Last Time Built Hello World java in Blue

Last Time • Built “Hello. World. java” in Blue. J (and Eclipse). • Looked at Java keywords. • Primitive types. • Expressions: Variables, operators, literal values, operator precedence. Spring 2006 CISC 101 - Prof. Mc. Leod 1

Today • Lab 1 and Assignment 1 are posted. • A few more things to do with expressions: – – Mixed type expressions Casting – automatic (implicit) and explicit Unary operators Other assignment operators • Screen Input/Output. • Variable Scope • Conditional (or “if” statements) – if we have time… Spring 2006 CISC 101 - Prof. Mc. Leod 2

Mixed Type Expressions • So far, we have been careful to have expressions where all the types match. • For example: double rect. Area = 4. 1 * 5. 2; • rect. Area contains 21. 32 • Operators will only work when the values on both sides of the operator are of the same type. Spring 2006 CISC 101 - Prof. Mc. Leod 3

Mixed Type Expressions, Cont. • Suppose the expression is: double rect. Area = 4 * 5. 2; • 4 is an int literal and 5. 2 is a double literal. • What do you think will happen? • 4 is changed - “cast” - to be a double literal and the evaluation proceeds. This is called “implicit” casting, and it happens automatically. Spring 2006 CISC 101 - Prof. Mc. Leod 4

Mixed Type Expressions, Cont. • What will happen with: double rect. Area = 4 * 5; • Both 4 and 5 are int literals. • The * operation results in 20 • The int value 20 is cast to 20. 0 and stored in rect. Area. Spring 2006 CISC 101 - Prof. Mc. Leod 5

Mixed Type Expressions, Cont. • So mixing int literals in expressions with double’s is not a problem, as long as the result is being saved in a double. • int’s will be automatically cast to double’s (an “implicit cast”), this is also called a “widening” cast. • What about: int area = 4 * 5. 2; • 4 * 5. 2 will evaluate to a double. • Java will not compile this statement. Why not? Spring 2006 CISC 101 - Prof. Mc. Leod 6

Mixed Type Expressions, Cont. • Java will refuse to try and store the double (10. 4) in a variable of type int. • Information would be lost. • To do this, the code will have to tell Java that it is OK to lose this information - this is called a “narrowing” cast. • Your code has to carry out a “narrowing” cast explicitly, it will not be done automatically: int area = (int)(4 * 5. 2); Spring 2006 CISC 101 - Prof. Mc. Leod 7

Casting • The code: “(int)” is called a casting operator. • Whatever follows the (int) will be cast to an int value. • Note that casting truncates, it does not round a value. So, (int)10. 9 is 10, not 11. • Casting takes precedence over the arithmetic operators. • (The “unary” operators take precedence over casting - but we have not discussed unary operators, yet. ) Spring 2006 CISC 101 - Prof. Mc. Leod 8

Casting - Cont. • “Implicit” casting is automatic, and will always go in the direction: byte > short > int > long > float > double • Anytime you want to cast in the opposite direction, you must use an “explicit” cast, using the appropriate casting operator. Spring 2006 CISC 101 - Prof. Mc. Leod 9

Aside - Integer Arithmetic • Integer arithmetic gets interesting for division. • What is the value in a. Val for: int a. Val = 5 / 2; • 2 • Not 2. 5, which would be a double! • 5 / 2 yields an int, not a double since both 5 and 2 are int literals. Spring 2006 CISC 101 - Prof. Mc. Leod 10

Integer Arithmetic, Cont. • What is the value in a. Val for: int a. Val = 20 / 3; • 6 • Not 7! • Truncates, not rounds! Spring 2006 CISC 101 - Prof. Mc. Leod 11

Integer Arithmetic, Cont. • Similarly, what is: int a. Val = 1 / 5; a. Val is 0 • How about: double a. Num = 7 / 2; Spring 2006 a. Num is 3. 0 CISC 101 - Prof. Mc. Leod 12

Unary Operators • Binary operators required values on both sides of the operator: * / - + % < > <= >= == != && || = • Unary operators operate on just one value, or variable: – – (casting) - (negation) ++ -- (increment and decrement) ! (logical “NOT”) Spring 2006 CISC 101 - Prof. Mc. Leod 13

Unary Arithmetic Operators • Unary operators include “-”, where “-a. Num” negates the value produced by a. Num, for example. • They also include the increment (++) and decrement (--) operators which increase or decrease an integer value by 1. • Preincrement and predecrement operators appear before a variable. They increment or decrement the value of the variable before it is used in the expression. • Example: int i = 4, j = 2, k; k = ++i - j; // i = 5, j = 2, k = 3 Spring 2006 CISC 101 - Prof. Mc. Leod 14

Unary Arithmetic Operators - Cont. • Postincrement and postdecrement operators appear after a variable. They increment or decrement the value of the variable after it is used in the expression. • Example: int i = 4, j = 2, k; k = i++ - j; // i = 5, j = 2, k = 2 • Keep expressions involving increment and decrement operators simple! Spring 2006 CISC 101 - Prof. Mc. Leod 15

Unary Logical Operator • The one “unary” logical operator is “!”. • Called the “Not” operator. • It reverses the logical value of a boolean. • For example: !(5 > 3) evaluates to false Spring 2006 CISC 101 - Prof. Mc. Leod 16

The Other Assignment Operators Syntax: variable. Name = expression; = set equal to (we’ve seen this one!) *= /= -= += multiply and set equal to divide and set equal to subtract and set equal to add and set equal to Spring 2006 CISC 101 - Prof. Mc. Leod 17

Assignment Operators - Cont. • For example, variable. Name += expression; is equivalent to variable. Name = variable. Name + expression; Spring 2006 CISC 101 - Prof. Mc. Leod 18

Screen I/O • Your program would not be much use if it could not communicate with the user! • Screen input is your program obtaining information - “values” - from the user. • Screen output is your program sending information to the screen for the user to read. • Screen input in Java is not as simple as screen output. Spring 2006 CISC 101 - Prof. Mc. Leod 19

Screen Output • Three methods in the System. out class for Console window output: – println() – printf() Spring 2006 (only in Java 5. 0) CISC 101 - Prof. Mc. Leod 20

Screen Output – Cont. • (Also called “Text Window”…) • Easy: System. out. print( Stuff_to_Print ); System. out. println( Stuff_to_Print ); • “Stuff_to_Print” can be anything - not just a String - these two methods are heavily overloaded. Spring 2006 CISC 101 - Prof. Mc. Leod 21

Screen Output - Cont. • Note that: System. out. print(“n”); is the same as: System. out. println(); • So, println() appends a carriage return/linefeed character sequence to the end of the output, where print() does not. Spring 2006 CISC 101 - Prof. Mc. Leod 22

Screen Output - Cont. • “Escape sequences” can be used to control the appearance of the output: ” ’ \ n r t a double quote a single quote a backslash a linefeed a carriage return a tab character • These can be embedded anywhere in a String. Spring 2006 CISC 101 - Prof. Mc. Leod 23

Screen Output - Cont. • For example the following code: System. out. println(""Tabbed" output: t 1t 2t 3t 4"); Prints the following to the screen: "Tabbed" output: Spring 2006 1 2 CISC 101 - Prof. Mc. Leod 3 4 24

printf Method • Which statement provides the most sensible output: ? double a. Num = 17. 0 / 3. 0; System. out. println("Using println: " + a. Num); System. out. printf("Using printf: %5. 2 f", a. Num); Using println: 5. 66666667 Using printf: 5. 67 Spring 2006 CISC 101 - Prof. Mc. Leod 25

printf Method, Cont. • For more information follow the link: “Format String Syntax” in the API listing for the printf method. • Basically use %width. decimalsf for float and double numbers, and %d for int’s. • You can have multiple numbers listed by using multiple format placeholders. Spring 2006 CISC 101 - Prof. Mc. Leod 26

Aside - Methods and Classes • I’ve been talking about “methods” and “classes” without really explaining them. • For example, printf() is a method in the System. out class. • Methods cannot exist on their own - they must “belong” to a class. • In order for Java to find the method, you must tell Java the name of the class that “owns” the method. Spring 2006 CISC 101 - Prof. Mc. Leod 27

Aside - Methods and Classes - Cont. • Telling the Java compiler who owns the method is done by using the “dot operator” - a fancy name for a period. • The period is used to separate the class name from the method name: • For example: System. out. println() Class name Spring 2006 Method name CISC 101 - Prof. Mc. Leod 28

Aside - Methods and Classes - Cont. • (If you are calling a method that exists within the same class as your main method, you do not need a class name or a period. We will see more about this later!) Spring 2006 CISC 101 - Prof. Mc. Leod 29

Console Input in Java 5. 0 • Use the “Scanner” class. It must be imported using: import java. util. Scanner; • In your program: Scanner console = new Scanner(System. in); Spring 2006 CISC 101 - Prof. Mc. Leod 30

Console Input in Java 5. 0, Cont. • Then, you can use the Object “console”, as many times as you want calling the appropriate method for whatever primitive type or String you want to get from the user. • For example: System. out. print("Please enter a number: "); double a. Num = console. next. Double(); Spring 2006 CISC 101 - Prof. Mc. Leod 31

Exercise 1. Write a program that accepts a double value from the user and then displays the volume of a cube with sides of the length provided. • Hints: – above public class: import java. util. Scanner; – inside main: • Scanner console = new Scanner(System. in); • prompt the user • Accept the input using console. next. Double() • Calculate and display the volume. Spring 2006 CISC 101 - Prof. Mc. Leod 32

import java. util. Scanner; public class Volume. Calculation { public static void main (String[] args) { double side. Length; double volume; Scanner console = new Scanner(System. in); System. out. print("Enter side length: "); side. Length = console. next. Double(); volume = side. Length * side. Length; System. out. println("Volume is " + volume); } // end main } // end Volume. Calculation Spring 2006 CISC 101 - Prof. Mc. Leod 33

Exercise, Cont. 2. Once your program works OK, try entering something that is not a number, to see what happens. Spring 2006 CISC 101 - Prof. Mc. Leod 34

Other Scanner Class Input Methods • Has an input method for each primitive type. (See the API) • Use. next. Line() to get the entire line as a String. Spring 2006 CISC 101 - Prof. Mc. Leod 35

Aside – “GUI” • GUI (“Graphical User Interface”) is all about the use of frames, laying out passive and interactive components (text boxes, command buttons, labels, etc. ), and then attaching methods to listener events, for user interaction. • This might be fun? … But, we will not get sidetracked in this course into what really is design and not so much programming. • Besides, an application with a nice GUI interface can still be a lousy program if the underlying code does not work well! Spring 2006 CISC 101 - Prof. Mc. Leod 36

Aside, GUI – Cont. • For screen output, you can use the JOption. Pane class, and invoke the show. Message. Dialog() method. JOption. Pane. show. Message. Dialog(null, “Volume is " + volume); • The JOption. Pane class is part of the javax. swing package, so you must have the following line at the top of your program: import javax. swing. JOption. Pane; Spring 2006 CISC 101 - Prof. Mc. Leod 37

Aside, GUI – Cont. • Produces the cute little window: Spring 2006 CISC 101 - Prof. Mc. Leod 38

GUI Input • Use the show. Input. Dialog() method from the JOption. Pane class: String message; message = JOption. Pane. show. Input. Dialog("Enter side length: "); • OK, but we have a String and we want a number… Spring 2006 CISC 101 - Prof. Mc. Leod 39

GUI Input - Cont. • Use Scanner class again: Scanner get. Number = new Scanner(message); side. Length = get. Number. next. Double(); • If message does not contain a double number, then an exception will be thrown. Spring 2006 CISC 101 - Prof. Mc. Leod 40

GUI Input, Cont. • So, to prevent crashing our program, we need to catch an Exception, specifically a “java. lang. Number. Format. Exception”. • Once we catch it we need to do something sensible to prevent the problem from crashing the rest of our program. Our program would be more robust! • (Except we do not know how to catch exceptions – yet…) Spring 2006 CISC 101 - Prof. Mc. Leod 41

import java. util. Scanner; import javax. swing. JOption. Pane; public class Volume. Calculation. GUI { public static void main (String[] args) { double side. Length; double volume; String message; message = JOption. Pane. show. Input. Dialog("Enter side length: "); Scanner get. Number = new Scanner(message); side. Length = get. Number. next. Double(); volume = side. Length * side. Length; JOption. Pane. show. Message. Dialog(null, "Volume is " + volume); } // end main Spring 2006 } // end Volume. Calculation. GUI CISC 101 - Prof. Mc. Leod 42

Screen I/O, Summary • For this course, you can user either console I/O or JOption. Pane I/O. • Don’t use both in a single program! • For now, assume that the user will supply the type requested. Later you can make more robust programs using exceptions. • We will not spend the time require to learn how to write full GUI programs in this course. Spring 2006 CISC 101 - Prof. Mc. Leod 43

Variable Scope • Variables are only “known” within the block in which they are declared. • If a block is inside another block, then a variable declared in the outer block is known to all blocks inside the outer one. • “known” means that outside the block in which they are declared, it is as if the variable does not exist. • For example: Spring 2006 CISC 101 - Prof. Mc. Leod 44

public class Scope. Test { public static void main (String[] args) { // block 1 { int a. Val = 6; } // block 2 { a. Val = 10; } } // end main method } // end Scope. Test Spring 2006 CISC 101 - Prof. Mc. Leod 45

Variable Scope – Cont. • Note that both block 1 and block 2 are contained within the main method’s block, which itself is contained within the block for the class definition. • This program will not even compile. • The “a. Val = 10; ” line will be highlighted and the following error message will be shown: Error: No entity named “a. Val” was found in this environment. • Next example: Spring 2006 CISC 101 - Prof. Mc. Leod 46

public class Scope. Test { public static void main (String[] args) { int a. Val; // block { a. Val } 1 = 6; 2 = 10; } // end main method } // end Scope. Test Spring 2006 CISC 101 - Prof. Mc. Leod 47

Variable Scope – Cont. • This program compiles and runs fine. • a. Val is declared in the same block that contains block 1 and block 2, so a. Val is known inside both blocks. • One more example: Spring 2006 CISC 101 - Prof. Mc. Leod 48

public class Scope. Test { public static void main (String[] args) { // block 1 { int a. Val = 6; } // block 2 { int a. Val = 10; } } // end main method } // end Scope. Test Spring 2006 CISC 101 - Prof. Mc. Leod 49

Variable Scope – Cont. • This program compiles and runs fine, but is considered Very Bad Form! • You should never re-declare a variable within a method! • These two variables are completely separate as far as the compiler knows, but any person reading your code will be confused! Spring 2006 CISC 101 - Prof. Mc. Leod 50

So Far… • So far, all our programs have been linear: etc. Spring 2006 CISC 101 - Prof. Mc. Leod 51

Branching Algorithms • If a program could test a condition, then it would have a choice as to its path of execution: if true if false etc. Spring 2006 CISC 101 - Prof. Mc. Leod 52

Conditionals or “Selection Statements” • Java has “if”, “if-else” and “switch” statements. • Simple if statement syntax: if (boolean_expression) statement_when_true; • Example: if (capacitance < 0) System. out. println(“Illegal capacitance”); Spring 2006 CISC 101 - Prof. Mc. Leod 53

if Statement • You might want to carry out more than one statement when a condition is true. • For example if the capacitance is negative (an “illegal” value) then re-prompt the user for positive value: if (capacitance < 0) { System. out. print(“Illegal value, please re-enter: ”); // code to obtain another value from the user } // end if • So, you can enclose a block of statements within “{}”. Spring 2006 CISC 101 - Prof. Mc. Leod 54

if-else Statement • Syntax of “if-else” statement: if (boolean_expression) statement_when_true; else statement_when_false; • Example: if (stress > max. Stress / 1. 5) result = “failure”; else result = “pass”; Spring 2006 CISC 101 - Prof. Mc. Leod 55

if-else Statement, Cont. • With statement blocks: if (boolean_expression) { block_of_code_when_true } else { block_of_code_when_false } Spring 2006 CISC 101 - Prof. Mc. Leod 56

if Statement, Cont. • You will often have to nest if statements: etc. Spring 2006 CISC 101 - Prof. Mc. Leod 57

Nested if Statements • For example, if you have three numbers, a, b, & c and you want to print them to the screen in order: T T abc b<c T acb Spring 2006 F a<b T b<c a<c F F F a<c cab T bac CISC 101 - Prof. Mc. Leod F cba bca 58

Nested if Statements - Cont. • Nested if statements can be tricky to code make sure you create test cases that will test every possible path through all the conditionals. Spring 2006 CISC 101 - Prof. Mc. Leod 59

“Chained” if Statements • Syntax: if (condition 1) { block 1 } else if (condition 2) { block 2 } else if (condition 3) { block 3 } else if (condition 4) { block 4 } … else { blockn } • Each condition is tested in turn, until one is evaluated to true. If none of them are true then the else block is executed. Spring 2006 CISC 101 - Prof. Mc. Leod 60

switch Statement • Syntax: switch (expression) { case val 1: // statements if expression produces val 1 break; case val 2: // statements if expression produces val 2 break; case val 3: … default: // statements if none of the above is true break; } // end switch Spring 2006 CISC 101 - Prof. Mc. Leod 61

switch Statement - Cont. • The code to be run depends on which val# value matches expression. • If none match, the statements after the default: clause are run. • The expression and val# values (or “Case Labels”) must all be of the same integer type. • The break; statements make sure that following cases are not executed after a match has been made. • It is possible to do multiple cases on one line, but it is clumsy: Spring 2006 CISC 101 - Prof. Mc. Leod 62

switch Statement - Cont. switch (expression) { case val 1: case val 2: case val 3: // statements if expression is val 1, val 2 or val 3 break; case val 4: case val 5: // statements if expression is val 4 or val 5 break; case val 6: … default: // statements if none of the above is true break; } // end switch Spring 2006 CISC 101 - Prof. Mc. Leod 63

switch Statement - Cont. • Not too useful a construct. • Menu coding, for example, is a possible use: – Provide a number of options to the user, like “(A)dd, (E)dit or (D)elete”. – The user presses a, e, d, A, E, D, or some other key. – In a switch statement, you would have: Spring 2006 CISC 101 - Prof. Mc. Leod 64

switch Statement - Cont. switch (user. Response) { // user. Response is a char case ‘a’: case ‘A’: // Add operation code break; case ‘e’: case ‘E’: // Edit operation code break; case ‘d’: case ‘D’: // Delete operation code break; default: // Tell user wrong key pressed break; } // end switch Spring 2006 CISC 101 - Prof. Mc. Leod 65

Exercise 1. Obtain an outdoor temperature (in degrees Centigrade) from the user. 2. If the temperature is less than -40, or greater than +40 tell him that the temperature is not legal and exit the program. 3. If the temperature is >= -40, but less than 0, display “It is cold! Wear a parka. ”. 4. If the temperature is >= 0, but less than 15, display “It is cool. Wear a jacket. ”. 5. If the temperature is >= 15 and less than 25, display “It is nice! Wear shorts. ”. 6. If the temperatuer is >=25 and less than 40, display “It is hot! Seek the beach!”. Spring 2006 CISC 101 - Prof. Mc. Leod 66

Repetition or Using “Loops” • We will discuss: – while – do/while – for – The “for each” loop in Java 5. 0 – Use of “break” and “continue” Spring 2006 CISC 101 - Prof. Mc. Leod 67

Repetition or “Loops” • Suppose we combine a boolean test with some kind of structure that allows us to branch back up to an earlier piece of code: if true if false etc. Spring 2006 CISC 101 - Prof. Mc. Leod 68

Repetition - Cont. • The boolean test determines when to stop the repetition - as long as the condition is true, the loop keeps going. • Something inside the looping part must affect what is tested in the condition - right? What if it did not - what would happen? Spring 2006 CISC 101 - Prof. Mc. Leod 69

Repetition - Cont. • A simple example - suppose we wanted a loop to execute only 20 times: i=1 if true if false i < 21 i = i+1 etc. Spring 2006 CISC 101 - Prof. Mc. Leod 70

Repetition - Cont. • The number of repetitions is controlled by changing the limit value for the loop counter - “i” in the example on the previous slide. • That example had i increasing by one each time. The loop counter was being incremented by one. • It could have been incremented by some other value, 2, 3, or whatever. • You could use something like “i = i * 2” to increment the counter. • If the counter is decreased in value each time, it is being “decremented”. Spring 2006 CISC 101 - Prof. Mc. Leod 71

Repetition - Cont. • Suppose, in the previous example, i was decremented by one instead. What would happen? i=1 if true if false i < 21 i=i-1 etc. Spring 2006 CISC 101 - Prof. Mc. Leod 72

Repetition - Cont. • The dreaded “infinite loop”! • The java compiler will not prevent you from coding a loop like the one shown - it will compile, and it will run! • And run, and run, and run, and run… • As a programmer, you must be “on guard” for such logic errors in your code. Spring 2006 CISC 101 - Prof. Mc. Leod 73

Aside - Increment and Decrement Operators • In loops, expressions like: j = j + 1; and k = k - 1; are used so often, it is typical to see the postincrement operator: j++; is the same as j = j + 1; k--; is the same as k = k - 1; • You can use either notation. Spring 2006 CISC 101 - Prof. Mc. Leod 74

“while” loop • A java while loop can be used to code the structure shown in the flowchart above (the “increment” one on slide 27): int i = 1; while (i < 21) { // other statements i = i + 1; // or you could use i++: } // end while • The “{ }” brackets enclose the statements that are repeated. • (A single statement to be repeated in the loop does not require the “{}”. ) Spring 2006 CISC 101 - Prof. Mc. Leod 75

“while” loop - Cont. • Note that java (thank goodness!!!) does not have anything equivalent to a “goto” statement. • (And if it did, I would not tell you about it, anyways!!) • So, you cannot construct a loop with an “if” statement and a goto. • An “if” statement cannot give you repetition, it only allows you to decide on a single pass through a branch of code. Spring 2006 CISC 101 - Prof. Mc. Leod 76

“while” loop - Cont. • while loop syntax: while ( boolean_expression ) { block_of_code } • As long as boolean_expression evaluates to true the statements in the block_of_code continue to execute. • By mistake, you might write the following - what would happen? while ( boolean_expression ); { block_of_code } Spring 2006 CISC 101 - Prof. Mc. Leod 77

“while” loop - Cont. • The boolean expression tested in a while loop could be false to start with: int i = 40; while (i < 21) { // other statements i = i + 1; } • In this case, the loop would not execute at all. • Use a “do/while” loop if you need a loop that will always run at least once: Spring 2006 CISC 101 - Prof. Mc. Leod 78

“do/while” loop • Syntax: do { block_of_code } while ( boolean_expression ); • Note the “; ” at the end of the while statement. • Since the conditional test is at the end of the loop, it will always execute the loop at least once. Spring 2006 CISC 101 - Prof. Mc. Leod 79

“do/while” loop - Cont. • For example, suppose we must obtain a value between 1 and 100, inclusive, from the user: int a. Val = 0; // The compiler will force us to // initialize a. Val do { System. out. print(“Enter value between 1 and 100: ”); // code to obtain a value from the user } while (a. Val < 1 || a. Val > 100); • As long as the user does not do what he is told, the loop will continue to re-prompt him for the correct value. Spring 2006 CISC 101 - Prof. Mc. Leod 80

“for” loop • The kind of while loop shown above: int i = 1; while (i < 21) { // other statements i = i + 1; } is used so often, that Java has provided another looping structure that does all that is shown above, but needs only one line: for (int i = 1; i < 21; i = i + 1) { // other statements } Spring 2006 CISC 101 - Prof. Mc. Leod 81

“for” loop - Cont. • Or, as written with an increment operator: for (int i = 1; i < 21; i++) { // other statements } • Syntax: for (initialization; boolean_expression; update) { block_of_code } • for loops are used when you know, in advance, the number of repetitions desired. Spring 2006 CISC 101 - Prof. Mc. Leod 82

“for” loop - Cont. • You don’t have to declare the counter inside the for loop, if you have declared it earlier in your program. • But if you do declare it in the “for” statement then the scope of that variable will only be inside the loop block. Spring 2006 CISC 101 - Prof. Mc. Leod 83

“for each” Loop in Java 5. 0 • Often, you will want to “visit” every element in a collection, not just a part. • Syntax of the “for each” loop: for (type_variable : collection) { // statements } • These loops are only used with collections. Spring 2006 CISC 101 - Prof. Mc. Leod 84

“for each” Loop in Java 5. 0, Cont. • (We don’t know what arrays are yet, but just for now: ) • For example, suppose we have an array called “data”, containing a collection of double type numbers, and you want to add them all up: double sum = 0; for (double e : data) { sum = sum + e; // or sum += e; } Spring 2006 CISC 101 - Prof. Mc. Leod 85

“for each” Loop in Java 5. 0, Cont. • Equivalent normal “for” loop: double sum = 0; for (int i = 0; i < data. length; i++) { sum = sum + data[i]; //or sum += data[i]; } • The “for each” loop is a bit easier with arrays, but is even better suited for other kinds of collections. Spring 2006 CISC 101 - Prof. Mc. Leod 86

Loops - Misc. • Don’t declare variables inside loops, as the repeated declaration process uses up time and memory unnecessarily. • Loops are often nested - to usually not more than three levels. For example: int i, j; int sum = 0; for (i = 1; i <= 100; i++) for (j = 1; j <= 10; j++) sum++; • sum would be 1000. Spring 2006 CISC 101 - Prof. Mc. Leod 87

Loops - Misc. - Cont. • There is no limit in Java to how many levels you can nest loops. • It is customary, but not necessary, to use the variables i, j, k as loop counters. • Loops really demonstrate the strength of computers as they allow the machine to complete mind-numbingly boring tasks with perfect accuracy! • Loops will always be used with any file I/O and array operations. Spring 2006 CISC 101 - Prof. Mc. Leod 88

Other Java Keywords Used With Loops • break and continue • The continue statement interrupts the execution of a loop, and returns control to the top of the loop. • The break statement interrupts the execution of a loop, and transfers control to the first statement after the loop. Spring 2006 CISC 101 - Prof. Mc. Leod 89

Use of “continue” • For example, • Would print: Spring 2006 CISC 101 - Prof. Mc. Leod 90

Use of “break” • For example, • Would print: 16 September 2002 Fall 2002 CISC 121 - Prof. Mc. Leod 91

Use of “break” & “continue” • Only use these keywords when it makes your code easier to read. • Avoid the use of more than one break or continue inside a loop. • If you use a condition to issue a break statement, then why can’t you put that condition in the loop test? • Overuse of break statements can lead to “spaghetti” code - just like the use of “goto” statements! Spring 2006 CISC 101 - Prof. Mc. Leod 92