Chapter 3:  Program Statements

	Presentation slides for
	Java Software Solutions
	Foundations of Program Design
	Third Edition
	by John Lewis and William Loftus
	Java Software Solutions is published by Addison-Wesley
	Presentation slides are copyright 2002 by John Lewis and William Loftus. All rights reserved.
	Instructors using the textbook may use and modify these slides for pedagogical purposes.

Program Statements

	Now we will examine some other program statements
	Chapter 3 focuses on:
		program development stages
		the flow of control through a method
		decision-making statements
		expressions for making complex decisions
		repetition statements
		drawing with conditionals and loops

Program Development

	The creation of software involves four basic activities:
		establishing the requirements
		creating a design
		implementing the code
		testing the implementation
	The development process is much more involved than this, but these are the four basic development activities

Requirements

	Software requirements specify the tasks a program must accomplish (what to do, not how to do it)
	They often include a description of the user interface
	An initial set of requirements often are provided, but usually must be critiqued, modified, and expanded
	Often it is difficult to establish detailed, unambiguous, complete requirements
	Careful attention to the requirements can save significant time and expense in the overall project

Design

	A software design specifies how a program will accomplish its requirements
	A design includes one or more algorithms to accomplish its goal
	An algorithm is a step-by-step process for solving a problem
	An algorithm may be expressed in pseudocode, which is code-like, but does not necessarily follow any specific syntax
	In object-oriented development, the design establishes the classes, objects,  methods, and data that are required

Implementation

	Implementation is the process of translating a design into source code
	Most novice programmers think that writing code is the heart of software development, but actually it should be the least creative step
	Almost all important decisions are made during requirements and design stages
	Implementation should focus on coding details, including style guidelines and documentation

Testing

	A program should be executed multiple times with various input in an attempt to find errors
	Debugging is the process of discovering the causes of  problems and fixing them
	Programmers often think erroneously that there is "only one more bug" to fix
	Tests should consider design details as well as overall requirements

Flow of Control

	Unless specified otherwise, the order of statement execution through a method is linear: one statement after the other in sequence
	Some programming statements modify that order, allowing us to:
		decide whether or not to execute a particular statement, or
		perform a statement over and over, repetitively
	These decisions are based on a boolean expression (also called a condition) that evaluates to true or false
	The order of statement execution is called the flow of control

Conditional Statements

	A conditional statement lets us choose which statement will be executed next
	Therefore they are sometimes called selection statements
	Conditional statements give us the power to make basic decisions
	Java's conditional statements are
		the if statement
		the if-else statement
		the switch statement

The if Statement

	The if statement has the following syntax:

The if Statement

	An example of an if statement:

Logic of an if statement

Boolean Expressions

	A condition often uses one of Java's equality operators or relational operators, which all return boolean results:
				== equal to
				!= not equal to
				< less than
				> greater than
				<= less than or equal to
				>= greater than or equal to
	Note the difference between the equality operator (==) and the assignment operator (=)

The if-else Statement

	An else clause can be added to an if statement to make an if-else statement

Logic of an if-else statement

Block Statements

	Several statements can be grouped together into a block statement
	A block is delimited by braces :  { … }
	A block statement can be used wherever a statement is called for by the Java syntax
	For example, in an if-else statement, the if portion, or the else portion, or both, could be block statements
	See Guessing.java (page 141)

Nested if Statements

	The statement executed as a result of an if statement or else clause could be another if statement
	These are called nested if statements
	See MinOfThree.java (page 143)
	An else clause is matched to the last unmatched if (no matter what the indentation implies)
	Braces can be used to specify the if statement to which an else clause belongs

The switch Statement

	The switch statement provides another means 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
	Each case contains a value and a list of statements
	The flow of control transfers to statement associated with the first value that matches

The switch Statement

	The general syntax of a switch statement is:

The switch Statement

	Often a break statement is 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
	Sometimes this can be appropriate, but usually we want to execute only the statements associated with one case

The switch Statement

	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
	Though the default case can be positioned anywhere in the switch, usually it is placed at the end
	If there is no default case, and no other value matches, control falls through to the statement after the switch

The switch Statement

	The expression of a switch statement must result in an integral type, meaning an int or a char
	It cannot be a boolean value, a floating point value (float or double), a byte, a short, or a long
	The implicit boolean condition in a switch statement is equality - it tries to match the expression with a value
	You cannot perform relational checks with a switch statement
	See GradeReport.java (page 147)

Logical Operators

	Boolean expressions can use the following logical operators:
		! Logical NOT
		&& Logical AND
		|| Logical OR
	They all take boolean operands and produce boolean results
	Logical NOT is a unary operator (it operates on one operand)
	Logical AND and logical OR are binary operators (each operates on two operands)

Logical NOT

	The logical NOT operation is also called logical negation or logical complement
	If some boolean condition a is true, then !a is false;  if a is false, then !a is true
	Logical expressions can be shown using truth tables

Logical AND and Logical OR

	The logical AND expression
	a && b
	is true if both a and b are true, and false otherwise
	The logical OR expression
	a || b
	is true if a or b or both are true, and false otherwise

Truth Tables

	A truth table shows the possible true/false combinations of the terms
	Since && and || each have two operands, there are four possible combinations of conditions a and b

Logical Operators

	Conditions can use logical operators to form complex expressions

Short Circuited Operators

	The processing of logical AND and logical OR is “short-circuited”
	If the left operand is sufficient to determine the result, the right operand is not evaluated

Truth Tables

	Specific expressions can be evaluated using truth tables

Comparing Characters

	We can use the relational operators on character data
	The results are based on the Unicode character set
	The following condition is true because the character + comes before the character J in the Unicode character set:

Comparing Strings

	Remember that a character string in Java is an object
	We cannot use the relational operators to compare strings
	The equals method can be called with strings to determine if two strings contain exactly the same characters in the same order
	The String class also contains a method called compareTo to determine if one string comes before another (based on the Unicode character set)

Lexicographic Ordering

	Because comparing characters and strings is based on a character set, it is called a lexicographic ordering
	This is not strictly alphabetical when uppercase and lowercase characters are mixed
	For example, the string "Great" comes before the string "fantastic" because all of the uppercase letters come before all of the lowercase letters in Unicode
	Also, short strings come before longer strings with the same prefix (lexicographically)
	Therefore "book" comes before "bookcase"

Comparing Float Values

	We also have to be careful when comparing two floating point values (float or double) for equality
	You should rarely use the equality operator (==) when comparing two floats
	In many situations, you might consider two floating point numbers to be "close enough" even if they aren't exactly equal
	Therefore, to determine the equality of two floats, you may want to use the following technique:

More Operators

	To round out our knowledge of Java operators, let's examine a few more
	In particular, we will examine
		the increment and decrement operators
		the assignment operators
		the conditional operator

Increment and Decrement

	The increment and decrement operators are arithmetic and operate on one operand
	The increment operator (++) adds one to its operand
	The decrement operator (--) subtracts one from its operand
	The statement
	count++;
	is functionally equivalent to
	count = count + 1;

Increment and Decrement

	The increment and decrement operators can be applied in prefix form (before the operand) or postfix form (after the operand)
	When used alone in a statement, the prefix and postfix forms are functionally equivalent.  That is,
	count++;
	is equivalent to
	++count;

Increment and Decrement

	When used in a larger expression, the prefix and postfix forms have different effects
	In both cases the variable is incremented (decremented)
	But the value used in the larger expression depends on the form used:

Increment and Decrement

	If count currently contains 45, then the statement
	total = count++;
	assigns 45 to total and 46 to count
	If count currently contains 45, then the statement
	total = ++count;
	assigns the value 46 to both total and count

Assignment Operators

	Often we perform an operation on a variable, and then store the result back into that variable
	Java provides assignment operators to simplify that process
	For example, the statement
	num += count;
	is equivalent to
	num = num + count;

Assignment Operators

	There are many assignment operators, including the following:

Assignment Operators

	The right hand side of an assignment operator can be a complex expression
	The entire right-hand expression is evaluated first, then the result is combined with the original variable
	Therefore
	result /= (total-MIN) % num;
	is equivalent to
	result = result / ((total-MIN) % num);

Assignment Operators

	The behavior of some assignment operators depends on the types of the operands
	If the operands to the += operator are strings, the assignment operator performs string concatenation
	The behavior of an assignment operator (+=) is always consistent with the behavior of the "regular" operator (+)

The Conditional Operator

	Java has a conditional operator that evaluates a boolean condition that determines which of two other expressions is evaluated
	The result of the chosen expression is the result of the entire conditional operator
	Its syntax is:
	condition ? expression1 : expression2
	If the condition is true, expression1 is evaluated;  if it is false, expression2 is evaluated

The Conditional Operator

	The conditional operator is similar to an if-else statement, except that it forms an expression that returns a value
	For example:
	larger = ((num1 > num2) ? num1 : num2);
	If num1 is greater that num2, then num1 is assigned to larger;  otherwise, num2 is assigned to larger
	The conditional operator is ternary because it requires three operands

The Conditional Operator

	Another example:

Repetition Statements

	Repetition statements allow us to execute a statement multiple times
	Often they are referred to as loops
	Like conditional statements, they are controlled by boolean expressions
	Java has three kinds of repetition statements:
		the while loop
		the do loop
		the for loop
	The programmer should choose the right kind of loop for the situation

The while Statement

	The while statement has the following syntax:

Logic of a while Loop

The while Statement

	Note that if the condition of a while statement is false initially, the statement is never executed
	Therefore, the body of a while loop will execute zero or more times
	See Counter.java (page 159)
	See Average.java (page 161)
		A sentinel value indicates the end of the input
		The variable sum maintains a running sum
	See WinPercentage.java (page 163)
		A loop is used to validate the input, making the program more robust

Infinite Loops

	The body of a while loop eventually must make the condition false
	If not, it is an infinite loop, which will execute until the user interrupts the program
	This is a common logical error
	You should always double check to ensure that your loops will terminate normally
	See Forever.java (page 165)

Nested Loops

	Similar to nested if statements, loops can be nested as well
	That is, the body of a loop can contain another loop
	Each time through the outer loop, the inner loop goes through its full set of iterations
	See PalindromeTester.java (page 167)

The StringTokenizer Class

	The elements that comprise a string are referred to as tokens
	The process of extracting these elements is called tokenizing
	Characters that separate one token from another are called delimiters
	The StringTokenizer class, which is defined in the java.util package, is used to separate a string into tokens

The StringTokenizer Class

	The default delimiters are space, tab, carriage return, and the new line characters
	The nextToken method returns the next token (substring) from the string
	The hasMoreTokens returns a boolean indicating if there are more tokens to process
	See CountWords.java (page 172)

The do Statement

	The do statement has the following syntax:

Logic of a do Loop

The do Statement

	A do loop is similar to a while loop, except that the condition is evaluated after the body of the loop is executed
	Therefore the body of a do loop will execute at least once
	See Counter2.java (page 175)
	See ReverseNumber.java (page 176)

Comparing while and do

The for Statement

	The for statement has the following syntax:

The for Statement

	A for loop is functionally equivalent to the following while loop structure:

Logic of a for loop

The for Statement

	Like a while loop, the condition of a for statement is tested prior to executing the loop body
	Therefore, the body of a for loop will execute zero or more times
	It is well suited for executing a loop a specific number of times that can be determined in advance
	See Counter3.java (page 178)
	See Multiples.java (page 180)
	See Stars.java (page 182)

The for Statement

	Each expression in the header of a for loop is optional
		If the initialization is left out, no initialization is performed
		If the condition is left out, it is always considered to be true, and therefore creates an infinite loop
		If the increment is left out, no increment operation is performed
	Both semi-colons are always required in the for loop header

Choosing a Loop Structure

	When you can’t determine how many times you want to execute the loop body, use a while statement or a do statement
		If it might be zero or more times, use a while statement
		If it will be at least once, use a do statement
	If you can determine how many times you want to execute the loop body, use a for statement

Program Development

	We now have several additional statements and operators at our disposal
	Following proper development steps is important
	Suppose you were given some initial requirements:
		accept a series of test scores
		compute the average test score
		determine the highest and lowest test scores
		display the average, highest, and lowest test scores

Program Development

	Requirements Analysis – clarify and flesh out specific requirements
		How much data will there be?
		How should data be accepted?
		Is there a specific output format required?
	After conferring with the client, we determine:
		the program must process an arbitrary number of test scores
		the program should accept input interactively
		the average should be presented to two decimal places
	The process of requirements analysis may take a long time

Program Development

	Design – determine a possible general solution
		Input strategy? (Sentinel value?)
		Calculations needed?
	An initial algorithm might be expressed in pseudocode
	Multiple versions of the solution might be needed to refine it
	Alternatives to the solution should be carefully considered

Program Development

	Implementation – translate the design into source code
	Make sure to follow coding and style guidelines
	Implementation should be integrated with compiling and testing your solution
	This process mirrors a more complex development model we'll eventually need to develop more complex software
	The result is a final implementation
	See ExamScores.java (page 186)

Program Development

	Testing – attempt to find errors that may exist in your programmed solution
	Compare your code to the design and resolve any discrepancies
	Determine test cases that will stress the limits and boundaries of your solution
	Carefully retest after finding and fixing an error

More Drawing Techniques

	Conditionals and loops can greatly enhance our ability to control graphics
	See Bullseye.java (page 189)
	See Boxes.java (page 191)
	See BarHeights.java (page 193)

Summary

	Chapter 3 has focused on:
		program development stages
		the flow of control through a method
		decision-making statements
		expressions for making complex decisions
		repetition statements
		drawing with conditionals and loops