Notes

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Chapter 2:  Objects and Primitive Data Presentation slides for Java Software Solutions Foundations of Program Design Third Edition by John Lewis and William Loftus Java Software Solutions is published by Addison-Wesley Presentation slides are copyright 2002 by John Lewis and William Loftus. All rights reserved. Instructors using the textbook may use and modify these slides for pedagogical purposes.

Objects and Primitive Data Now we can explore some more fundamental programming concepts Chapter 2 focuses on: predefined objects primitive data the declaration and use of variables expressions and operator precedence creating and using objects class libraries Java applets drawing shapes

Introduction to Objects An object represents something with which we can interact in a program An object provides a collection of services that we can tell it to perform for us The services are defined by methods in a class that defines the object A class represents a concept, and an object represents the embodiment of a class A class can be used to create multiple objects

Objects and Classes

Inheritance One class can be used to derive another via inheritance Classes can be organized into inheritance hierarchies

Using Objects The System.out object represents a destination to which we can send output In the Lincoln program, we invoked the println method of the System.out object:

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 (page 65)

Abstraction An abstraction hides (or suppresses) the right details at the right time An object is abstract in that we don't have to think about its internal details in order to use it For example, we don't have to know how the println method works in order to invoke it A human being can manage only seven (plus or minus 2) pieces of information at one time But if we group information into chunks (such as objects) we can manage many complicated pieces at once Classes and objects help us write complex software

Character Strings Every character string is an object in Java, defined by the String class Every string literal, delimited by double quotation marks, represents a String object The string concatenation operator (+) is used to append one string to the end of another 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 (page 68)

String Concatenation The plus operator (+) is also used for arithmetic addition The function that the + operator 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 Parentheses can be used to force the operation order See Addition.java (page 70)

Escape Sequences What if we wanted to print a double 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 (\), which indicates that the character(s) that follow should be treated in a special way System.out.println ("I said \"Hello\" to you.");

Escape Sequences Some Java escape sequences:

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

Variables A variable can be given an initial value in the declaration

Assignment An assignment statement changes the value of a variable The assignment operator is the = sign

Constants A constant is an identifier that is similar to a variable except that it holds one value while the program is active The compiler will issue an error if you try to change the value of a constant during execution In Java, we use the final modifier to declare a constant final int MIN_HEIGHT = 69; Constants: give names to otherwise unclear literal values facilitate updates of values used throughout a program prevent inadvertent attempts to change a value

Primitive Data There are exactly 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

Numeric Primitive Data The difference between the various numeric primitive types is their size, and therefore the values they can store:

Characters A char variable stores a single character from the Unicode character set A character set is an ordered list of characters, and each character corresponds to a unique number 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 Character literals are delimited by single quotes: 'a'   'X'    '7'    '$'    ','    '\n'

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:

Boolean A boolean value represents a true or false condition A boolean also can be used to represent any two states, such as a light bulb being on or off The reserved words true and false are the only valid values for a boolean type boolean done = false;

Arithmetic Expressions An expression is a combination of one or more operands and their operators Arithmetic expressions compute numeric results and make use of the arithmetic operators:

Division and Remainder If both operands to the division operator (/) are integers, the result is an integer (the fractional part is discarded)

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 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 Parentheses can be used to force the evaluation order

Operator Precedence What is the order of evaluation in the following expressions?

Assignment Revisited The assignment operator has a lower precedence than the arithmetic operators

Assignment Revisited The right and left hand sides of an assignment statement can contain the same variable

Data Conversions Sometimes it is convenient to convert data from one type to another For example, we may want to treat an integer as a floating point value during a computation 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 they tend to go from a large data type to a smaller one (such as an int to a short)

Data Conversions In Java, data conversions can occur in three ways: assignment conversion arithmetic promotion casting Assignment conversion occurs when a value of one type is assigned to a variable of another Only widening conversions can happen via assignment Arithmetic promotion happens automatically when operators in expressions convert their operands

Data Conversions 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 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;

Creating Objects A variable holds either a primitive type or a reference to an object A class name can be used as a type to declare an object reference variable String title; No object is created with this declaration An object reference variable holds the address of an object The object itself must be created separately

Creating Objects Generally, we use the new operator to create an object

Creating Objects Because strings are so common, we don't have to use the new operator to create a String object title = "Java Software Solutions"; This is special syntax that works only for strings Once an object has been instantiated, we can use the dot operator to invoke its methods title.length()

String Methods The String class has several methods that are useful for manipulating strings Many of the methods return a value, such as an integer or a new String object See the list of String methods on page 89 and in Appendix M See StringMutation.java (page 90)

Class Libraries A class library is a collection of classes that we can use when developing programs The Java standard class library is part of any Java development environment Its classes are not part of the Java language per se, but we rely on them heavily The System class and the String class are part of the Java standard class library Other class libraries can be obtained through third party vendors, or you can create them yourself

Packages The classes of the Java standard class library are organized into packages Some of the packages in the standard class library are:

The import Declaration When you want to use a class from a package, you could use its fully qualified name java.util.Random Or you can import the class, and then use just the class name import java.util.Random; To import all classes in a particular package, you can use the * wildcard character import java.util.*;

The import Declaration All classes of the java.lang package are imported automatically into all programs That's why we didn't have to import the System or String classes explicitly in earlier programs The Random class is part of the java.util package It provides methods that generate pseudorandom numbers See RandomNumbers.java (page 97)

Class Methods Some methods can be invoked through the class name, instead of through an object of the class These methods are called class methods or static methods The Math class contains many static methods, providing various mathematical functions, such as absolute value, trigonometry functions, square root, etc. temp = Math.cos(90) + Math.sqrt(delta);

The Keyboard Class The Keyboard class is NOT part of the Java standard class library It is provided by the authors of the textbook to make reading input from the keyboard easy Details of the Keyboard class are explored in Chapter 5 The Keyboard class is part of a package called cs1 It contains several static methods for reading particular types of data See Echo.java (page 101) See Quadratic.java (page 102)

Formatting Output The NumberFormat class has static methods that return a formatter object getCurrencyInstance() getPercentInstance() Each formatter object has a method called format that returns a string with the specified information in the appropriate format See Price.java (page 104)

Formatting Output The DecimalFormat class can be used to format a floating point value in generic ways For example, you can specify that the number should be printed to three decimal places The constructor of the DecimalFormat class takes a string that represents a pattern for the formatted number See CircleStats.java (page 107)

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 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 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 7 See Einstein.java (page 109) An applet is embedded into an HTML file using a tag that references the bytecode file of the applet class 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

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 sizes Recall from Chapter 1 that the Java coordinate system has the origin in the top left corner 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

Drawing a Rectangle

Drawing an Oval

The Color Class A color is defined in a Java program using an object created from the Color class The Color class also contains several static predefined colors, including:

The Color Class Every drawing surface has a background color Every graphics context has a current foreground color Both can be set explicitly See Snowman.java (page115)

Summary Chapter 2 has focused on: predefined objects primitive data the declaration and use of variables expressions and operator precedence creating and using objects class libraries Java applets drawing shapes