2. Java Programming Basics

What We Will Cover Announcements Illuminations 2.1: Data and Expressions 2.1.1: Data Types 2.1.2: Variables 2.1.3: Expressions 2.1.4: Casting 2.1.5: Strings 2.1.6: Summary Exercise 2.1 2.2: Console I/O 2.2.1: Printing to a Console 2.2.2: Importing Classes 2.2.3: Using System.in 2.2.4: Reading Numbers 2.2.5: Summary Exercise 2.2 2.3: Making Decisions 2.3.1: Flow of Control 2.3.2: Relational Operators 2.3.3: Logical Operators 2.3.4: Branching and Boolean Expressions 2.3.5: Using the if Statement 2.3.6: Using the if-else Statement 2.3.7: if-else if-else if-...-else Statements 2.3.8: Using the switch Statement Exercise 2.3 2.4: Repetition 2.4.1: while Loops 2.4.2: do-while Loops 2.4.3: for Loops 2.4.4: Exiting Early 2.4.5: Nested Loops 2.4.6: Practical Advice Exercise 2.4 Wrap Up Assignments Due By Next Meeting: A2: Decimal Palindromes (2/25/04) Illuminations Homework Questions? A1: Information Please! (2/18/04) Questions from last class? What to do when WebCT has problems Room Policies ^ top 2.1: Data and Expressions Objectives At the end of the lesson the student will be able to: Create variables and assign them values Define the data type of a variable Use expressions in programs ^ top 2.1.1: Data Types Every variable has a data type associated with it Tells the compiler how to store the data Two main kinds of types in Java: Primitive and Class Primitive Data Types Simplest types Atomic values -- cannot decompose into other types Integer data types: byte, short, int, long Default type is int Literal values assigned with "=" sign long integer types have "L" or "l" after the number For example: int i = -215; long max = 12147483648L; Character data type: char char data type is actually a type of integer Represents Unicode character set (letter, digits, symbols) Unicode includes all ASCII codes plus additional ones for languages with an alphabet other than English For example: char letterA = 65; char letterB = 0x0042; char letterC = 'C'; Floating-point types: float, double Floating-point numbers follow standard IEEE 754-1985 double is the default floating-point type Type float needs an "F" or "f" during assignment For example: float pi = 3.14159F; double fraction = 1.0 / 3.0; Boolean type: truth values (i.e. true, false) For example: boolean truth = true; Type Name Kind of Value Memory Used Size Range byte integer 1 byte -128 to 127 short integer 2 bytes -32768 to 32767 int integer 4 bytes -2,147,483,648 to 2,147,483,647 long integer 8 bytes -9,223,372,036,854,775,808 to 9,223,374,036,854,775,807 char single character (Unicode) 2 bytes "all" Unicode characters float floating point 4 bytes +/- 3.4028... x 1038 to +/- 1.4023... x 10-45 double floating point 8 bytes +/- 1.767... x 10308 to +/- 4.940... x 10-324 boolean true or false 1 byte Not applicable ^ top 2.1.2: Variables About Variables A variable is a named location (container) to store data Name corresponds to a location in memory Variables can hold only one type of data (such as int or char) Using variables requires two steps: Declaration Assignment These two steps can be combined into one program statement Declaring Variables All Java program variables must be declared before using them Variable declaration associates a name with a storage location in memory Also specifies the type of data it will store Type Variable_1, Variable_2, ...; For example, the following declares three integer variables: int sum, number1, number2; Assigning Values to Variables Use the assignment operator ("equals sign"): = variable = expression; Assigns value of expression (right side) to the variable (left side) Can have the same variable on both sides of an equals sign int count = 10; // initialize counter to ten count = count + 1; // increment counter Note that the value of the variable changed in the second line Reading variables from memory does not change them Values placed into a variable replace previous values Cannot erase a memory location Assigning Initial Values to Variables Initial values may or may not be assigned when variables are declared: //These may not be initialized when declared //and have unknown values int sum, number1, number2; //These are initialized when declared int sum = 0; int number1 = 5, number2 = 10; Java assigns default values to variables, except local variables inside methods Integer (byte, short, int, long): 0 Floating-point (float, double): 0.0 Character (char): '\u0000' Boolean: false Objects: null Good programming practice: explicitly initialize variables when declared. Constant Variables Java uses the keyword final to indicate a variable that cannot change Usually implemented as a public static final public static final int MY_CONSTANT = 10; static means the variable is part of a class and not an object Will discuss more later ^ top 2.1.3: Expressions Expression -- a part of a program that returns a value Consists of constants, variables, operators, parentheses and methods. Will discuss methods next week int sum, number1, number2; number1 = 5; number2 = 10; sum = number1 + number2; In the last line number1 returns the value 5 and number2 returns the value 10 number1 + number2 is an expression that returns the integer value 15 Examples of Expressions Math Expression Java Expression mc2 m * c * c 2(salary + bonus) 2 * (salary + bonus) 1 time + 3mass 1 / (time + 3 * mass) a - 7 t + 9v (a - 7) / (t + 9 * v) Arithmetic Arithmetic operators: + and - for addition and subtraction * for multiplication / for division % for modulus (integer remainder) No operator for exponentiation (use Math.pow()) Integer division truncates remainder 7 / 5 evaluates to 1 Modulus operator % returns the remainder Used with integer types Returns the remainder of the division of a by b For example: int a = 15; b = 12, c; c = a % b; c has the value 3, the remainder when 15 is divided by 12 Abbreviated Assignment Expressions Any statement of form: variable = variable <op> (expression); May be written in an abbreviated form: variable <op>= expression; <op> is one of: +, -, *, /, or % For example: sum = sum + 5; Can be written as: sum += 5; Increment and Decrement Operators Unary increment operator: ++ Increments variable’s value by 1 Preincrement adds 1 before using value in an expression For example: ++i Post-increment uses current value of expression then adds 1 For example: j++ Unary decrement operator: -- Predecrement subtracts 1 before using value in an expression For example: --i Post-decrement uses current value of expression then subtracts 1 For example: j-- Precedence and Associativity Some arithmetic operators act before others (e.g., multiplication before addition) Java expressions follow rules similar to real-number algebra Arithmetic operators processed in algebraic order: Parentheses ( ) Unary operators: +, -, ++, -- Multiplication, division, modulus: *, /, % Addition, subtraction: +, - Binary operators of same precedence evaluated left to right Use parentheses to force precedence Do not clutter expressions with parentheses when the precedence is correct and obvious However, when in doubt use parentheses Example: Find the average of three variables a, b and c Do not use: a + b + c / 3 Use: (a + b + c ) / 3 Numerical Accuracy Computers store numbers using a fixed number of bits Not every real (floating-point) number can be encoded precisely Infinite number of bits would be required to precisely represent any real number For example, if a computer can represent up to 10 decimal digits, the number 0.9 may be stored as 0.90000004 if that is the closest it can come to 0.9 System.out.println(.8F + .1F); System.out.println(.8 + .1); Integers, on the other hand, are encoded precisely If the value 2 is assigned to an int variable, its value is exactly 2 However, integers can run out of room Causes values to roll-over from highest to lowest What is displayed by the following code? public class Rollover { public static void main(String[] args) { int x = 2147483647; System.out.println(x); x = x + 1; System.out.println(x); } } Java has math classes for numbers of any desired accuracy ^ top 2.1.4: Casting Cast: change the data type of the returned value of an expression Cast means to change the data type of a value returned by an expression Java is a strongly-typed language and checks for compatible data types when compiling To put a value of a different type in a variable, must convert it Casting only changes the type of the returned value -- not the type of the variable For example: double x; int n = 5; x = n; Since n is an integer and x is a double, the value returned by n must be converted to type double before it is assigned to x Implicit Casting Casting is done automatically (implicitly) when a narrower-type is assigned to a broader-type Data type hierarchy (from narrowest to broadest): byte => short => int => long => float => double / char =/ For example: double x; int n = 5; x = n; Value returned by n is cast to type double, then assigned to x x contains 5.000... Called implicit casting because it is done automatically Data type of variable n is unchanged -- still an int Implicit Casting in an Expression Some expressions have a mix of data types All values implicitly cast to the highest level before the calculation For example: double a; int n = 2; float x = 5.1F; double y = 1.33; a = (n * x) / y; n and x are automatically cast to type double before performing the arithmetic Explicit Casting Explicit casting changes the data type for a single use of the variable Precede the variable name with the new data type in parentheses: (<data type>) variableName Data type is changed only for the single use of the returned value For example: int n; double x = 2.0; n = (int) x; Value of x is converted from type double to integer before assigning the value to n Explicit casting is required to assign a higher type to a lower type ILLEGAL: Implicit casting to a lower data type int n; double x = 2.1; n = x; // illegal in java Illegal since x is double, n is an int, and double is a higher data type than int LEGAL: Explicit casting to a lower data type int n; double x = 2.1; n = (int) x; // legal in java Can use an explicit cast even when an implicit one will be done Casting a char to an int Casting a char value to int produces the ASCII/Unicode value For example, what would the following display? char answer = 'Y'; System.out.println(answer); System.out.println((int) answer); Answer: >y >89 Truncation When Casting double to Integer type Converting (casting) a double to integer type does not round -- it truncates Fractional part is lost (discarded, ignored, thrown away) For example: int n; double x = 3.99999; n = (int) x; // x truncated Value of n is 3 Truncation in Integer Division No truncation if at least one of the values is of type float or double All values are promoted to the highest data type Truncation does occur if all the values in a division are integers For example: int a = 4, b = 5, c; double x = 1.5, y; y = b / x; //value returned by b is cast to double //value of y is about 3.33333 c = b / a; //all values are ints so the division //truncates: the value of c is 1 ^ top 2.1.5: Strings Strings are objects in Java Will discuss later how this affects their use However, they have some special sytax to make their use easier String literals are made by enclosing a sequence of characters in double quotes For example: "abc" "b" "3.14159" "$3.95" "My name is Ed" You can assign a value to a String like a primitive type String message = "Please enter an identification code"; Also, you can concatenate two strings using the "+" operator "Java" + " rules!" ^ top 2.1.6: Summary Java has eight primitive data types: Type Bytes Use byte 1 Very short integers from -128 to 127. short 2 Short integers from -32,768 to 32,767. int 4 Integers from -2,147,483,648 to 2,147,483,647. long 8 Long integers from -9,223,372,036,854,775,808 to 9,223,372,036,854,775,807. float 4 Single-precision, floating-point numbers from -3.4E38 to 3.4E38 with 6 or 7 significant digits. double 8 Double-precision, floating-point numbers from -1.7E308 to 1.7E-324 with from 14 to 15 significant digits. char 2 A single Unicode character that’s stored in two bytes. boolean 1 A true (1) or false (0) value. Java uses the following operators for arithmetic + for addition - for subtraction * for multiplication / for division % for modulus (remainder) Results of integer division is truncated Must use modulus operator (%) to get the remainder value Operators have the same precedence as in algebra Parenthesis: ( ) Unary operators: +, - Multiplication, division, modulus: *, /, % Addition, subtraction: +, - More complex mathematical operations are available using the Math class Variables store a value that can change as a program executes. Constants store a value that cannot change. Variables and constants must be declared before use. To initialize a variable or constant, declare a name and assign a value. Simple assignment statements have a variable, equals sign and an expression. variable = expression; The expression is computed before the assignment Java has assignment variations of the form: variable <op>= expression; You can explicitly cast values of an expression to a different type. You must use an explicit cast when assigning a broader type to a narrower type. ^ top Exercise 2.1 Lets go through these one at a time. Take one minute to prepare an answer and then we will discuss it. Given the following code, which statement is true? int a, b = 1; Variable a is not declared Variable b is not declared Variable a is declared but not initialized Variable b is declared but not initialized Neither variable is declared nor initialized What is the value returned by the following expressions? - -1-3 * 10 / 5-1; -8 -6 7 8 10 Starting with the common code: int n = 3; int m = 4; int result; What will be the value of m and result after each of these executes? result = n * ++m; //preincrement m result = n * m++; //postincrement m result = n * --m; //predecrement m result = n * m--; //postdecrement m ^ top 2.2: Console I/O Objectives At the end of the lesson the student will be able to: Display information to a console Get user input from the command line Convert strings into numbers ^ top 2.2.1: Printing to a Console Two useful prewritten methods are print() and println() Their function is to display data The difference between the two methods is that: print() leaves the cursor positioned after the last character println() positions the cursor on a new line Cursor position determines where the next character is displayed System.out.println("Hello, world!"); In this statement, out is the name of an object in a Java class named System System is automatically created whenever a Java program is executed System.out lets a program know to send data to the standard output stream Usually the video display screen The data to display is passed to the method within parenthesis ^ top 2.2.2: Importing Classes About Classes and Packages To make it easier to write programs, Java has many libraries These libraries contain prewritten code Referred to as the Java Application Programming Interface (API) Recall that all Java code is stored in classes Groups of related classes are organized into packages Some Commonly Used Packages Package Name Description java.lang Provides classes fundamental to Java. java.io Provides classes to read and write files. java.txt Provides classes to handle text, dates, and numbers java.util Provides classes to work with collections and miscellaneous utilities. javax.swing Provides classes to create graphical user interfaces and applets. Importing Classes and Packages Classes in the java.lang package are available automatically To use other classes, you must import them import packagename.ClassName; or import packagename.*; Using the '*' wildcard will import all classes in a package For example: import java.text.NumberFormat; import javax.swing.JOptionPane; import javax.swing.JFrame; import javax.swing.*; ^ top 2.2.3: Using System.in System.in is the foundation object for entering data into a program Recall that System.out is used to send data to the standard output stream Usually the video display screen Similarly, System.in is used to get data from the standard input stream Usually the keyboard Streams deliver data as a stream of individual data bytes to the program Java uses a number of objects to manage the streams of data To use the stream objects, we need to import the java.io package: import java.io.*; System.in is an InputStream object that has basic read methods Processes the input data by accepting one or more bytes at a time Each byte is processed as an int Allows using -1 as an end-of-file (EOF) marker Use an InputStreamReader to convert int values of InputStream to char values To program this, we "wrap" an InputStreamReader around System.in InputStreamReader isr = new InputStreamReader(System.in); Although we can now work with the char data types, easier to use strings Class BufferedReader provides a method to read one line at a time To program this, we "wrap" a BufferedReader around our InputStreamReader BufferedReader br = new BufferedReader(isr); Can now use the readLine() method to read one line at a time String data = br.readLine(); Method readLine() collects all the input until the Enter key is pressed Putting everything together into one program, we have: import java.io.*; public class LineInput { public static void main(String[] args) throws IOException { // set up for basic input stream InputStreamReader isr = new InputStreamReader(System.in); // need to use readLine() BufferedReader br = new BufferedReader(isr); System.out.print("Type a line and press Enter: "); String data = br.readLine(); System.out.println("You entered: " + data); } } Note that we added a line of code after the main() method header throws IOException This code is required at this point for us to use the readLine() method Will review the reason later in this lesson ^ top 2.2.4: Reading Numbers Method readLine() accepts input from the user as strings Often need to convert String values to numeric types Java provides some very handy String conversion methods Can use these methods to convert our string input into numbers Conversion to Primitive Values Return Type Conversion Method Example int Integer.parseInt(String) Integer.parseInt("1234") long Long.parseLong(String) Long.parseLong("123456") float Float.parseFloat(String) Float.parseFloat("123.45") double Double.parseDouble(String) Double.parseDouble("123.45") An Example Consider an application to sum (add) two numbers import java.io.*; public class SumOf2 { public static void main(String[] args) throws IOException { double num1, num2; String data; InputStreamReader isr = new InputStreamReader(System.in); BufferedReader br = new BufferedReader(isr); System.out.print("First number: "); data = br.readLine(); num1 = Double.parseDouble(data); System.out.print("Second number: "); data = br.readLine(); num2 = Double.parseDouble(data); System.out.println(num1 + " + " + num2 + " = " + (num1 + num2)); } } We setup for reading from the command line as usual InputStreamReader isr = new InputStreamReader(System.in); BufferedReader br = new BufferedReader(isr); We read each number as a string System.out.print("First number: "); data = br.readLine(); Final step is to convert the String to a double num1 = Double.parseDouble(data); Then we can use the numbers using arithmetic operators System.out.println(num1 + " + " + num2 + " = " + (num1 + num2)); ^ top 2.2.5: Summary You can use the System.out object to print to the console System.out.print() leaves the cursor positioned after the last character System.out.println() positions the cursor on a new line To make it easier to write programs, Java has many libraries of prewritten code You can make use of this code by importing packages import packagename.ClassName; or import packagename.*; For example, to import all the java.io libraries import java.io.*; Alternatively, you can use the fully qualified package name You can use System.in to read data from the command line To use System.in, you need to need to import the java.io.IOException class To convert bytes to chars, you wrap System.in in an InputStreamReader To buffer the input and use the readLine() method, you also wrap the stream in a BufferedReader Thus most code to read from the command line starts like: public static void main(String[] args) throws IOException { InputStreamReader isr = new InputStreamReader(System.in); BufferedReader br = new BufferedReader(isr); ... } To read a string from the command line, you use the readLine() method String input = br.readLine(); If you need a number as an input, you convert the String to a number Return Type Conversion Method Example int Integer.parseInt(String) Integer.parseInt("1234") long Long.parseLong(String) Long.parseLong("123456") float Float.parseFloat(String) Float.parseFloat("123.45") double Double.parseDouble(String) Double.parseDouble("123.45") ^ top Exercise 2.2 Take one minute to prepare an answer the following question. Write a main() method to read an int from the keyboard and display it to the console. ^ top 2.3: Making Decisions Objectives At the end of the lesson the student will be able to: Use relational expressions to produce boolean values Use if Statements Use if-else Statements Use switch Statements ^ top 2.3.1: Flow of Control Determines the order a program executes instructions All programs can be written with three control flow elements: Sequence - continue with the next instruction Sequence is the default operation Selection - a choice between at least two Either continue with the next instruction Or jump to some other instruction if if-else if-else if-else if- ... - else switch Repetition - a loop (repeats a block of code) At the end of the loop: Either go back and repeat the block of code Or continue with the next instruction after the block while do-while for First we will consider Java's selection statements Since selection requires choosing between alternatives, we now examine Java's selection criteria ^ top 2.3.2: Relational Operators Relational operators used to produce a boolean value Math Name Java Example Result = Equal to == 5 == 10 2 == 2 false true ≠ Not equal to != 5 != 10 2 != 2 true false < Less than < 5 < 10 5 < 5 5 < 2 true false false ≤ Less than or equal to <= 5 <= 10 5 <= 5 5 <= 2 true true false > Greater than > 5 > 10 5 > 5 5 > 2 false false true ≥ Greater than or equal to >= 5 >= 10 5 >= 5 5 >= 2 false true true Java Comparison Methods for String Class "==" does not act like you may think for String objects "==" tests to see if the storage addresses of the two objects are the same Must use .equals method to test if the strings contents are equal String s1 = new String("Fred"); String s2 = new String("Fred"); System.out.println(s1 == s2); // prints false System.out.println(s1.equals(s2)); // prints true Method equals() is case sensitive Use equalsIgnoreCase() to ignore case Numerical Accuracy Problem Problems can occur with relational expressions using floating-point numbers Tests for equality (==) should be avoided using floating-point numbers Reason is that many decimal numbers (e.g. 0.1) cannot be represented exactly For example: Cannot find file: examples/Inexact1.java Thus, testing for equality for such numbers may fail When equality of non-integer values is needed, there is a possible solution Can test that the absolute value of the difference is less than some small number Instead of: operandOne == operandTwo Replace the condition with: Math.abs(operandOne - operandTwo) < EPSILON For example, instead of: x/y == .35 Replace the condition with: Math.abs(x/y - .35) < .0000001 ^ top 2.3.3: Logical Operators Sometimes must consider several logical values at same time Logical operators allow you to consider multiple cases and to change conditions Use ! operator to reverse value of expression Use && to AND two or more conditions Use || to OR two or more conditions ! Operator If expr is... Then ! expr is... Example Result true false !true false false true !(5 < 2) true && operator returns true only if expressions on both sides are true && Operator If expr1 is... And expr2 is... Then expr1 && expr2 is... Example Result true true true 5 < 10 && 5 > 2 true true false false 5 < 10 && 5 < 2 false false true false 5 > 10 && 5 > 2 false false false false 5 > 10 && 5 < 2 false || operator returns true if either expression is true || Operator If expr1 is... || expr2 is... Then expr1 || expr2 is... Example Result true true true 5 < 10 || 5 > 2 true true false true 5 < 10 || 5 < 2 true false true true 5 > 10 || 5 > 2 true false false false 5 > 10 || 5 < 2 false Common Mistake with Logical Expressions Logical expressions often read like "normal" English. However, Java syntax requires more detail. For example we might say in English: A equals B or C, but the corresponding boolean expression is A == B || A == C ^ top 2.3.4: Branching and Boolean Expressions Branching implies there is more than one choice for the next instruction Which branch is taken depends on a test condition A test condition evaluates to either true or false If a test is true then do this otherwise it is false so do something else Variables that are either true or false are called boolean variables Boolean expressions return either true or false ^ top 2.3.5: Using the if Statement Executes a section of code if a condition is true if (boolean expression is true) { Action1 //execute only if true } If condition is not true, Java skips the code What is displayed by running the following code? import java.awt.*; import javax.swing.*; public class IfThen extends JApplet { private JTextArea out = new JTextArea(); public void init() { getContentPane().add(out, BorderLayout.CENTER); //----------------------------- int errorCount = 1; out.append("OK so far\n"); if (errorCount > 0) { out.append("Oops! We got an error!\n"); } out.append("What happened?\n"); } } See it! Note that code is indented between the if and } for readability What would be displayed if intErrorCount = 0? Multiple Conditions With logical operators (&&, ||, !), can test for multiple conditions For example, can create condition that displays only if two conditions are true What is displayed by running the following code? import java.awt.*; import javax.swing.*; public class IfThen2 extends JApplet { private JTextArea out = new JTextArea(); public void init() { getContentPane().add(out, BorderLayout.CENTER); //----------------------------- int x = 1, y = 2; out.append("OK so far\n"); if (x == 1 && y == 2) { out.append("Its true!\n"); } out.append("What happened?"); } } See it! ^ top 2.3.6: Using the if-else Statement Tests a condition to determine if true or false if (boolean expression is true) { Action1 //execute only if true } else { Action2 //execute only if false } If true, executes the first block {...} of code If false, executes second block of code What is displayed by running the following code? import java.awt.*; import javax.swing.*; public class IfElse extends JApplet { private JTextArea out = new JTextArea(); public void init() { getContentPane().add(out, BorderLayout.CENTER); //----------------------------- int errorCount = 0; out.append("OK so far\n"); if (errorCount > 0) { out.append("Oops! We got an error!\n"); } else { out.append("Everything is still OK\n"); } out.append("What happened?"); //----------------------------- } } See it! Nested If Statements Can include if statements within other if statements Each if statement evaluated only if outer condition is met What is displayed for the following? import java.awt.*; import javax.swing.*; public class IfNested extends JApplet { private JTextArea out = new JTextArea(); public void init() { getContentPane().add(out, BorderLayout.CENTER); //----------------------------- double temp = 77.0; if (temp > 65) { if (temp > 75) { out.append("Temperature is too hot\n"); } else { out.append("Temperature is just right\n"); } } else { out.append("Its cold!\n"); } //----------------------------- } } See it! Nested conditionals can be confusing if too deep Rule of thumb: no more than three deep ^ top 2.3.7: if-else if-else if-...-else Statement Tests multiple conditions until the first true condition is found If true condition is found, executes code within the block What is displayed by running the following code? import java.awt.*; import javax.swing.*; public class IfElseIf extends JApplet { private JTextArea out = new JTextArea(); public void init() { getContentPane().add(out, BorderLayout.CENTER); //----------------------------- double temp = 77.0; if (temp > 75) { out.append("Temperature is too hot!"); } else if (temp > 65) { out.append("Temperature is just right."); } else { out.append("Its cold!"); } //----------------------------- } } See it! Note nesting is now only one level deep "Dangling Else" Problem if (Expression1) if (Expression2) Statement1 else Statement2 Indentation is misleading if (Expression1) if (Expression2) Statement1 else Statement2 Common mistake is not matching else with the correct if Rule: else always matches the last unmatched if Use braces to force a different grouping ^ top 2.3.8: Using the switch Statement Executes a section of code depending on value of a variable switch (integerExpression) { case label1: statements break; case label2: statements break; ... case labeln: statements break; default: statements break; } Code executes if variable matches the case label value Use default for default conditions What is displayed by running the following code? import java.awt.*; import javax.swing.*; public class Switch extends JApplet { private JTextArea out = new JTextArea(); public void init() { getContentPane().add(out, BorderLayout.CENTER); //----------------------------- final int PARCEL_POST = 0; final int UPS = 1; final int FED_EX = 2; int shippingOption = UPS; switch (shippingOption) { case PARCEL_POST: out.append("Delivery in two weeks."); break; case UPS: out.append("Delivery in three days."); break; case FED_EX: out.append("Overnight delivery."); break; default: out.append("It may never get there!"); } //----------------------------- } } See it! ^ top Exercise 2.3 Take one minute to prepare an answer the following questions. What will happen if we attempt to compile and run the following code? class IfTest { public static void main(String[] args) { if (true) if (false) System.out.println("first"); else System.out.println("second"); } } What, if anything, is wrong with the following code? class SwitchTest { public static void main(String[] args) { int x = 1; switch(x) { case 1: case 2: case 0: default: case 5: } } } ^ top 2.4: Repetition Objectives At the end of the lesson the student will be able to: Select the appropriate repetition structure in a Java program Use while Loops Use do-while Loops Use for Loops Programs often need to repeat series of steps This process called looping ^ top 2.4.1: while Loops General loop structure: Initialization code Loop condition -- evaluated during the loop Loop body while loop Syntax: //Initialization ... while (loopTest) { //loop condition //loop body ... } Initialization statements usually precede the loop for while loops Checks loopTest at start of every loop iteration Continues as long as loopTest is true Note that if the loop condition is initially false then loop body never executes Several logical organizations and uses of loops Counter-controlled loops Sentinel-controlled loops Infinite loops Counter-Controlled Loop Executed a specified number of times Use a variable in the loop test called the counter Counter must be incremented every time the body of the loop is executed What is displayed for the following? class Counter5 { public static void main(String[] args) { //Initialization int count = 0; while (count < 5) { //loop condition //loop body ++count; System.out.println(count); } System.out.println("After count= " + count); } } Sentinel-Controlled Loop Used when we do not know in advance how many iterations are needed Commonly used for reading input from a user or file Use a sentinel value to tell the program that there is no more data Looping continues as long as the data value read is not the sentinel When the sentinel is read, execution continues at the statement after the loop Following example loops to sum positive numbers entered by the user import java.io.*; public class SumInput { public static void main(String[] args) throws IOException { InputStreamReader isr = new InputStreamReader(System.in); BufferedReader br = new BufferedReader(isr); System.out.println("Enter positive numbers"); // Initialization double input = 0.0; double total = 0.0; while (input >= 0.0) { // Loop condition // Loop body total = total + input; String data = br.readLine(); input = Double.parseDouble(data); } System.out.println("Total:" + total); } } Sentinel variable is input Sentinel value is any negative number Terminates when the user enters a negative number Infinite loops Common error: infinite loop Sometimes we do want loops to continue indefinitely How would we intentionally write an infinite while-loop? ^ top 2.4.2: do-while Loops Syntax: //Initialization ... do { //loop body ... } while (loopTest); //loop condition Initialization code may precede loop body Loop test is after loop body Loop must execute at least once (minimum of at least one iteration) May be either counting- or sentinel-controlled loop Use when you want to force a minimum of one iteration Sentinel-Controlled-Loop Example import java.io.*; public class SumInputDo { public static void main(String[] args) throws IOException { InputStreamReader isr = new InputStreamReader(System.in); BufferedReader br = new BufferedReader(isr); System.out.println("Enter positive numbers"); // Initialization double input = 0.0; double total = 0.0; do { // Loop body total += input; String data = br.readLine(); input = Double.parseDouble(data); } while (input >= 0.0); // Loop condition System.out.println("Total:" + total); } } ^ top 2.4.3: for Loops Syntax: for (initialization; loopTest; incrementExpression) { //loop body ... } Initialization, loop-test, and increment-expression are part of the syntax Mostly used for counter-controlled loops Execution sequence as follows: When for statement reached -- initialization executes Check if loop-test is true if true then continue with Step 3 Otherwise, continue with Step 6 Execute block containing the loop body When end of loop body is reached, execute the incrementExpression Return to Step 2 Loop is finished: Continue with statements after the loop Counter-Controlled-Loop Example What is displayed by the following code? class CounterFor { public static void main(String[] args) { for (int count = 0; count < 5; ++count) { System.out.println(count); } } } Note that code is indented in the loop body for readability Stepping Down Can also count downwards Positive numbers step higher (+inf) and negative numbers step lower (-inf) What is displayed with the following? class CountDown { public static void main(String[] args) { int count = 0; for (count = 5; count > 0 ; --count) { System.out.println(count); } System.out.println("After count= " + count); } } ^ top 2.4.4: Exiting Early break statement can be used to exit loops Transfers control to statement following the loop Provides a way to break out of an intentional infinite loop class BreakOut { public static void main(String[] args) { int count = 0; while (true) { ++count; System.out.println(count); if (count >= 5) break; } System.out.println("After count= " + count); } } ^ top 2.4.5: Nested Loops Often necessary to have one loop inside another Following example shows this structure Let's follow the execution sequence before checking the result class Nested { public static void main(String[] args) { for (int outer = 1; outer < 4; outer++) { for (int inner = 1; inner < 4; inner++) { System.out.println(outer + " " + inner); } } } } ^ top 2.4.6: Practical Advice Loopy Errors Most common errors with loops: Unintended infinite loops Off-by-one errors in counter-controlled loops Everyone writes an unintentional infinite loop To get out of an unintended infinite loop: ^C (control-C) keys Thoroughly test loops -- especially at the boundaries of the loop test Check for off-by-one and other error conditions ^ top Exercise 2.4 Take one minute to prepare an answer the following: for loops and while loops can be used interchangeably. Convert the following code from a for-loop to a while-loop. class CountDown { public static void main(String[] args) { int count = 0; for (count = 5; count > 0 ; --count) { System.out.println(count); } System.out.println("After count= " + count); } } ^ top Wrap Up Due Next Class: A2: Decimal Palindromes (2/25/04) ^ top Home | WebCT | Announcements | Schedule | Expectations | Course info Help | FAQ's | HowTo's | Links Last Updated: March 02 2004 @14:02:24