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Chapter 3Using Classes and ObjectsUsing Classes and ObjectsWe can create more interesting programs using predefined classes and related objectsChapter 3 focuses on:Object creation and object referencesThe string class and its methodsThe java standard class libraryThe random and math classesFormatting outputEnumerated typesWrapper classes 2004 Pearson Addison-Wesley. All rights reserved3-2Outline 2004 Pearson Addison-Wesley. All rights reserved3-3Creating ObjectsThe String ClassPackagesFormatting OutputEnumerated TypesWrapper ClassesCreating ObjectsA variable holds either a primitive type or a reference to an objectA class name can be used as a type to declare an object reference variableString title;No object is created with this declarationAn object reference variable holds the address of an objectThe object itself must be created separately 2004 Pearson Addison-Wesley. All rights reserved3-4Creating ObjectsGenerally, we use the new operator to create an object 2004 Pearson Addison-Wesley. All rights reserved3-5title = new String ("Java Software Solutions");This calls the String constructor, which isa special method that sets up the objectCreating an object is called instantiationAn object is an instance of a particular classInvoking MethodsWe've seen that once an object has been instantiated, we can use the dot operator to invoke its methodscount = title.length()A method may return a value, which can be used in an assignment or expressionA method invocation can be thought of as asking an object to perform a service 2004 Pearson Addison-Wesley. All rights reserved3-6ReferencesNote that a primitive variable contains the value itself, but an object variable contains the address of the objectAn object reference can be thought of as a pointer to the location of the objectRather than dealing with arbitrary addresses, we often depict a reference graphically 2004 Pearson Addison-Wesley. All rights reserved3-7"Steve Jobs"name1num138Assignment RevisitedThe act of assignment takes a copy of a value and stores it in a variableFor primitive types: 2004 Pearson Addison-Wesley. All rights reserved3-8num138num296Before:num2 = num1;num138num238After:Reference AssignmentFor object references, assignment copies the address: 2004 Pearson Addison-Wesley. All rights reserved3-9name2 = name1;name1name2Before:"Steve Jobs""Steve Wozniak"name1name2After:"Steve Jobs"AliasesTwo or more references that refer to the same object are called aliases of each otherThat creates an interesting situation: one object can be accessed using multiple reference variablesAliases can be useful, but should be managed carefullyChanging an object through one reference changes it for all of its aliases, because there is really only one object 2004 Pearson Addison-Wesley. All rights reserved3-10Garbage CollectionWhen an object no longer has any valid references to it, it can no longer be accessed by the programThe object is useless, and therefore is called garbageJava performs automatic garbage collection periodically, returning an object's memory to the system for future useIn other languages, the programmer is responsible for performing garbage collection 2004 Pearson Addison-Wesley. All rights reserved3-11Outline 2004 Pearson Addison-Wesley. All rights reserved3-12Creating ObjectsThe String ClassPackagesFormatting OutputEnumerated TypesWrapper ClassesThe String ClassBecause strings are so common, we don't have to use the new operator to create a String objecttitle = "Java Software Solutions";This is special syntax that works only for stringsEach string literal (enclosed in double quotes) represents a String object 2004 Pearson Addison-Wesley. All rights reserved3-13String MethodsOnce a String object has been created, neither its value nor its length can be changedThus we say that an object of the String class is immutableHowever, several methods of the String class return new String objects that are modified versions of the originalSee the list of String methods on page 119 and in Appendix M 2004 Pearson Addison-Wesley. All rights reserved3-14String IndexesIt is occasionally helpful to refer to a particular character within a stringThis can be done by specifying the character's numeric indexThe indexes begin at zero in each stringIn the string "Hello", the character 'H' is at index 0 and the 'o' is at index 4See StringMutation.java (page 120) 2004 Pearson Addison-Wesley. All rights reserved3-15Outline 2004 Pearson Addison-Wesley. All rights reserved3-16Creating ObjectsThe String ClassPackagesFormatting OutputEnumerated TypesWrapper ClassesClass LibrariesA class library is a collection of classes that we can use when developing programsThe Java standard class library is part of any Java development environmentIts classes are not part of the Java language per se, but we rely on them heavilyVarious classes we've already used (System , Scanner, String) are part of the Java standard class libraryOther class libraries can be obtained through third party vendors, or you can create them yourself 2004 Pearson Addison-Wesley. All rights reserved3-17PackagesThe classes of the Java standard class library are organized into packagesSome of the packages in the standard class library are: 2004 Pearson Addison-Wesley. All rights reserved3-18Packagejava.langjava.appletjava.awtjavax.swingjava.netjava.utiljavax.xml.parsersPurposeGeneral supportCreating applets for the webGraphics and graphical user interfacesAdditional graphics capabilitiesNetwork communicationUtilitiesXML document processingThe import DeclarationWhen you want to use a class from a package, you could use its fully qualified namejava.util.ScannerOr you can import the class, and then use just the class nameimport java.util.Scanner;To import all classes in a particular package, you can use the * wildcard characterimport java.util.*;

2004 Pearson Addison-Wesley. All rights reserved3-19The import DeclarationAll classes of the java.lang package are imported automatically into all programsIt's as if all programs contain the following line:import java.lang.*;That's why we didn't have to import the System or String classes explicitly in earlier programsThe Scanner class, on the other hand, is part of the java.util package, and therefore must be imported 2004 Pearson Addison-Wesley. All rights reserved3-20The Random ClassThe Random class is part of the java.util packageIt provides methods that generate pseudorandom numbersA Random object performs complicated calculations based on a seed value to produce a stream of seemingly random valuesSee RandomNumbers.java (page 126)

2004 Pearson Addison-Wesley. All rights reserved3-21The Math ClassThe Math class is part of the java.lang packageThe Math class contains methods that perform various mathematical functionsThese include:absolute valuesquare rootexponentiationtrigonometric functions 2004 Pearson Addison-Wesley. All rights reserved3-22The Math ClassThe methods of the Math class are static methods (also called class methods)Static methods can be invoked through the class name no object of the Math class is neededvalue = Math.cos(90) + Math.sqrt(delta);See Quadratic.java (page 129)We discuss static methods further in Chapter 6

2004 Pearson Addison-Wesley. All rights reserved3-23Outline 2004 Pearson Addison-Wesley. All rights reserved3-24Creating ObjectsThe String ClassPackagesFormatting OutputEnumerated TypesWrapper ClassesFormatting OutputIt is often necessary to format values in certain ways so that they can be presented properlyThe Java standard class library contains classes that provide formatting capabilitiesThe NumberFormat class allows you to format values as currency or percentagesThe DecimalFormat class allows you to format values based on a patternBoth are part of the java.text package 2004 Pearson Addison-Wesley. All rights reserved3-25Formatting OutputThe NumberFormat class has static methods that return a formatter objectgetCurrencyInstance()getPercentInstance()Each formatter object has a method called format that returns a string with the specified information in the appropriate formatSee Purchase.java (page 131) 2004 Pearson Addison-Wesley. All rights reserved3-26Formatting OutputThe DecimalFormat class can be used to format a floating point value in various waysFor example, you can specify that the number should be truncated to three decimal placesThe constructor of the DecimalFormat class takes a string that represents a pattern for the formatted numberSee CircleStats.java (page 134) 2004 Pearson Addison-Wesley. All rights reserved3-27Outline 2004 Pearson Addison-Wesley. All rights reserved3-28Creating ObjectsThe String ClassPackagesFormatting OutputEnumerated TypesWrapper ClassesEnumerated TypesJava allows you to define an enumerated type, which can then be used to declare variablesAn enumerated type establishes all possible values for a variable of that typeThe values are identifiers of your own choosingThe following declaration creates an enumerated type called Seasonenum Season {winter, spring, summer, fall};Any number of values can be listed 2004 Pearson Addison-Wesley. All rights reserved3-29Enumerated TypesOnce a type is defined, a variable of that type can be declaredSeason time;and it can be assigned a valuetime = Season.fall;The values are specified through the name of the typeEnumerated types are type-safe you cannot assign any value other than those listed

2004 Pearson Addison-Wesley. All rights reserved3-30Ordinal ValuesInternally, each value of an enumerated type is stored as an integer, called its ordinal valueThe first value in an enumerated type has an ordinal value of zero, the second one, and so onHowever, you cannot assign a numeric value to an enumerated type, even if it corresponds to a valid ordinal value 2004 Pearson Addison-Wesley. All rights reserved3-31Enumerated TypesThe declaration of an enumerated type is a special type of class, and each variable of that type is an objectThe ordinal method returns the ordinal value of the objectThe name method returns the name of the identifier corresponding to the object's valueSee IceCream.java (page 137) 2004 Pearson Addison-Wesley. All rights reserved3-32Outline 2004 Pearson Addison-Wesley. All rights reserved3-33Creating ObjectsThe String ClassPackagesFormatting OutputEnumerated TypesWrapper ClassesWrapper ClassesThe java.lang package contains wrapper classes that correspond to each primitive type:Primitive TypeWrapper ClassbyteByteshortShortintIntegerlongLongfloatFloatdoubleDoublecharCharacterbooleanBooleanvoidVoid 2004 Pearson Addison-Wesley. All rights reserved3-34Wrapper ClassesThe following declaration creates an Integer object which represents the integer 40 as an object Integer age = new Integer(40);An object of a wrapper class can be used in any situation where a primitive value will not sufficeFor example, some objects serve as containers of other objectsPrimitive values could not be stored in such containers, but wrapper objects could be 2004 Pearson Addison-Wesley. All rights reserved3-35Wrapper ClassesWrapper classes also contain static methods that help manage the associated typeFor example, the Integer class contains a method to convert an integer stored in a String to an int value:num = Integer.parseInt(str);The wrapper classes often contain useful constants as wellFor example, the Integer class contains MIN_VALUE and MAX_VALUE which hold the smallest and largest int values 2004 Pearson Addison-Wesley. All rights reserved3-36AutoboxingAutoboxing is the automatic conversion of a primitive value to a corresponding wrapper object:Integer obj;int num = 42;obj = num;The assignment creates the appropriate Integer objectThe reverse conversion (called unboxing) also occurs automatically as needed 2004 Pearson Addison-Wesley. All rights reserved3-37SummaryChapter 3 focused on:object creation and object referencesthe String class and its methodsthe Java standard class librarythe Random and Math classesformatting outputenumerated typeswrapper classes 2004 Pearson Addison-Wesley. All rights reserved3-38