statements are of three types, the if statement, the if…else statement and the switch statement. The syntax for each is as follows:

if (boolean expression);

{

statements

}//end if

if (boolean expression);

{

statements

}//end if

else

{

statements

}//end else

switch (variable of type int, char byte or short)

{

case variable of type int, char byte or short:

{statements}

break;

case variable of type int, char byte or short:

{statements}

break;

case variable of type int, char byte or short:

{statements}

break;

. . . .

case variable of type int, char byte or short:

{statements}

break;

}//end switch

In the case of a conditional statement, the statements immediately following the if are executed immediately if the condition or Boolean expression is evaluated to be true. A Boolean expression is two values or variables separated by a relational operator. The Boolean expression is enclosed in parentheses. The relational operators, their meaning and examples of use are in the following table:

Operator	Meaning	Example
==	equal to	if (y == 4)
!=	not equal to	if (age != 65)
>	greater than	if (salary > 100000)
<	less than	if (grade < 90)
>=	greater than or equal to	if (height >= 60)
<=	less than or equal to	if (pay <= 5)

Boolean expressions can also be combined with logical operators so that multiple statements are tested. The logical operators are listed in the following table:

Operator	Logical Meaning	Example
&&	and	if (y == 4) && (a < 3)
!	not	if (age != 65) ! (hair != gray)
||	or	if (salary > 10) || (salary < 100)

Conditional statements can also be nested. This means that the statements being executed after an if statement can also contain additional if statements

if (boolean expression);

{

if (boolean expression);

{

statements

}//end if

}//end if

else

{

if (boolean expression);

{

statements

}//end if

}//end else

Notice how in nesting statements that are grouped together in a block, that the curly braces associated with the block have the same level of indentation. The statements within the block should also have the same level of indentation. The reason for this is that it makes the code easier for someone to examine or review.

Procedure

GUI’s or Graphical User Interfaces

1. Up until now we have been using the Keyboard class for inputting data. The methods of the Keyboard class allow the computer to parse the input and assign it to the proper primitive data type. However, Java has a set of dialog boxes that have been constructed (i.e programmed) within the “swing” class the permit a much better looking way to input data. To use the dialog boxes you have to import the Java swing class methods at the beginning of your program. Since the input data type is a string, you will have to convert the string input to a data type of int.

import javax.swing.*;

.

.

.

int x;

String input; //input string from dialog box

String output; //output string from computations

.

.

input = JOptionPane.showInputDialog( prompt string );

x = Integer.parseInt(input);

.

.

JOptionPane.showMessageDialog (null, output);

2. Write a short program that uses the above interfaces to input and output an integer. Use the string concatenation ability of Java to construct the output string, e.g.:

output = output + “The number is: “ + x;

Programming Assignment

Using the Internet find a formula for calculating the date of the Christian holiday Easter in any given year. Note that Easter can be in March or April, so your program will have to distinguish between whether Easter should be in March or April. Your program should also calculate the day for Ash Wednesday (46 days before Easter) and Ascension Day (39 days after Easter) You should make a new project. Use the GUI interfaces discussed above for your input and output. Turn in a program listing your Easter Date source code. Source code should use proper formatting. Also turn in a copy of your output. Output should be test with the years 1960, 1985 and 2003. This can be obtained by running the program and then using AltPrintScreen to copy the window of the output onto the clipboard. Then paste the contents of the clipboard into WordPad and print out the document.