Chapter2.pdf

Java Software Solutions Foundations of Program Design Sixth Edition Chapter 2: Data and Expressions Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley by Lewis & Loftus Data and Expressions ? Let's explore some other fundamental programming concepts ? Chapter 2 focuses on: ? character strings ? primitive data Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-2 ? the declaration and use of variables ? expressions and operator precedence ? data conversions ? accepting input from the user ? Java applets ? introduction to graphics Outline Character Strings Variables and Assignment Primitive Data Types Expressions Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-3 Data Conversion Interactive Programs Graphics Applets Drawing Shapes Character Strings ? A string of characters can be represented as a string literal by putting double quotes around the text: ? Examples: Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-4 "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 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 Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-5 send output System.out.println ("Whatever you are, be a good one."); 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 Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-6 ? Therefore anything printed after a print statement will appear on the same line ? See Countdown.java 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 Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-7 ? A string literal cannot be broken across two lines in a program ? See Facts.java 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 Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-8 ? 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 Escape Sequences ? What if we wanted to print a 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 Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-9 represents a special character ? An escape sequence begins with a backslash character (\) System.out.println ("I said \"Hello\" to you."); Escape Sequences ? Some Java escape sequences: \b \t \n Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-10 See Roses.java \r \" \' \\ Outline Character Strings Variables and Assignment Primitive Data Types Expressions Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-11 Data Conversion Interactive Programs Graphics Applets Drawing Shapes Variables ? A variable is a name for a location in memory ? A variable must be declared by specifying the variable's name and the type of information that it will hold Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-12 int total; int count, temp, result; Variable Initialization ? A variable can be given an initial value in the declaration When a variable is referenced in a program, its current value is int sum = 0; int base = 32, max = 149; Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-13 used See PianoKeys.java Assignment ? An assignment statement changes the value of a variable ? The assignment operator is the = sign total = 55; Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-14 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 The expression on the right is evaluated and the result is stored in the variable on the left 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 Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-15 ? 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; Constants ? Constants are useful for three important reasons ? First, they give meaning to otherwise unclear literal values ? For example, MAX_LOAD means more than the literal 250 Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-16 ? Second, they facilitate program maintenance ? If a constant is used in multiple places, its value need only be updated in one place ? Third, they formally establish that a value should not change, avoiding inadvertent errors by other programmers Outline Character Strings Variables and Assignment Primitive Data Types Expressions Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-17 Data Conversion Interactive Programs Graphics Applets Drawing Shapes 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: Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-18 ? float, double ? One of them represents characters: ? char ? And one of them represents boolean values: ? boolean Numeric Primitive Data ? The difference between the various numeric primitive types is their size, and therefore the values they can store: Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-19 byte short int long float double Characters ? A char variable stores a single character ? Character literals are delimited by single quotes: 'a' 'X' '7' '$' ',' '\n' ? Example declarations: Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-20 char topGrade = 'A'; char terminator = ';', separator = ' '; ? Note the distinction between a primitive character variable, which holds only one character, and a String object, which can hold multiple characters 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 Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-21 ? 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 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: Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-22 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 Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-23 boolean done = false; ? A boolean variable can also be used to represent any two states, such as a light bulb being on or off Outline Character Strings Variables and Assignment Primitive Data Types Expressions Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-24 Data Conversion Interactive Programs Graphics Applets Drawing Shapes 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: Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-25 If either or both operands used by an arithmetic operator are floating point, then the result is a floating point Division and Remainder ? If both operands to the division operator (/) are integers, the result is an integer (the fractional part is discarded) 14 / 3 8 / 12 4 0 Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-26 The remainder operator (%) returns the remainder after dividing the second operand into the first 14 % 3 8 % 12 2 8 Operator Precedence ? Operators can be combined into complex expressions result = total + count / max - offset; ? Operators have a well-defined precedence which determines the order in which they are evaluated Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-27 ? Multiplication, division, and remainder are evaluated prior to 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 Operator Precedence ? What is the order of evaluation in the following expressions? a + b + c + d + e 1 432 a + b * c - d / e 3 241 Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-28 a / (b + c) - d % e 2 341 a / (b * (c + (d - e))) 4 123 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 + Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-29 a + (b ? c) / d a / - d b c Assignment Revisited ? The assignment operator has a lower precedence than the arithmetic operators answer = sum / 4 + MAX * lowest; Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-30 14 3 2 Assignment Revisited ? The right and left hand sides of an assignment statement can contain the same variable count = count + 1; Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-31 Increment and Decrement ? The increment and decrement operators use only one operand ? The increment operator (++) adds one to its operand ? The decrement operator (--) subtracts one from Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-32 its operand ? The statement count++; is functionally equivalent to count = count + 1; Increment and Decrement ? The increment and decrement operators can be applied in postfix form: count++ ? or prefix form: Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-33 ++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 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 Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-34 ? For example, the statement num += count; is equivalent to num = num + count; Assignment Operators ? There are many assignment operators in Java, including the following: += x += y x = x + y Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-35 -= *= /= %= x -= y x *= y x /= y x %= y x = x - y x = x * y x = x / y x = x % y 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 Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-36 ? Therefore result /= (total-MIN) % num; is equivalent to result = result / ((total-MIN) % num); Assignment Operators ? The behavior of some assignment operators depends on the types of the operands ? If the operands to the += operator are strings, the assignment operator performs string concatenation Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-37 ? The behavior of an assignment operator (+=) is always consistent with the behavior of the corresponding operator (+) Outline Character Strings Variables and Assignment Primitive Data Types Expressions Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-38 Data Conversion Interactive Programs Graphics Applets Drawing Shapes 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 Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-39 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 Data Conversion ? Conversions must be handled carefully to avoid losing information ? 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 Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-40 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 Assignment Conversion ? Assignment conversion occurs when a value of one type is assigned to a variable of another ? If money is a float variable and dollars is an int variable, the following assignment converts the value in dollars to a float Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-41 money = dollars ? Only widening conversions can happen via assignment ? Note that the value or type of dollars did not change Data Conversion ? Promotion happens automatically when operators in expressions convert their operands ? For example, if sum is a float and count is an int, the value of count is converted to a floating point value to perform the following Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-42 calculation: result = sum / count; 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 Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-43 of the value being converted ? For example, if total and count are integers, but we want a floating point result when dividing them, we can cast total: result = (float) total / count; Outline Character Strings Variables and Assignment Primitive Data Types Expressions Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-44 Data Conversion Interactive Programs Graphics Applets Drawing Shapes Interactive Programs ? Programs generally need input on which to operate ? The Scanner class provides convenient methods for reading input values of various types Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-45 ? 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 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 Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-46 ? Once created, the Scanner object can be used to invoke various input methods, such as: answer = scan.nextLine(); Reading Input ? The Scanner class is part of the java.util class library, and must be imported into a program to be used ? See Echo.java Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-47 ? The nextLine method reads all of the input until the end of the line is found ? The details of object creation and class libraries are discussed further in Chapter 3 Input Tokens ? Unless specified otherwise, white space is used to separate the elements (called tokens) of the input ? White space includes space characters, tabs, new line characters Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-48 ? The next method of the Scanner class reads the next input token and returns it as a string ? Methods such as nextInt and nextDouble read data of particular types ? See GasMileage.java Outline Character Strings Variables and Assignment Primitive Data Types Expressions Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-49 Data Conversion Interactive Programs Graphics Applets Drawing Shapes Introduction to Graphics ? The last few sections of each chapter of the textbook focus on graphics and graphical user interfaces ? A picture or drawing must be digitized for storage on a computer ? A picture is made up of pixels (picture elements), and Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-50 each pixel is stored separately ? The number of pixels used to represent a picture is called the picture resolution ? The number of pixels that can be displayed by a monitor is called the monitor resolution Coordinate Systems ? Each pixel can be identified using a two- dimensional coordinate system ? When referring to a pixel in a Java program, we use a coordinate system with the origin in the top-left corner Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-51 Y X(0, 0) (112, 40) 112 40 Representing Color ? A black and white picture could be stored using one bit per pixel (0 = white and 1 = black) ? A colored picture requires more information; there are several techniques for representing colors ? For example, every color can be represented as a Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-52 mixture of the three additive primary colors Red, Green, and Blue ? Each color is represented by three numbers between 0 and 255 that collectively are called an RGB value The Color Class ? A color in a Java program is represented as an object created from the Color class ? The Color class also contains several predefined colors, including the following: Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-53 Color.black Color.blue Color.cyan Color.orange Color.white Color.yellow 0, 0, 0 0, 0, 255 0, 255, 255 255, 200, 0 255, 255, 255 255, 255, 0 Outline Character Strings Variables and Assignment Primitive Data Types Expressions Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-54 Data Conversion Interactive Programs Graphics Applets Drawing Shapes Applets ? A Java application is a stand-alone program with a main method (like the ones we've seen so far) ? A Java applet is a program that is intended to transported over the Web and executed using a web browser Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-55 ? An applet also can be executed using the appletviewer tool of the Java Software Development Kit ? An applet doesn't have a main method ? Instead, there are several special methods that serve specific purposes Applets ? The paint method, for instance, is executed automatically and is used to draw the applet?s contents ? The paint method accepts a parameter that is an object of the Graphics class Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-56 ? A Graphics object defines a graphics context on which we can draw shapes and text ? The Graphics class has several methods for drawing shapes Applets ? The class that defines an applet extends the Applet class ? This makes use of inheritance, which is explored in more detail in Chapter 8 ? See Einstein.java Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-57 ? An applet is embedded into an HTML file using a tag that references the bytecode file of the applet ? The bytecode version of the program is transported across the web and executed by a Java interpreter that is part of the browser The HTML applet Tag Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-58 Outline Character Strings Variables and Assignment Primitive Data Types Expressions Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-59 Data Conversion Interactive Programs Graphics Applets Drawing Shapes Drawing Shapes ? Let's explore some of the methods of the Graphics class that draw shapes in more detail ? A shape can be filled or unfilled, depending on which method is invoked ? The method parameters specify coordinates and Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-60 sizes ? Shapes with curves, like an oval, are usually drawn by specifying the shape?s bounding rectangle ? An arc can be thought of as a section of an oval Drawing a Line X 10 20 150 Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-61 Y 45 page.drawLine (10, 20, 150, 45); page.drawLine (150, 45, 10, 20); Drawing a Rectangle X 50 20 40 Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-62 Y page.drawRect (50, 20, 100, 40); 100 Drawing an Oval X 175 20 80 Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-63 Y page.drawOval (175, 20, 50, 80); 50 Drawing Shapes ? Every drawing surface has a background color ? Every graphics context has a current foreground color ? Both can be set explicitly Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-64 ? See Snowman.java Summary ? Chapter 2 focused on: ? character strings ? primitive data ? the declaration and use of variables ? expressions and operator precedence Copyright © 2009 Pearson Education, Inc. Publishing as Pearson Addison-Wesley 2-65 ? data conversions ? accepting input from the user ? Java applets ? introduction to graphics Richard Carver Microsoft PowerPoint - M02_LEWIS2058_06_PP_C02.ppt [Compatibility Mode]