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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