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Chapter 5 Conditionals and Loops Java Software Solutions Foundations of Program Design Seventh Edition John Lewis William Loftus Copyright © 2012 Pearson Education, Inc. Conditionals and Loops • Now we will examine programming statements that allow us to: – make decisions – repeat processing steps in a loop • Chapter 5 focuses on: – – – – – – – boolean expressions the if and if-else statements comparing data while loops iterators more drawing techniques more GUI components Copyright © 2012 Pearson Education, Inc. Outline Boolean Expressions The if Statement Comparing Data The while Statement Iterators The ArrayList Class Determining Event Sources Check Boxes and Radio Buttons Copyright © 2012 Pearson Education, Inc. Flow of Control • Unless specified otherwise, the order of statement execution through a method is linear: one after another • Some programming statements allow us to make decisions and perform repetitions • These decisions are based on boolean expressions (also called conditions) that evaluate to true or false • The order of statement execution is called the flow of control Copyright © 2012 Pearson Education, Inc. Conditional Statements • A conditional statement lets us choose which statement will be executed next • They are sometimes called selection statements • Conditional statements give us the power to make basic decisions • The Java conditional statements are the: – if and if-else statement – switch statement • We'll explore the switch statement in Chapter 6 Copyright © 2012 Pearson Education, Inc. 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 (=) Copyright © 2012 Pearson Education, Inc. Boolean Expressions • An if statement with its boolean condition: if (sum > MAX) delta = sum – MAX; • First, the condition is evaluated: the value of sum is either greater than the value of MAX, or it is not • If the condition is true, the assignment statement is executed; if it isn't, it is skipped • See Age.java Copyright © 2012 Pearson Education, Inc. //******************************************************************** // Age.java Author: Lewis/Loftus // // Demonstrates the use of an if statement. //******************************************************************** import java.util.Scanner; public class Age { //----------------------------------------------------------------// Reads the user's age and prints comments accordingly. //----------------------------------------------------------------public static void main (String[] args) { final int MINOR = 21; Scanner scan = new Scanner (System.in); System.out.print ("Enter your age: "); int age = scan.nextInt(); continue Copyright © 2012 Pearson Education, Inc. continue System.out.println ("You entered: " + age); if (age < MINOR) System.out.println ("Youth is a wonderful thing. Enjoy."); System.out.println ("Age is a state of mind."); } } Copyright © 2012 Pearson Education, Inc. Sample Run continue Enter your age: 47 You entered: 47 Age is a state of mind. System.out.println ("You entered: " + age); if (age < MINOR) System.out.println ("Youth is a wonderful thing. Enjoy."); System.out.println ("Age is a state of mind."); } } Another Sample Run Enter your age: 12 You entered: 12 Youth is a wonderful thing. Enjoy. Age is a state of mind. Copyright © 2012 Pearson Education, Inc. Logical Operators • Boolean expressions can also 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) Copyright © 2012 Pearson Education, Inc. 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 a truth table: a true false !a false true Copyright © 2012 Pearson Education, Inc. 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 Copyright © 2012 Pearson Education, Inc. Logical AND and Logical OR • A truth table shows all possible true-false combinations of the terms • Since && and || each have two operands, there are four possible combinations of conditions a and b a true true false false b true false true false a && b true false false false a || b true true true false Copyright © 2012 Pearson Education, Inc. Logical Operators • Expressions that use logical operators can form complex conditions if (total < MAX+5 && !found) System.out.println ("Processing…"); • All logical operators have lower precedence than the relational operators • The ! operator has higher precedence than && and || Copyright © 2012 Pearson Education, Inc. Boolean Expressions • Specific expressions can be evaluated using truth tables total < MAX found !found total < MAX && !found false false true true false true false true true false true false false false true false Copyright © 2012 Pearson Education, Inc. Short-Circuited Operators • The processing of && and || is “short-circuited” • If the left operand is sufficient to determine the result, the right operand is not evaluated if (count != 0 && total/count > MAX) System.out.println ("Testing."); • This type of processing should be used carefully Copyright © 2012 Pearson Education, Inc. Outline Boolean Expressions The if Statement Comparing Data The while Statement Iterators The ArrayList Class Determining Event Sources Check Boxes and Radio Buttons Copyright © 2012 Pearson Education, Inc. The if Statement • Let's now look at the if statement in more detail • The if statement has the following syntax: if is a Java reserved word The condition must be a boolean expression. It must evaluate to either true or false. if ( condition ) statement; If the condition is true, the statement is executed. If it is false, the statement is skipped. Copyright © 2012 Pearson Education, Inc. Logic of an if statement condition evaluated true false statement Copyright © 2012 Pearson Education, Inc. Indentation • The statement controlled by the if statement is indented to indicate that relationship • The use of a consistent indentation style makes a program easier to read and understand • The compiler ignores indentation, which can lead to errors if the indentation is not correct "Always code as if the person who ends up maintaining your code will be a violent psychopath who knows where you live." -- Martin Golding Copyright © 2012 Pearson Education, Inc. Quick Check What do the following statements do? if (total != stock + warehouse) inventoryError = true; if (found || !done) System.out.println("Ok"); Copyright © 2012 Pearson Education, Inc. Quick Check What do the following statements do? if (total != stock + warehouse) inventoryError = true; Sets the boolean variable to true if the value of total is not equal to the sum of stock and warehouse if (found || !done) System.out.println("Ok"); Prints "Ok" if found is true or done is false Copyright © 2012 Pearson Education, Inc. The if-else Statement • An else clause can be added to an if statement to make an if-else statement if ( condition ) statement1; else statement2; • If the condition is true, statement1 is executed; if the condition is false, statement2 is executed • One or the other will be executed, but not both • See Wages.java Copyright © 2012 Pearson Education, Inc. //******************************************************************** // Wages.java Author: Lewis/Loftus // // Demonstrates the use of an if-else statement. //******************************************************************** import java.text.NumberFormat; import java.util.Scanner; public class Wages { //----------------------------------------------------------------// Reads the number of hours worked and calculates wages. //----------------------------------------------------------------public static void main (String[] args) { final double RATE = 8.25; // regular pay rate final int STANDARD = 40; // standard hours in a work week Scanner scan = new Scanner (System.in); double pay = 0.0; continue Copyright © 2012 Pearson Education, Inc. continue System.out.print ("Enter the number of hours worked: "); int hours = scan.nextInt(); System.out.println (); // Pay overtime at "time and a half" if (hours > STANDARD) pay = STANDARD * RATE + (hours-STANDARD) * (RATE * 1.5); else pay = hours * RATE; NumberFormat fmt = NumberFormat.getCurrencyInstance(); System.out.println ("Gross earnings: " + fmt.format(pay)); } } Copyright © 2012 Pearson Education, Inc. continue Sample Run System.out.print ("Enter the of number of worked: hours worked: Enter the number hours 46 "); int hours = scan.nextInt(); Gross earnings: $404.25 System.out.println (); // Pay overtime at "time and a half" if (hours > STANDARD) pay = STANDARD * RATE + (hours-STANDARD) * (RATE * 1.5); else pay = hours * RATE; NumberFormat fmt = NumberFormat.getCurrencyInstance(); System.out.println ("Gross earnings: " + fmt.format(pay)); } } Copyright © 2012 Pearson Education, Inc. Logic of an if-else statement condition evaluated true statement1 false statement2 Copyright © 2012 Pearson Education, Inc. The Coin Class • Let's look at an example that uses a class that represents a coin that can be flipped • Instance data is used to indicate which face (heads or tails) is currently showing • See CoinFlip.java • See Coin.java Copyright © 2012 Pearson Education, Inc. //******************************************************************** // CoinFlip.java Author: Lewis/Loftus // // Demonstrates the use of an if-else statement. //******************************************************************** public class CoinFlip { //----------------------------------------------------------------// Creates a Coin object, flips it, and prints the results. //----------------------------------------------------------------public static void main (String[] args) { Coin myCoin = new Coin(); myCoin.flip(); System.out.println (myCoin); if (myCoin.isHeads()) System.out.println ("You win."); else System.out.println ("Better luck next time."); } } Copyright © 2012 Pearson Education, Inc. Sample Run //******************************************************************** // CoinFlip.java Author: Lewis/Loftus Tails // // Demonstrates the use of anluck if-else Better nextstatement. time. //******************************************************************** public class CoinFlip { //----------------------------------------------------------------// Creates a Coin object, flips it, and prints the results. //----------------------------------------------------------------public static void main (String[] args) { Coin myCoin = new Coin(); myCoin.flip(); System.out.println (myCoin); if (myCoin.isHeads()) System.out.println ("You win."); else System.out.println ("Better luck next time."); } } Copyright © 2012 Pearson Education, Inc. //******************************************************************** // Coin.java Author: Lewis/Loftus // // Represents a coin with two sides that can be flipped. //******************************************************************** public class Coin { private final int HEADS = 0; private final int TAILS = 1; private int face; //----------------------------------------------------------------// Sets up the coin by flipping it initially. //----------------------------------------------------------------public Coin () { flip(); } continue Copyright © 2012 Pearson Education, Inc. continue //----------------------------------------------------------------// Flips the coin by randomly choosing a face value. //----------------------------------------------------------------public void flip () { face = (int) (Math.random() * 2); } //----------------------------------------------------------------// Returns true if the current face of the coin is heads. //----------------------------------------------------------------public boolean isHeads () { return (face == HEADS); } continue Copyright © 2012 Pearson Education, Inc. continue //----------------------------------------------------------------// Returns the current face of the coin as a string. //----------------------------------------------------------------public String toString() { String faceName; if (face == HEADS) faceName = "Heads"; else faceName = "Tails"; return faceName; } } Copyright © 2012 Pearson Education, Inc. Indentation Revisited • Remember that indentation is for the human reader, and is ignored by the compiler if (depth >= UPPER_LIMIT) delta = 100; else System.out.println("Reseting Delta"); delta = 0; • Despite what the indentation implies, delta will be set to 0 no matter what Copyright © 2012 Pearson Education, Inc. Block Statements • Several statements can be grouped together into a block statement delimited by braces • A block statement can be used wherever a statement is called for in the Java syntax rules if (total > MAX) { System.out.println ("Error!!"); errorCount++; } Copyright © 2012 Pearson Education, Inc. Block Statements • The if clause, or the else clause, or both, could govern block statements if (total > MAX) { System.out.println ("Error!!"); errorCount++; } else { System.out.println ("Total: " + total); current = total*2; } • See Guessing.java Copyright © 2012 Pearson Education, Inc. //******************************************************************** // Guessing.java Author: Lewis/Loftus // // Demonstrates the use of a block statement in an if-else. //******************************************************************** import java.util.*; public class Guessing { //----------------------------------------------------------------// Plays a simple guessing game with the user. //----------------------------------------------------------------public static void main (String[] args) { final int MAX = 10; int answer, guess; Scanner scan = new Scanner (System.in); Random generator = new Random(); answer = generator.nextInt(MAX) + 1; continue Copyright © 2012 Pearson Education, Inc. continue System.out.print ("I'm thinking of a number between 1 and " + MAX + ". Guess what it is: "); guess = scan.nextInt(); if (guess == answer) System.out.println ("You got it! Good guessing!"); else { System.out.println ("That is not correct, sorry."); System.out.println ("The number was " + answer); } } } Copyright © 2012 Pearson Education, Inc. continue Sample Run System.out.print ("I'mbetween thinking1ofand a number between 1 and I'm thinking of a number 10. Guess what it "is: 6 + MAX + ". Guess what it is: "); That is not correct, sorry. The number was 9 guess = scan.nextInt(); if (guess == answer) System.out.println ("You got it! Good guessing!"); else { System.out.println ("That is not correct, sorry."); System.out.println ("The number was " + answer); } } } Copyright © 2012 Pearson Education, Inc. Nested if Statements • The statement executed as a result of an if or else clause could be another if statement • These are called nested if statements • 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 • See MinOfThree.java Copyright © 2012 Pearson Education, Inc. //******************************************************************** // MinOfThree.java Author: Lewis/Loftus // // Demonstrates the use of nested if statements. //******************************************************************** import java.util.Scanner; public class MinOfThree { //----------------------------------------------------------------// Reads three integers from the user and determines the smallest // value. //----------------------------------------------------------------public static void main (String[] args) { int num1, num2, num3, min = 0; Scanner scan = new Scanner (System.in); System.out.println ("Enter three integers: "); num1 = scan.nextInt(); num2 = scan.nextInt(); num3 = scan.nextInt(); continue Copyright © 2012 Pearson Education, Inc. continue if (num1 < num2) if (num1 < num3) min = num1; else min = num3; else if (num2 < num3) min = num2; else min = num3; System.out.println ("Minimum value: " + min); } } Copyright © 2012 Pearson Education, Inc. continue if (num1 < num2) if (num1 < num3) min = num1; else min = num3; else if (num2 < num3) min = num2; else min = num3; Sample Run Enter three integers: 84 69 90 Minimum value: 69 System.out.println ("Minimum value: " + min); } } Copyright © 2012 Pearson Education, Inc. Outline Boolean Expressions The if Statement Comparing Data The while Statement Iterators The ArrayList Class Determining Event Sources Check Boxes and Radio Buttons Copyright © 2012 Pearson Education, Inc. Comparing Data • When comparing data using boolean expressions, it's important to understand the nuances of certain data types • Let's examine some key situations: – – – – Comparing floating point values for equality Comparing characters Comparing strings (alphabetical order) Comparing object vs. comparing object references Copyright © 2012 Pearson Education, Inc. Comparing Float Values • You should rarely use the equality operator (==) when comparing two floating point values (float or double) • Two floating point values are equal only if their underlying binary representations match exactly • Computations often result in slight differences that may be irrelevant • In many situations, you might consider two floating point numbers to be "close enough" even if they aren't exactly equal Copyright © 2012 Pearson Education, Inc. Comparing Float Values • To determine the equality of two floats, use the following technique: if (Math.abs(f1 - f2) < TOLERANCE) System.out.println ("Essentially equal"); • If the difference between the two floating point values is less than the tolerance, they are considered to be equal • The tolerance could be set to any appropriate level, such as 0.000001 Copyright © 2012 Pearson Education, Inc. Comparing Characters • As we've discussed, Java character data is based on the Unicode character set • Unicode establishes a particular numeric value for each character, and therefore an ordering • We can use relational operators on character data based on this ordering • For example, the character '+' is less than the character 'J' because it comes before it in the Unicode character set • Appendix C provides an overview of Unicode Copyright © 2012 Pearson Education, Inc. Comparing Characters • In Unicode, the digit characters (0-9) are contiguous and in order • Likewise, the uppercase letters (A-Z) and lowercase letters (a-z) are contiguous and in order Characters 0–9 A–Z a–z Unicode Values 48 through 57 65 through 90 97 through 122 Copyright © 2012 Pearson Education, Inc. Comparing Strings • Remember that in Java a character string is an object • The equals method can be called with strings to determine if two strings contain exactly the same characters in the same order • The equals method returns a boolean result if (name1.equals(name2)) System.out.println ("Same name"); Copyright © 2012 Pearson Education, Inc. Comparing Strings • We cannot use the relational operators to compare strings • The String class contains the compareTo method for determining if one string comes before another • A call to name1.compareTo(name2) – returns zero if name1 and name2 are equal (contain the same characters) – returns a negative value if name1 is less than name2 – returns a positive value if name1 is greater than name2 Copyright © 2012 Pearson Education, Inc. Comparing Strings • Because comparing characters and strings is based on a character set, it is called a lexicographic ordering int result = name1.comareTo(name2); if (result < 0) System.out.println (name1 + "comes first"); else if (result == 0) System.out.println ("Same name"); else System.out.println (name2 + "comes first"); Copyright © 2012 Pearson Education, Inc. Lexicographic Ordering • Lexicographic ordering 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" Copyright © 2012 Pearson Education, Inc. Comparing Objects • The == operator can be applied to objects – it returns true if the two references are aliases of each other • The equals method is defined for all objects, but unless we redefine it when we write a class, it has the same semantics as the == operator • It has been redefined in the String class to compare the characters in the two strings • When you write a class, you can redefine the equals method to return true under whatever conditions are appropriate Copyright © 2012 Pearson Education, Inc. Outline Boolean Expressions The if Statement Comparing Data The while Statement Iterators The ArrayList Class Determining Event Sources Check Boxes and Radio Buttons Copyright © 2012 Pearson Education, Inc. 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: while, do, and for loops • The do and for loops are discussed in Chapter 6 Copyright © 2012 Pearson Education, Inc. The while Statement • A while statement has the following syntax: while ( condition ) statement; • If the condition is true, the statement is executed • Then the condition is evaluated again, and if it is still true, the statement is executed again • The statement is executed repeatedly until the condition becomes false Copyright © 2012 Pearson Education, Inc. Logic of a while Loop condition evaluated true false statement Copyright © 2012 Pearson Education, Inc. The while Statement • An example of a while statement: int count = 1; while (count <= 5) { System.out.println (count); count++; } • If the condition of a while loop is false initially, the statement is never executed • Therefore, the body of a while loop will execute zero or more times Copyright © 2012 Pearson Education, Inc. Sentinel Values • Let's look at some examples of loop processing • A loop can be used to maintain a running sum • A sentinel value is a special input value that represents the end of input • See Average.java Copyright © 2012 Pearson Education, Inc. //******************************************************************** // Average.java Author: Lewis/Loftus // // Demonstrates the use of a while loop, a sentinel value, and a // running sum. //******************************************************************** import java.text.DecimalFormat; import java.util.Scanner; public class Average { //----------------------------------------------------------------// Computes the average of a set of values entered by the user. // The running sum is printed as the numbers are entered. //----------------------------------------------------------------public static void main (String[] args) { int sum = 0, value, count = 0; double average; Scanner scan = new Scanner (System.in); System.out.print ("Enter an integer (0 to quit): "); value = scan.nextInt(); continue Copyright © 2012 Pearson Education, Inc. continue while (value != 0) { count++; // sentinel value of 0 to terminate loop sum += value; System.out.println ("The sum so far is " + sum); System.out.print ("Enter an integer (0 to quit): "); value = scan.nextInt(); } continue Copyright © 2012 Pearson Education, Inc. continue System.out.println (); if (count == 0) System.out.println ("No values were entered."); else { average = (double)sum / count; DecimalFormat fmt = new DecimalFormat ("0.###"); System.out.println ("The average is " + fmt.format(average)); } } } Copyright © 2012 Pearson Education, Inc. continue Sample Run Enter an integer (0 to quit): 25 The sum so far is 25 an integer (0 to quit): 164 if (count ==Enter 0) The sum so("No farvalues is 189 System.out.println were entered."); else Enter an integer (0 to quit): -14 { The sum so far is 175 average =Enter (double)sum / count; an integer (0 to quit): 84 The sum so far is 259 DecimalFormat fmt = new DecimalFormat ("0.###"); Enter an integer (0 toisquit): 12 System.out.println ("The average " + fmt.format(average)); The sum so far is 271 } Enter an integer (0 to quit): -35 The sum so far is 236 Enter an integer (0 to quit): 0 System.out.println (); } } The average is 39.333 Copyright © 2012 Pearson Education, Inc. Input Validation • A loop can also be used for input validation, making a program more robust • It's generally a good idea to verify that input is valid (in whatever sense) when possible • See WinPercentage.java Copyright © 2012 Pearson Education, Inc. //******************************************************************** // WinPercentage.java Author: Lewis/Loftus // // Demonstrates the use of a while loop for input validation. //******************************************************************** import java.text.NumberFormat; import java.util.Scanner; public class WinPercentage { //----------------------------------------------------------------// Computes the percentage of games won by a team. //----------------------------------------------------------------public static void main (String[] args) { final int NUM_GAMES = 12; int won; double ratio; Scanner scan = new Scanner (System.in); System.out.print ("Enter the number of games won (0 to " + NUM_GAMES + "): "); won = scan.nextInt(); continue Copyright © 2012 Pearson Education, Inc. continue while (won < 0 || won > NUM_GAMES) { System.out.print ("Invalid input. Please reenter: "); won = scan.nextInt(); } ratio = (double)won / NUM_GAMES; NumberFormat fmt = NumberFormat.getPercentInstance(); System.out.println (); System.out.println ("Winning percentage: " + fmt.format(ratio)); } } Copyright © 2012 Pearson Education, Inc. continue Sample Run Enter the number of games won (0 to 12): -5 while (won < 0 || won > NUM_GAMES) Invalid input. Please reenter: 13 { Invalid input. Pleaseinput. reenter: 7 reenter: "); System.out.print ("Invalid Please won = scan.nextInt(); } Winning percentage: 58% ratio = (double)won / NUM_GAMES; NumberFormat fmt = NumberFormat.getPercentInstance(); System.out.println (); System.out.println ("Winning percentage: " + fmt.format(ratio)); } } Copyright © 2012 Pearson Education, Inc. Infinite Loops • The body of a while loop eventually must make the condition false • If not, it is called an infinite loop, which will execute until the user interrupts the program • This is a common logical error • You should always double check the logic of a program to ensure that your loops will terminate normally Copyright © 2012 Pearson Education, Inc. Infinite Loops • An example of an infinite loop: int count = 1; while (count <= 25) { System.out.println (count); count = count - 1; } • This loop will continue executing until interrupted (Control-C) or until an underflow error occurs Copyright © 2012 Pearson Education, Inc. Nested Loops • Similar to nested if statements, loops can be nested as well • That is, the body of a loop can contain another loop • For each iteration of the outer loop, the inner loop iterates completely • See PalindromeTester.java Copyright © 2012 Pearson Education, Inc. //******************************************************************** // PalindromeTester.java Author: Lewis/Loftus // // Demonstrates the use of nested while loops. //******************************************************************** import java.util.Scanner; public class PalindromeTester { //----------------------------------------------------------------// Tests strings to see if they are palindromes. //----------------------------------------------------------------public static void main (String[] args) { String str, another = "y"; int left, right; Scanner scan = new Scanner (System.in); while (another.equalsIgnoreCase("y")) // allows y or Y { System.out.println ("Enter a potential palindrome:"); str = scan.nextLine(); left = 0; right = str.length() - 1; continue Copyright © 2012 Pearson Education, Inc. continue while (str.charAt(left) == str.charAt(right) && left < right) { left++; right--; } System.out.println(); if (left < right) System.out.println ("That string is NOT a palindrome."); else System.out.println ("That string IS a palindrome."); System.out.println(); System.out.print ("Test another palindrome (y/n)? "); another = scan.nextLine(); } } } Copyright © 2012 Pearson Education, Inc. continue Sample Run Enter a potential palindrome: while (str.charAt(left) == str.charAt(right) && left < right) radar { left++; right--; That string IS a palindrome. } Test another palindrome (y/n)? y Enter a potential palindrome: was I ere I saw elba if (leftable < right) System.out.println(); System.out.println ("That string is NOT a palindrome."); else That string IS a palindrome. System.out.println ("That string IS a palindrome."); Test another palindrome (y/n)? y System.out.println(); Enter a potential palindrome: System.out.print ("Test another palindrome (y/n)? "); another abracadabra = scan.nextLine(); } } That string is NOT a palindrome. } Test another palindrome (y/n)? n Copyright © 2012 Pearson Education, Inc. Quick Check How many times will the string "Here" be printed? count1 = 1; while (count1 <= 10) { count2 = 1; while (count2 < 20) { System.out.println ("Here"); count2++; } count1++; } Copyright © 2012 Pearson Education, Inc. Quick Check How many times will the string "Here" be printed? count1 = 1; while (count1 <= 10) { 10 * 19 = 190 count2 = 1; while (count2 < 20) { System.out.println ("Here"); count2++; } count1++; } Copyright © 2012 Pearson Education, Inc. Outline Boolean Expressions The if Statement Comparing Data The while Statement Iterators The ArrayList Class Determining Event Sources Check Boxes and Radio Buttons Copyright © 2012 Pearson Education, Inc. Iterators • An iterator is an object that allows you to process a collection of items one at a time • It lets you step through each item in turn and process it as needed • An iterator has a hasNext method that returns true if there is at least one more item to process • The next method returns the next item • Iterator objects are defined using the Iterator interface, which is discussed further in Chapter 7 Copyright © 2012 Pearson Education, Inc. Iterators • Several classes in the Java standard class library are iterators • The Scanner class is an iterator – the hasNext method returns true if there is more data to be scanned – the next method returns the next scanned token as a string • The Scanner class also has variations on the hasNext method for specific data types (such as hasNextInt) Copyright © 2012 Pearson Education, Inc. Iterators • The fact that a Scanner is an iterator is particularly helpful when reading input from a file • Suppose we wanted to read and process a list of URLs stored in a file • One scanner can be set up to read each line of the input until the end of the file is encountered • Another scanner can be set up for each URL to process each part of the path • See URLDissector.java Copyright © 2012 Pearson Education, Inc. //******************************************************************** // URLDissector.java Author: Lewis/Loftus // // Demonstrates the use of Scanner to read file input and parse it // using alternative delimiters. //******************************************************************** import java.util.Scanner; import java.io.*; public class URLDissector { //----------------------------------------------------------------// Reads urls from a file and prints their path components. //----------------------------------------------------------------public static void main (String[] args) throws IOException { String url; Scanner fileScan, urlScan; fileScan = new Scanner (new File("urls.inp")); continue Copyright © 2012 Pearson Education, Inc. continue // Read and process each line of the file while (fileScan.hasNext()) { url = fileScan.nextLine(); System.out.println ("URL: " + url); urlScan = new Scanner (url); urlScan.useDelimiter("/"); // Print each part of the url while (urlScan.hasNext()) System.out.println (" " + urlScan.next()); System.out.println(); } } } Copyright © 2012 Pearson Education, Inc. Sample Run continue URL: www.google.com www.google.com // Read and process each line of the file URL: www.linux.org/info/gnu.html while (fileScan.hasNext()) www.linux.org { url = fileScan.nextLine(); info System.out.println gnu.html ("URL: " + url); urlScan = new Scanner (url); URL: thelyric.com/calendar/ urlScan.useDelimiter("/"); thelyric.com calendar Print each part of the url // while (urlScan.hasNext()) System.out.println (" " + urlScan.next()); URL: www.cs.vt.edu/undergraduate/about } } } www.cs.vt.edu System.out.println(); undergraduate about URL: youtube.com/watch?v=EHCRimwRGLs youtube.com watch?v=EHCRimwRGLs Copyright © 2012 Pearson Education, Inc. Outline Boolean Expressions The if Statement Comparing Data The while Statement Iterators The ArrayList Class Determining Event Sources Check Boxes and Radio Buttons Copyright © 2012 Pearson Education, Inc. The ArrayList Class • An ArrayList object stores a list of objects, and is often processed using a loop • The ArrayList class is part of the java.util package • You can reference each object in the list using a numeric index • An ArrayList object grows and shrinks as needed, adjusting its capacity as necessary Copyright © 2012 Pearson Education, Inc. The ArrayList Class • Index values of an ArrayList begin at 0 (not 1): 0 1 2 3 4 "Bashful" "Sleepy" "Happy" "Dopey" "Doc" • Elements can be inserted and removed • The indexes of the elements adjust accordingly Copyright © 2012 Pearson Education, Inc. ArrayList Methods • Some ArrayList methods: boolean add (E obj) void add (int index, E obj) Object remove (int index) Object get (int index) boolean isEmpty() int size() Copyright © 2012 Pearson Education, Inc. The ArrayList Class • The type of object stored in the list is established when the ArrayList object is created: ArrayList names = new ArrayList(); ArrayList list = new ArrayList(); • This makes use of Java generics, which provide additional type checking at compile time • An ArrayList object cannot store primitive types, but that's what wrapper classes are for • See Beatles.java Copyright © 2012 Pearson Education, Inc. //******************************************************************** // Beatles.java Author: Lewis/Loftus // // Demonstrates the use of a ArrayList object. //******************************************************************** import java.util.ArrayList; public class Beatles { //----------------------------------------------------------------// Stores and modifies a list of band members. //----------------------------------------------------------------public static void main (String[] args) { ArrayList band = new ArrayList(); band.add band.add band.add band.add ("Paul"); ("Pete"); ("John"); ("George"); continue Copyright © 2012 Pearson Education, Inc. continue System.out.println (band); int location = band.indexOf ("Pete"); band.remove (location); System.out.println (band); System.out.println ("At index 1: " + band.get(1)); band.add (2, "Ringo"); System.out.println ("Size of the band: " + band.size()); int index = 0; while (index < band.size()) { System.out.println (band.get(index)); index++; } } } Copyright © 2012 Pearson Education, Inc. continue Output System.out.println (band); [Paul, Pete, John, George] int location =[Paul, band.indexOf John, ("Pete"); George] band.remove (location); At index 1: John Size (band); of the band: 4 System.out.println Paul ("At index 1: " + band.get(1)); System.out.println John band.add (2, "Ringo"); Ringo System.out.println ("Size of the band: " + band.size()); George int index = 0; while (index < band.size()) { System.out.println (band.get(index)); index++; } } } Copyright © 2012 Pearson Education, Inc. Outline Boolean Expressions The if Statement Comparing Data The while Statement Iterators The ArrayList Class Determining Event Sources Check Boxes and Radio Buttons Copyright © 2012 Pearson Education, Inc. Determining Event Sources • Recall that interactive GUIs require establishing a relationship between components and the listeners that respond to component events • One listener object can be used to listen to two different components • The source of the event can be determined by using the getSource method of the event passed to the listener • See LeftRight.java • See LeftRightPanel.java Copyright © 2012 Pearson Education, Inc. //******************************************************************** // LeftRight.java Authors: Lewis/Loftus // // Demonstrates the use of one listener for multiple buttons. //******************************************************************** import javax.swing.JFrame; public class LeftRight { //----------------------------------------------------------------// Creates the main program frame. //----------------------------------------------------------------public static void main (String[] args) { JFrame frame = new JFrame ("Left Right"); frame.setDefaultCloseOperation (JFrame.EXIT_ON_CLOSE); frame.getContentPane().add(new LeftRightPanel()); frame.pack(); frame.setVisible(true); } } Copyright © 2012 Pearson Education, Inc. //******************************************************************** // LeftRight.java Authors: Lewis/Loftus // // Demonstrates the use of one listener for multiple buttons. //******************************************************************** import javax.swing.JFrame; public class LeftRight { //----------------------------------------------------------------// Creates the main program frame. //----------------------------------------------------------------public static void main (String[] args) { JFrame frame = new JFrame ("Left Right"); frame.setDefaultCloseOperation (JFrame.EXIT_ON_CLOSE); frame.getContentPane().add(new LeftRightPanel()); frame.pack(); frame.setVisible(true); } } Copyright © 2012 Pearson Education, Inc. //******************************************************************** // LeftRightPanel.java Authors: Lewis/Loftus // // Demonstrates the use of one listener for multiple buttons. //******************************************************************** import java.awt.*; import java.awt.event.*; import javax.swing.*; public class LeftRightPanel extends JPanel { private JButton left, right; private JLabel label; private JPanel buttonPanel; continue Copyright © 2012 Pearson Education, Inc. continue //----------------------------------------------------------------// Constructor: Sets up the GUI. //----------------------------------------------------------------public LeftRightPanel () { left = new JButton ("Left"); right = new JButton ("Right"); ButtonListener listener = new ButtonListener(); left.addActionListener (listener); right.addActionListener (listener); label = new JLabel ("Push a button"); buttonPanel = new JPanel(); buttonPanel.setPreferredSize (new Dimension(200, 40)); buttonPanel.setBackground (Color.blue); buttonPanel.add (left); buttonPanel.add (right); setPreferredSize (new Dimension(200, 80)); setBackground (Color.cyan); add (label); add (buttonPanel); } continue Copyright © 2012 Pearson Education, Inc. continue //***************************************************************** // Represents a listener for both buttons. //***************************************************************** private class ButtonListener implements ActionListener { //-------------------------------------------------------------// Determines which button was pressed and sets the label // text accordingly. //-------------------------------------------------------------public void actionPerformed (ActionEvent event) { if (event.getSource() == left) label.setText("Left"); else label.setText("Right"); } } } Copyright © 2012 Pearson Education, Inc. Outline Boolean Expressions The if Statement Comparing Data The while Statement Iterators The ArrayList Class Determining Event Sources Check Boxes and Radio Buttons Copyright © 2012 Pearson Education, Inc. Check Boxes • A check box is a button that can be toggled on or off • It is represented by the JCheckBox class • Unlike a push button, which generates an action event, a check box generates an item event whenever it changes state • The ItemListener interface is used to define item event listeners • A check box calls the itemStateChanged method of the listener when it is toggled Copyright © 2012 Pearson Education, Inc. Check Boxes • Let's examine a program that uses check boxes to determine the style of a label's text string • It uses the Font class, which embodies a character font's: – family name (such as Times or Courier) – style (bold, italic, or both) – font size • See StyleOptions.java • See StyleOptionsPanel.java Copyright © 2012 Pearson Education, Inc. //******************************************************************** // StyleOptions.java Author: Lewis/Loftus // // Demonstrates the use of check boxes. //******************************************************************** import javax.swing.JFrame; public class StyleOptions { //----------------------------------------------------------------// Creates and presents the program frame. //----------------------------------------------------------------public static void main (String[] args) { JFrame frame = new JFrame ("Style Options"); frame.setDefaultCloseOperation (JFrame.EXIT_ON_CLOSE); StyleOptionsPanel panel = new StyleOptionsPanel(); frame.getContentPane().add (panel); frame.pack(); frame.setVisible(true); } } Copyright © 2012 Pearson Education, Inc. //******************************************************************** // StyleOptions.java Author: Lewis/Loftus // // Demonstrates the use of check boxes. //******************************************************************** import javax.swing.JFrame; public class StyleOptions { //----------------------------------------------------------------// Creates and presents the program frame. //----------------------------------------------------------------public static void main (String[] args) { JFrame frame = new JFrame ("Style Options"); frame.setDefaultCloseOperation (JFrame.EXIT_ON_CLOSE); StyleOptionsPanel panel = new StyleOptionsPanel(); frame.getContentPane().add (panel); frame.pack(); frame.setVisible(true); } } Copyright © 2012 Pearson Education, Inc. //******************************************************************** // StyleOptionsPanel.java Author: Lewis/Loftus // // Demonstrates the use of check boxes. //******************************************************************** import javax.swing.*; import java.awt.*; import java.awt.event.*; public class StyleOptionsPanel extends JPanel { private JLabel saying; private JCheckBox bold, italic; continue Copyright © 2012 Pearson Education, Inc. continue //----------------------------------------------------------------// Sets up a panel with a label and some check boxes that // control the style of the label's font. //----------------------------------------------------------------public StyleOptionsPanel() { saying = new JLabel ("Say it with style!"); saying.setFont (new Font ("Helvetica", Font.PLAIN, 36)); bold = new JCheckBox ("Bold"); bold.setBackground (Color.cyan); italic = new JCheckBox ("Italic"); italic.setBackground (Color.cyan); StyleListener listener = new StyleListener(); bold.addItemListener (listener); italic.addItemListener (listener); add (saying); add (bold); add (italic); setBackground (Color.cyan); setPreferredSize (new Dimension(300, 100)); } continue Copyright © 2012 Pearson Education, Inc. continue //***************************************************************** // Represents the listener for both check boxes. //***************************************************************** private class StyleListener implements ItemListener { //-------------------------------------------------------------// Updates the style of the label font style. //-------------------------------------------------------------public void itemStateChanged (ItemEvent event) { int style = Font.PLAIN; if (bold.isSelected()) style = Font.BOLD; if (italic.isSelected()) style += Font.ITALIC; saying.setFont (new Font ("Helvetica", style, 36)); } } } Copyright © 2012 Pearson Education, Inc. Radio Buttons • A group of radio buttons represents a set of mutually exclusive options – only one can be selected at any given time • When a radio button from a group is selected, the button that is currently "on" in the group is automatically toggled off • To define the group of radio buttons that will work together, each radio button is added to a ButtonGroup object • A radio button generates an action event Copyright © 2012 Pearson Education, Inc. Radio Buttons • Let's look at a program that uses radio buttons to determine which line of text to display • See QuoteOptions.java • See QuoteOptionsPanel.java Copyright © 2012 Pearson Education, Inc. //******************************************************************** // QuoteOptions.java Author: Lewis/Loftus // // Demonstrates the use of radio buttons. //******************************************************************** import javax.swing.JFrame; public class QuoteOptions { //----------------------------------------------------------------// Creates and presents the program frame. //----------------------------------------------------------------public static void main (String[] args) { JFrame frame = new JFrame ("Quote Options"); frame.setDefaultCloseOperation (JFrame.EXIT_ON_CLOSE); QuoteOptionsPanel panel = new QuoteOptionsPanel(); frame.getContentPane().add (panel); frame.pack(); frame.setVisible(true); } } Copyright © 2012 Pearson Education, Inc. //******************************************************************** // QuoteOptions.java Author: Lewis/Loftus // // Demonstrates the use of radio buttons. //******************************************************************** import javax.swing.JFrame; public class QuoteOptions { //----------------------------------------------------------------// Creates and presents the program frame. //----------------------------------------------------------------public static void main (String[] args) { JFrame frame = new JFrame ("Quote Options"); frame.setDefaultCloseOperation (JFrame.EXIT_ON_CLOSE); QuoteOptionsPanel panel = new QuoteOptionsPanel(); frame.getContentPane().add (panel); frame.pack(); frame.setVisible(true); } } Copyright © 2012 Pearson Education, Inc. //******************************************************************** // QuoteOptionsPanel.java Author: Lewis/Loftus // // Demonstrates the use of radio buttons. //******************************************************************** import javax.swing.*; import java.awt.*; import java.awt.event.*; public class QuoteOptionsPanel extends JPanel { private JLabel quote; private JRadioButton comedy, philosophy, carpentry; private String comedyQuote, philosophyQuote, carpentryQuote; //----------------------------------------------------------------// Sets up a panel with a label and a set of radio buttons // that control its text. //----------------------------------------------------------------public QuoteOptionsPanel() { comedyQuote = "Take my wife, please."; philosophyQuote = "I think, therefore I am."; carpentryQuote = "Measure twice. Cut once."; quote = new JLabel (comedyQuote); quote.setFont (new Font ("Helvetica", Font.BOLD, 24)); continue Copyright © 2012 Pearson Education, Inc. continue comedy = new JRadioButton ("Comedy", true); comedy.setBackground (Color.green); philosophy = new JRadioButton ("Philosophy"); philosophy.setBackground (Color.green); carpentry = new JRadioButton ("Carpentry"); carpentry.setBackground (Color.green); ButtonGroup group = new ButtonGroup(); group.add (comedy); group.add (philosophy); group.add (carpentry); QuoteListener listener = new QuoteListener(); comedy.addActionListener (listener); philosophy.addActionListener (listener); carpentry.addActionListener (listener); add add add add (quote); (comedy); (philosophy); (carpentry); setBackground (Color.green); setPreferredSize (new Dimension(300, 100)); } continue Copyright © 2012 Pearson Education, Inc. continue //***************************************************************** // Represents the listener for all radio buttons //***************************************************************** private class QuoteListener implements ActionListener { //-------------------------------------------------------------// Sets the text of the label depending on which radio // button was pressed. //-------------------------------------------------------------public void actionPerformed (ActionEvent event) { Object source = event.getSource(); if (source == comedy) quote.setText (comedyQuote); else if (source == philosophy) quote.setText (philosophyQuote); else quote.setText (carpentryQuote); } } } Copyright © 2012 Pearson Education, Inc. Summary • Chapter 5 focused on: – boolean expressions – the if and if-else statements – comparing data – while loops – iterators – more drawing techniques – more GUI components Copyright © 2012 Pearson Education, Inc.