CSC 243 Chapter 2, Spring 2015

Copyright © 2014 Pearson Education, Inc. Chapter 2 Data and Expressions Java Software Solutions Foundations of Program Design John Lewis William Loftus Outline Character Strings Variables and Assignment Primitive Data Types Expressions Data Conversion Interactive Programs Copyright © 2014 Pearson Education, Inc. Character Strings • A string literal is represented by putting double quotes around the text • Examples: "This is a string literal." "123 Main Street" "X" • Every character string is an object in Java, defined by the String class • Every string literal represents a String object Copyright © 2014 Pearson Education, Inc. The println Method • In the Lincoln program from Chapter 1, we invoked the println method to print a character string • The System.out object represents a destination (the monitor screen) to which we can send output System.out.println ("Whatever you are, be a good one."); object object object object method method method method name name name name information provided to the method information provided to the method information provided to the method information provided to the method (parameters) (parameters) (parameters) (parameters) Copyright © 2014 Pearson Education, Inc. The print Method • The System.out object provides another service as well • The print method is similar to the println method, except that it does not advance to the next line • Therefore anything printed after a print statement will appear on the same line • See Countdown.java Copyright © 2014 Pearson Education, Inc. Copyright © 2014 Pearson Education, Inc. //******************************************************************** // Countdown.java Author: Lewis/Loftus // // Demonstrates the difference between print and println. //******************************************************************** public class Countdown { //----------------------------------------------------------------- // Prints two lines of output representing a rocket countdown. //----------------------------------------------------------------- public static void main(String[] args) { System.out.print("Three... "); System.out.print("Two... "); System.out.print("One... "); System.out.print("Zero... "); System.out.println("Liftoff!"); // appears on first output line System.out.println("Houston, we have a problem."); } } Copyright © 2014 Pearson Education, Inc. //******************************************************************** // Countdown.java Author: Lewis/Loftus // // Demonstrates the difference between print and println. //******************************************************************** public class Countdown { //----------------------------------------------------------------- // Prints two lines of output representing a rocket countdown. //----------------------------------------------------------------- public static void main(String[] args) { System.out.print("Three... "); System.out.print("Two... "); System.out.print("One... "); System.out.print("Zero... "); System.out.println("Liftoff!"); // appears on first output line System.out.println("Houston, we have a problem."); } } Output Three... Two... One... Zero... Liftoff! Houston, we have a problem. String Concatenation • The string concatenation operator (+) is used to append one string to the end of another "Peanut butter " + "and jelly" • It can also be used to append a number to a string • A string literal cannot be broken across two lines in a program • See Facts.java Copyright © 2014 Pearson Education, Inc. Copyright © 2014 Pearson Education, Inc. //******************************************************************** // Facts.java Author: Lewis/Loftus // // Demonstrates the use of the string concatenation operator and the // automatic conversion of an integer to a string. //******************************************************************** public class Facts { //----------------------------------------------------------------- // Prints various facts. //----------------------------------------------------------------- public static void main(String[] args) { // Strings can be concatenated into one long string System.out.println("We present the following facts for your " + "extracurricular edification:"); System.out.println(); // A string can contain numeric digits System.out.println("Letters in the Hawaiian alphabet: 12"); continue Copyright © 2014 Pearson Education, Inc. continue // A numeric value can be concatenated to a string System.out.println("Dialing code for Antarctica: " + 672); System.out.println("Year in which Leonardo da Vinci invented " + "the parachute: " + 1515); System.out.println("Speed of ketchup: " + 40 + " km per year"); } } Copyright © 2014 Pearson Education, Inc. continue // A numeric value can be concatenated to a string System.out.println ("Dialing code for Antarctica: " + 672); System.out.println ("Year in which Leonardo da Vinci invented " + "the parachute: " + 1515); System.out.println("Speed of ketchup: " + 40 + " km per year"); } } Output We present the following facts for your extracurricular edification: Letters in the Hawaiian alphabet: 12 Dialing code for Antarctica: 672 Year in which Leonardo da Vinci invented the parachute: 1515 Speed of ketchup: 40 km per year String Concatenation • The + operator is also used for arithmetic addition • The function that it performs depends on the type of the information on which it operates • If both operands are strings, or if one is a string and one is a number, it performs string concatenation • If both operands are numeric, it adds them • The + operator is evaluated left to right, but parentheses can be used to force the order • See Addition.java Copyright © 2014 Pearson Education, Inc. Copyright © 2014 Pearson Education, Inc. //******************************************************************** // Addition.java Author: Lewis/Loftus // // Demonstrates the difference between the addition and string // concatenation operators. //******************************************************************** public class Addition { //----------------------------------------------------------------- // Concatenates and adds two numbers and prints the results. //----------------------------------------------------------------- public static void main(String[] args) { System.out.println("24 and 45 concatenated: " + 24 + 45); System.out.println("24 and 45 added: " + (24 + 45)); } } Copyright © 2014 Pearson Education, Inc. //******************************************************************** // Addition.java Author: Lewis/Loftus // // Demonstrates the difference between the addition and string // concatenation operators. //******************************************************************** public class Addition { //----------------------------------------------------------------- // Concatenates and adds two numbers and prints the results. //----------------------------------------------------------------- public static void main(String[] args) { System.out.println("24 and 45 concatenated: " + 24 + 45); System.out.println("24 and 45 added: " + (24 + 45)); } } Output 24 and 45 concatenated: 2445 24 and 45 added: 69 Quick Check Copyright © 2014 Pearson Education, Inc. What output is produced by the following? System.out.println("X: " + 25); System.out.println("Y: " + (15 + 50)); System.out.println("Z: " + 300 + 50); Quick Check Copyright © 2014 Pearson Education, Inc. What output is produced by the following? System.out.println("X: " + 25); System.out.println("Y: " + (15 + 50)); System.out.println("Z: " + 300 + 50); X: 25 Y: 65 Z: 30050 Escape Sequences • What if we wanted to print the quote character? • The following line would confuse the compiler because it would interpret the second quote as the end of the string System.out.println("I said "Hello" to you."); • An escape sequence is a series of characters that represents a special character • An escape sequence begins with a backslash character (\) System.out.println("I said \"Hello\" to you."); Copyright © 2014 Pearson Education, Inc. Escape Sequences • Some Java escape sequences: Escape Sequence Escape Sequence Escape Sequence Escape Sequence \b \t \n \r \" \' \\ Meaning Meaning Meaning Meaning backspace backspace backspace backspace tab tab tab tab newline newline newline newline carriage return carriage return carriage return carriage return double quote double quote double quote double quote single quote single quote single quote single quote backslash backslash backslash backslash Copyright © 2014 Pearson Education, Inc. • See Roses.java Copyright © 2014 Pearson Education, Inc. //******************************************************************** // Roses.java Author: Lewis/Loftus // // Demonstrates the use of escape sequences. //******************************************************************** public class Roses { //----------------------------------------------------------------- // Prints a poem (of sorts) on multiple lines. //----------------------------------------------------------------- public static void main(String[] args) { System.out.println("Roses are red,\n\tViolets are blue,\n" + "Sugar is sweet,\n\tBut I have \"commitment issues\",\n\t" + "So I'd rather just be friends\n\tAt this point in our " + "relationship."); } } Copyright © 2014 Pearson Education, Inc. //******************************************************************** // Roses.java Author: Lewis/Loftus // // Demonstrates the use of escape sequences. //******************************************************************** public class Roses { //----------------------------------------------------------------- // Prints a poem (of sorts) on multiple lines. //----------------------------------------------------------------- public static void main (String[] args) { System.out.println ("Roses are red,\n\tViolets are blue,\n" + "Sugar is sweet,\n\tBut I have \"commitment issues\",\n\t" + "So I'd rather just be friends\n\tAt this point in our " + "relationship."); } } Output Roses are red, Violets are blue, Sugar is sweet, But I have "commitment issues", So I'd rather just be friends At this point in our relationship. Quick Check Copyright © 2014 Pearson Education, Inc. Write a single println statement that produces the following output: "Thank you all for coming to my home tonight," he said mysteriously. Quick Check Copyright © 2014 Pearson Education, Inc. Write a single println statement that produces the following output: "Thank you all for coming to my home tonight," he said mysteriously. System.out.println("\"Thank you all for " + "coming to my home\ntonight,\" he said " + "mysteriously."); Outline Character Strings Variables and Assignment Primitive Data Types Expressions Data Conversion Interactive Programs Graphics Applets Drawing Shapes Copyright © 2014 Pearson Education, Inc. Variables • A variable is a name for a location in memory that holds a value • A variable declaration specifies the variable's name and the type of information that it will hold int total; int count, temp, result; Multiple variables can be created in one declaration Multiple variables can be created in one declaration Multiple variables can be created in one declaration Multiple variables can be created in one declaration data type data type data type data type variable name variable name variable name variable name Copyright © 2014 Pearson Education, Inc. Variable Initialization • A variable can be given an initial value in the declaration int sum = 0; int base = 32, max = 149; Copyright © 2014 Pearson Education, Inc. • When a variable is referenced in a program, its current value is used • See PianoKeys.java Copyright © 2014 Pearson Education, Inc. //******************************************************************** // PianoKeys.java Author: Lewis/Loftus // // Demonstrates the declaration, initialization, and use of an // integer variable. //******************************************************************** public class PianoKeys { //----------------------------------------------------------------- // Prints the number of keys on a piano. //----------------------------------------------------------------- public static void main(String[] args) { int keys = 88; System.out.println("A piano has " + keys + " keys."); } } Copyright © 2014 Pearson Education, Inc. //******************************************************************** // PianoKeys.java Author: Lewis/Loftus // // Demonstrates the declaration, initialization, and use of an // integer variable. //******************************************************************** public class PianoKeys { //----------------------------------------------------------------- // Prints the number of keys on a piano. //----------------------------------------------------------------- public static void main(String[] args) { int keys = 88; System.out.println("A piano has " + keys + " keys."); } } Output A piano has 88 keys. Assignment • An assignment statement changes the value of a variable • The assignment operator is the = sign total = 55; Copyright © 2014 Pearson Education, Inc. • The value that was in total is overwritten • You can only assign a value to a variable that is consistent with the variable's declared type • See Geometry.java Copyright © 2014 Pearson Education, Inc. //******************************************************************** // Geometry.java Author: Lewis/Loftus // // Demonstrates the use of an assignment statement to change the // value stored in a variable. //******************************************************************** public class Geometry { //----------------------------------------------------------------- // Prints the number of sides of several geometric shapes. //----------------------------------------------------------------- public static void main(String[] args) { int sides = 7; // declaration with initialization System.out.println("A heptagon has " + sides + " sides."); sides = 10; // assignment statement System.out.println("A decagon has " + sides + " sides."); sides = 12; System.out.println("A dodecagon has " + sides + " sides."); } } Copyright © 2014 Pearson Education, Inc. //******************************************************************** // Geometry.java Author: Lewis/Loftus // // Demonstrates the use of an assignment statement to change the // value stored in a variable. //******************************************************************** public class Geometry { //----------------------------------------------------------------- // Prints the number of sides of several geometric shapes. //----------------------------------------------------------------- public static void main (String[] args) { int sides = 7; // declaration with initialization System.out.println ("A heptagon has " + sides + " sides."); sides = 10; // assignment statement System.out.println ("A decagon has " + sides + " sides."); sides = 12; System.out.println ("A dodecagon has " + sides + " sides."); } } Output A heptagon has 7 sides. A decagon has 10 sides. a dodecagon has 12 sides. Constants • A constant is an identifier that is similar to a variable except that it holds the same value during its entire existence • As the name implies, it is constant, not variable • The compiler will issue an error if you try to change the value of a constant • In Java, we use the final modifier to declare a constant final int MIN_HEIGHT = 69; Copyright © 2014 Pearson Education, Inc. Constants • Constants are useful for three important reasons • First, they give meaning to otherwise unclear literal values – Example: MAX_LOAD means more than the literal 250 • Second, they facilitate program maintenance – If a constant is used in multiple places, its value need only be set in one place • Third, they formally establish that a value should not change, avoiding inadvertent errors by other programmers Copyright © 2014 Pearson Education, Inc. Outline Character Strings Variables and Assignment Primitive Data Types Expressions Data Conversion Interactive Programs Copyright © 2014 Pearson Education, Inc. Primitive Data • There are eight primitive data types in Java • Four of them represent integers: – byte, short, int, long • Two of them represent floating point numbers: – float, double • One of them represents characters: – char • And one of them represents boolean values: – boolean Copyright © 2014 Pearson Education, Inc. Numeric Primitive Data • The difference between the numeric primitive types is their size and the values they can store: Type Type Type Type byte short int long float double Storage Storage Storage Storage 8 bits 8 bits 8 bits 8 bits 16 bits 16 bits 16 bits 16 bits 32 bits 32 bits 32 bits 32 bits 64 bits 64 bits 64 bits 64 bits 32 bits 32 bits 32 bits 32 bits 64 bits 64 bits 64 bits 64 bits Min Value Min Value Min Value Min Value ----128 128 128 128 ----32,768 32,768 32,768 32,768 ----2,147,483,648 2,147,483,648 2,147,483,648 2,147,483,648 < < < < ----9 x 10 9 x 10 9 x 10 9 x 1018 18 18 18 +/ +/ +/ +/---- 3.4 x 10 3.4 x 10 3.4 x 10 3.4 x 1038 38 38 38 with 7 significant digits with 7 significant digits with 7 significant digits with 7 significant digits +/ +/ +/ +/---- 1.7 x 10 1.7 x 10 1.7 x 10 1.7 x 10308 308 308 308 with 15 significant digits with 15 significant digits with 15 significant digits with 15 significant digits Max Value Max Value Max Value Max Value 127 127 127 127 32,767 32,767 32,767 32,767 2,147,483,647 2,147,483,647 2,147,483,647 2,147,483,647 > 9 x 10 > 9 x 10 > 9 x 10 > 9 x 1018 18 18 18 Copyright © 2014 Pearson Education, Inc. Characters • A char variable stores a single character • Character literals are delimited by single quotes: 'a' 'X' '7' '$' ',' '\n' • Example declarations: char topGrade = 'A'; char terminator = ';', separator = ' '; • Note the difference between a primitive character variable, which holds only one character, and a String object, which can hold multiple characters Copyright © 2014 Pearson Education, Inc. Character Sets • A character set is an ordered list of characters, with each character corresponding to a unique number • A char variable in Java can store any character from the Unicode character set • The Unicode character set uses sixteen bits per character, allowing for 65,536 unique characters • It is an international character set, containing symbols and characters from many world languages Copyright © 2014 Pearson Education, Inc. Characters • The ASCII character set is older and smaller than Unicode, but is still quite popular • The ASCII characters are a subset of the Unicode character set, including: uppercase letters uppercase letters uppercase letters uppercase letters lowercase letters lowercase letters lowercase letters lowercase letters punctuation punctuation punctuation punctuation digits digits digits digits special symbols special symbols special symbols special symbols control characters control characters control characters control characters A, B, C, … A, B, C, … A, B, C, … A, B, C, … a, b, c, … a, b, c, … a, b, c, … a, b, c, … period, semi period, semi period, semi period, semi----colon, … colon, … colon, … colon, … 0, 1, 2, … 0, 1, 2, … 0, 1, 2, … 0, 1, 2, … &, |, &, |, &, |, &, |, \\\\, … , … , … , … carriage return, tab, ... carriage return, tab, ... carriage return, tab, ... carriage return, tab, ... Copyright © 2014 Pearson Education, Inc. Boolean • A boolean value represents a true or false condition • The reserved words true and false are the only valid values for a boolean type boolean done = false; • A boolean variable can also be used to represent any two states, such as a light bulb being on or off Copyright © 2014 Pearson Education, Inc. Outline Character Strings Variables and Assignment Primitive Data Types Expressions Data Conversion Interactive Programs Copyright © 2014 Pearson Education, Inc. Expressions • An expression is a combination of one or more operators and operands • Arithmetic expressions compute numeric results and make use of the arithmetic operators: Addition Addition Addition Addition Subtraction Subtraction Subtraction Subtraction Multiplication Multiplication Multiplication Multiplication Division Division Division Division Remainder Remainder Remainder Remainder ++++ ---- **** //// % % % % Copyright © 2014 Pearson Education, Inc. • If either or both operands are floating point values, then the result is a floating point value Division and Remainder • If both operands to the division operator (/) are integers, the result is an integer (the fractional part is discarded) 14 / 3 equals equals equals equals 4444 8 / 12 equals equals equals equals 0000 Copyright © 2014 Pearson Education, Inc. • The remainder operator (%) returns the remainder after dividing the first operand by the second 14 % 3 equals equals equals equals 2222 8 % 12 equals equals equals equals 8888 Quick Check Copyright © 2014 Pearson Education, Inc. What are the results of the following expressions? 12 / 2 12.0 / 2.0 10 / 4 10 / 4.0 4 / 10 4.0 / 10 12 % 3 10 % 3 3 % 10 Quick Check Copyright © 2014 Pearson Education, Inc. What are the results of the following expressions? 12 / 2 12.0 / 2.0 10 / 4 10 / 4.0 4 / 10 4.0 / 10 12 % 3 10 % 3 3 % 10 = 6 = 6.0 = 2 = 2.5 = 0 = 0.4 = 0 = 1 = 0 Operator Precedence • Operators can be combined into larger expressions result = total + count / max - offset; • Operators have a well-defined precedence which determines the order in which they are evaluated • Multiplication, division, and remainder are evaluated before addition, subtraction, and string concatenation • Arithmetic operators with the same precedence are evaluated from left to right, but parentheses can be used to force the evaluation order Copyright © 2014 Pearson Education, Inc. Quick Check a + b + c + d + e a + b * c - d / e a / (b + c) - d % e a / (b * (c + (d - e))) Copyright © 2014 Pearson Education, Inc. In what order are the operators evaluated in the following expressions? Quick Check a + b + c + d + e a + b * c - d / e a / (b + c) - d % e a / (b * (c + (d - e))) 1 4 3 2 3 2 4 1 2 3 4 1 4 1 2 3 Copyright © 2014 Pearson Education, Inc. In what order are the operators evaluated in the following expressions? Expression Trees • The evaluation of a particular expression can be shown using an expression tree • The operators lower in the tree have higher precedence for that expression a + (b – c) / d a + / - d b c Copyright © 2014 Pearson Education, Inc. Assignment Revisited • The assignment operator has a lower precedence than the arithmetic operators First the expression on the right hand First the expression on the right hand First the expression on the right hand First the expression on the right hand side of the = operator is evaluated side of the = operator is evaluated side of the = operator is evaluated side of the = operator is evaluated Then the result is stored in the Then the result is stored in the Then the result is stored in the Then the result is stored in the variable on the left hand side variable on the left hand side variable on the left hand side variable on the left hand side answer = sum / 4 + MAX * lowest; 1 4 3 2 Copyright © 2014 Pearson Education, Inc. Assignment Revisited • The right and left hand sides of an assignment statement can contain the same variable First, one is added to the First, one is added to the First, one is added to the First, one is added to the original value of original value of original value of original value of count Then the result is stored back into Then the result is stored back into Then the result is stored back into Then the result is stored back into count (overwriting the original value) (overwriting the original value) (overwriting the original value) (overwriting the original value) count = count + 1; Copyright © 2014 Pearson Education, Inc. Increment and Decrement • The increment (++) and decrement (--) operators use only one operand • The statement count++; is functionally equivalent to count = count + 1; Copyright © 2014 Pearson Education, Inc. Increment and Decrement • The increment and decrement operators can be applied in postfix form: count++ • or prefix form: ++count • When used as part of a larger expression, the two forms can have different effects • Because of their subtleties, the increment and decrement operators should be used with care Copyright © 2014 Pearson Education, Inc. Assignment Operators • Often we perform an operation on a variable, and then store the result back into that variable • Java provides assignment operators to simplify that process • For example, the statement num += count; is equivalent to num = num + count; Copyright © 2014 Pearson Education, Inc. Assignment Operators • There are many assignment operators in Java, including the following: Operator Operator Operator Operator += -= *= /= %= Example Example Example Example x += y x -= y x *= y x /= y x %= y Equivalent To Equivalent To Equivalent To Equivalent To x = x + y x = x - y x = x * y x = x / y x = x % y Copyright © 2014 Pearson Education, Inc. Assignment Operators • The right hand side of an assignment operator can be a complex expression • The entire right-hand expression is evaluated first, then the result is combined with the original variable • Therefore result /= (total-MIN) % num; is equivalent to result = result / ((total-MIN) % num); Copyright © 2014 Pearson Education, Inc. Assignment Operators • The behavior of some assignment operators depends on the types of the operands • If the operands to the += operator are strings, the assignment operator performs string concatenation • The behavior of an assignment operator (+=) is always consistent with the behavior of the corresponding operator (+) Copyright © 2014 Pearson Education, Inc. Outline Character Strings Variables and Assignment Primitive Data Types Expressions Data Conversion Interactive Programs Copyright © 2014 Pearson Education, Inc. Data Conversion • Sometimes it is convenient to convert data from one type to another • For example, in a particular situation we may want to treat an integer as a floating point value • These conversions do not change the type of a variable or the value that's stored in it – they only convert a value as part of a computation Copyright © 2014 Pearson Education, Inc. Data Conversion • Widening conversions are safest because they tend to go from a small data type to a larger one (such as a short to an int) • Narrowing conversions can lose information because they tend to go from a large data type to a smaller one (such as an int to a short) • In Java, data conversions can occur in three ways: – assignment conversion – promotion – casting Copyright © 2014 Pearson Education, Inc. Data Conversion Copyright © 2014 Pearson Education, Inc. Widening Conversions Narrowing Conversions Assignment Conversion • Assignment conversion occurs when a value of one type is assigned to a variable of another • Example: int dollars = 20; double money = dollars; • Only widening conversions can happen via assignment • Note that the value or type of dollars did not change Copyright © 2014 Pearson Education, Inc. Promotion • Promotion happens automatically when operators in expressions convert their operands • Example: int count = 12; double sum = 490.27; result = sum / count; • The value of count is converted to a floating point value to perform the division calculation Copyright © 2014 Pearson Education, Inc. Casting • Casting is the most powerful, and dangerous, technique for conversion • Both widening and narrowing conversions can be accomplished by explicitly casting a value • To cast, the type is put in parentheses in front of the value being converted int total = 50; float result = (float) total / 6; • Without the cast, the fractional part of the answer would be lost Copyright © 2014 Pearson Education, Inc. Outline Character Strings Variables and Assignment Primitive Data Types Expressions Data Conversion Interactive Programs Copyright © 2014 Pearson Education, Inc. Interactive Programs • Programs generally need input on which to operate • The Scanner class provides convenient methods for reading input values of various types • A Scanner object can be set up to read input from various sources, including the user typing values on the keyboard • Keyboard input is represented by the System.in object Copyright © 2014 Pearson Education, Inc. Reading Input • The following line creates a Scanner object that reads from the keyboard: Scanner scan = new Scanner(System.in); • The new operator creates the Scanner object • Once created, the Scanner object can be used to invoke various input methods, such as: answer = scan.nextLine(); Copyright © 2014 Pearson Education, Inc. Reading Input • The Scanner class is part of the java.util class library, and must be imported into a program to be used • The nextLine method reads all of the input until the end of the line is found • See Echo.java • The details of object creation and class libraries are discussed further in Chapter 3 Copyright © 2014 Pearson Education, Inc. Copyright © 2014 Pearson Education, Inc. //******************************************************************** // Echo.java Author: Lewis/Loftus // // Demonstrates the use of the nextLine method of the Scanner class // to read a string from the user. //******************************************************************** import java.util.Scanner; public class Echo { //----------------------------------------------------------------- // Reads a character string from the user and prints it. //----------------------------------------------------------------- public static void main(String[] args) { String message; Scanner scan = new Scanner(System.in); System.out.println("Enter a line of text:"); message = scan.nextLine(); System.out.println("You entered: \"" + message + "\""); } } Copyright © 2014 Pearson Education, Inc. //******************************************************************** // Echo.java Author: Lewis/Loftus // // Demonstrate