1、3,Using Classes and Objects,5TH EDITIONLewis & LoftusjavaSoftware SolutionsFoundations of Program Design, 2007 Pearson Addison-Wesley. All rights reserved, 2007 Pearson Addison-Wesley. All rights reserved,3-2,Using Classes and Objects, 2007 Pearson Addison-Wesley. All rights reserved,3-3,Outline,Cre
2、ating Objects The String Class Packages Formatting Output Enumerated Types Wrapper Classes Components and Containers Images, 2007 Pearson Addison-Wesley. All rights reserved,3-4,Creating Objects,A variable holds either a primitive type or a reference to an object A class name can be used as a type t
3、o 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, 2007 Pearson Addison-Wesley. All rights reserved,3-5,Creating Objects,Generally, we use the new op
4、erator to create an object,title = new String (“Java Software Solutions“);,This calls the String constructor, which is a special method that sets up the object,Creating an object is called instantiation,An object is an instance of a particular class, 2007 Pearson Addison-Wesley. All rights reserved,
5、3-6,Invoking Methods,Weve seen that once an object has been instantiated, we can use the dot operator to invoke its methods count = title.length() A method may return a value, which can be used in an assignment or expression A method invocation can be thought of as asking an object to perform a serv
6、ice, 2007 Pearson Addison-Wesley. All rights reserved,3-7,References,Note that a primitive variable contains the value itself, but an object variable contains the address of the object An object reference can be thought of as a pointer to the location of the object Rather than dealing with arbitrary
7、 addresses, we often depict a reference graphically, 2007 Pearson Addison-Wesley. All rights reserved,3-8,Assignment Revisited,The act of assignment takes a copy of a value and stores it in a variable For primitive types:,num2 = num1;, 2007 Pearson Addison-Wesley. All rights reserved,3-9,Reference A
8、ssignment,For object references, assignment copies the address:,name2 = name1;, 2007 Pearson Addison-Wesley. All rights reserved,3-10,Aliases,Two or more references that refer to the same object are called aliases of each other That creates an interesting situation: one object can be accessed using
9、multiple reference variables Aliases can be useful, but should be managed carefully Changing an object through one reference changes it for all of its aliases, because there is really only one object, 2007 Pearson Addison-Wesley. All rights reserved,3-11,Garbage Collection,When an object no longer h
10、as any valid references to it, it can no longer be accessed by the program The object is useless, and therefore is called garbage Java performs automatic garbage collection periodically, returning an objects memory to the system for future use In other languages, the programmer is responsible for pe
11、rforming garbage collection, 2007 Pearson Addison-Wesley. All rights reserved,3-12,Outline,Creating Objects The String Class Packages Formatting Output Enumerated Types Wrapper Classes Components and Containers Images, 2007 Pearson Addison-Wesley. All rights reserved,3-13,The String Class,Because st
12、rings are so common, we dont have to use the new operator to create a String object title = “Java Software Solutions“; This is special syntax that works only for strings Each string literal (enclosed in double quotes) represents a String object, 2007 Pearson Addison-Wesley. All rights reserved,3-14,
13、String Methods,Once a String object has been created, neither its value nor its length can be changed Thus we say that an object of the String class is immutable However, several methods of the String class return new String objects that are modified versions of the original See the list of String m
14、ethods on page 119 and in Appendix M, 2007 Pearson Addison-Wesley. All rights reserved,3-15,String Indexes,It is occasionally helpful to refer to a particular character within a string This can be done by specifying the characters numeric index The indexes begin at zero in each string In the string
15、“Hello“, the character H is at index 0 and the o is at index 4 See StringMutation.java (page 120), 2007 Pearson Addison-Wesley. All rights reserved,3-16,Outline,Creating Objects The String Class Packages Formatting Output Enumerated Types Wrapper Classes Components and Containers Images, 2007 Pearso
16、n Addison-Wesley. All rights reserved,3-17,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
17、 Various classes weve already used (System , Scanner, String) are part of the Java standard class library Other class libraries can be obtained through third party vendors, or you can create them yourself, 2007 Pearson Addison-Wesley. All rights reserved,3-18,Packages,The classes of the Java standar
18、d class library are organized into packages Some of the packages in the standard class library are:, 2007 Pearson Addison-Wesley. All rights reserved,3-19,The import Declaration,When you want to use a class from a package, you could use its fully qualified name java.util.Scanner Or you can import th
19、e class, and then use just the class name import java.util.Scanner; To import all classes in a particular package, you can use the * wildcard character import java.util.*;, 2007 Pearson Addison-Wesley. All rights reserved,3-20,The import Declaration,All classes of the java.lang package are imported
20、automatically into all programs Its as if all programs contain the following line: import java.lang.*; Thats why we didnt have to import the System or String classes explicitly in earlier programs The Scanner class, on the other hand, is part of the java.util package, and therefore must be imported,
21、 2007 Pearson Addison-Wesley. All rights reserved,3-21,The Random Class,The Random class is part of the java.util package It provides methods that generate pseudorandom numbers A Random object performs complicated calculations based on a seed value to produce a stream of seemingly random values See
22、RandomNumbers.java (page 125), 2007 Pearson Addison-Wesley. All rights reserved,3-22,The Math Class,The Math class is part of the java.lang package The Math class contains methods that perform various mathematical functions These include: absolute value square root exponentiation trigonometric funct
23、ions, 2007 Pearson Addison-Wesley. All rights reserved,3-23,The Math Class,The 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 needed value = Math.cos(90) + Math.sqrt(delta); See Quadratic.ja
24、va (page 129) We discuss static methods further in Chapter 6, 2007 Pearson Addison-Wesley. All rights reserved,3-24,Outline,Creating Objects The String Class Packages Formatting Output Enumerated Types Wrapper Classes Components and Containers Images, 2007 Pearson Addison-Wesley. All rights reserved
25、,3-25,Formatting Output,It is often necessary to format values in certain ways so that they can be presented properly The Java standard class library contains classes that provide formatting capabilities The NumberFormat class allows you to format values as currency or percentages The DecimalFormat
26、class allows you to format values based on a pattern Both are part of the java.text package, 2007 Pearson Addison-Wesley. All rights reserved,3-26,Formatting Output,The NumberFormat class has static methods that return a formatter object getCurrencyInstance() getPercentInstance() Each formatter obje
27、ct has a method called format that returns a string with the specified information in the appropriate format See Purchase.java (page 131), 2007 Pearson Addison-Wesley. All rights reserved,3-27,Formatting Output,The DecimalFormat class can be used to format a floating point value in various ways For
28、example, you can specify that the number should be truncated 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 134), 2007 Pearson Addison-Wesley. All rights reserved,3-28,Outline,Creating Ob
29、jects The String Class Packages Formatting Output Enumerated Types Wrapper Classes Components and Containers Images, 2007 Pearson Addison-Wesley. All rights reserved,3-29,Enumerated Types,Java allows you to define an enumerated type, which can then be used to declare variables An enumerated type est
30、ablishes all possible values for a variable of that type The values are identifiers of your own choosing The following declaration creates an enumerated type called Season enum Season winter, spring, summer, fall; Any number of values can be listed, 2007 Pearson Addison-Wesley. All rights reserved,3
31、-30,Enumerated Types,Once a type is defined, a variable of that type can be declared Season time;and it can be assigned a value time = Season.fall; The values are specified through the name of the type Enumerated types are type-safe you cannot assign any value other than those listed, 2007 Pearson A
32、ddison-Wesley. All rights reserved,3-31,Ordinal Values,Internally, each value of an enumerated type is stored as an integer, called its ordinal value The first value in an enumerated type has an ordinal value of zero, the second one, and so on However, you cannot assign a numeric value to an enumera
33、ted type, even if it corresponds to a valid ordinal value, 2007 Pearson Addison-Wesley. All rights reserved,3-32,Enumerated Types,The declaration of an enumerated type is a special type of class, and each variable of that type is an object The ordinal method returns the ordinal value of the object T
34、he name method returns the name of the identifier corresponding to the objects value See IceCream.java (page 136), 2007 Pearson Addison-Wesley. All rights reserved,3-33,Outline,Creating Objects The String Class Packages Formatting Output Enumerated Types Wrapper Classes Components and Containers Ima
35、ges, 2007 Pearson Addison-Wesley. All rights reserved,3-34,Wrapper Classes,The java.lang package contains wrapper classes that correspond to each primitive type:, 2007 Pearson Addison-Wesley. All rights reserved,3-35,Wrapper Classes,The following declaration creates an Integer object which represent
36、s 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 suffice For example, some objects serve as containers of other objects Primitive values could not be stored in such containers, but wrapper objects
37、could be, 2007 Pearson Addison-Wesley. All rights reserved,3-36,Wrapper Classes,Wrapper classes also contain static methods that help manage the associated type For example, the Integer class contains a method to convert an integer stored in a String to an int value: num = Integer.parseInt(str); The
38、 wrapper classes often contain useful constants as well For example, the Integer class contains MIN_VALUE and MAX_VALUE which hold the smallest and largest int values, 2007 Pearson Addison-Wesley. All rights reserved,3-37,Autoboxing,Autoboxing is the automatic conversion of a primitive value to a co
39、rresponding wrapper object:Integer obj;int num = 42;obj = num; The assignment creates the appropriate Integer object The reverse conversion (called unboxing) also occurs automatically as needed, 2007 Pearson Addison-Wesley. All rights reserved,3-38,Outline,Creating Objects The String Class Packages
40、Formatting Output Enumerated Types Wrapper Classes Components and Containers Images, 2007 Pearson Addison-Wesley. All rights reserved,3-39,Graphical Applications,Except for the applets seen in Chapter 2, the example programs weve explored thus far have been text-based They are called command-line ap
41、plications, which interact with the user using simple text prompts Lets examine some Java applications that have graphical components These components will serve as a foundation to programs that have true graphical user interfaces (GUIs), 2007 Pearson Addison-Wesley. All rights reserved,3-40,GUI Com
42、ponents,A GUI component is an object that represents a screen element such as a button or a text field GUI-related classes are defined primarily in the java.awt and the javax.swing packages The Abstract Windowing Toolkit (AWT) was the original Java GUI package The Swing package provides additional a
43、nd more versatile components Both packages are needed to create a Java GUI-based program, 2007 Pearson Addison-Wesley. All rights reserved,3-41,GUI Containers,A GUI container is a component that is used to hold and organize other components A frame is a container that is used to display a GUI-based
44、Java application A frame is displayed as a separate window with a title bar it can be repositioned and resized on the screen as needed A panel is a container that cannot be displayed on its own but is used to organize other components A panel must be added to another container to be displayed, 2007
45、Pearson Addison-Wesley. All rights reserved,3-42,GUI Containers,A GUI container can be classified as either heavyweight or lightweight A heavyweight container is one that is managed by the underlying operating system A lightweight container is managed by the Java program itself Occasionally this dis
46、tinction is important A frame is a heavyweight container and a panel is a lightweight container, 2007 Pearson Addison-Wesley. All rights reserved,3-43,Labels,A label is a GUI component that displays a line of text Labels are usually used to display information or identify other components in the int
47、erface Lets look at a program that organizes two labels in a panel and displays that panel in a frame See Authority.java (page 143) This program is not interactive, but the frame can be repositioned and resized, 2007 Pearson Addison-Wesley. All rights reserved,3-44,Nested Panels,Containers that cont
48、ain other components make up the containment hierarchy of an interface This hierarchy can be as intricate as needed to create the visual effect desired The following example nests two panels inside a third panel note the effect this has as the frame is resized See NestedPanels.java (page 145), 2007
49、Pearson Addison-Wesley. All rights reserved,3-45,Outline,Creating Objects The String Class Packages Formatting Output Enumerated Types Wrapper Classes Components and Containers Images, 2007 Pearson Addison-Wesley. All rights reserved,3-46,Images,Images are often used in a programs with a graphical interface Java can manage images in both JPEG and GIF formats As weve seen, a JLabel object can be used to display a line of text It can also be used to display an image That is, a label can be composed of text, and image, or both at the same time,
