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.0).Those classes arein the java package and include all the classes you ve seen so far in this book, plus a whole lotmore classes you ll learn about later on in this book (and more you may not learn about at all).The Java Developer s Kit comes with documentation for all the Java class libraries, whichincludes descriptions of each class s instance variables, methods, constructors, interfaces, and soon.A shorter summary of the Java API is in Appendix B as well.Exploring the Java class libraries76SSFRWTMabcdand their methods and instance variables is a great way to figure out what Java can and cannotdo, as well as a starting point for your own development.Here are the class packages that are part of the Java class libraries:java.lang: Classes that apply to the language itself, which includes the Object class,the String class, and the System class.It also contains the special classes for theprimitive types (Integer, Character, Float, and so on).java.util: Utility classes, such as Date, as well as simple collection classes, such asVector and Hashtable.java.io: Input and output classes for writing to and reading from streams (such asstandard input and output) and for handling files.java.net: Classes for networking support, including Socket and URL (a class torepresent references to documents on the World Wide Web).java.awt: (the Abstract Window Toolkit): Classes to implement a graphical userinterface, including classes for Window, Menu, Button, Font, CheckBox, and so on.Thispackage also includes classes for processing images (the java.awt.Image package).java.applet: Classes to implement Java applets, including the Applet class itself, as4well as the AudioClip class.In addition to the Java classes, your development environment may also include additionalclasses that provide other utilities or functionality.Although these classes may be useful, becausethey are not part of the standard Java library, they won t be available to other people trying torun your Java program.This is particularly important for applets, because applets are expectedto be able to run on any platform, using any Java-aware browser.Only classes inside the javapackage are guaranteed to be available on all browsers and Java environments.SummaryObjects, objects everywhere.Today, you learned all about how to deal with objects: how tocreate them, how to find out and change the values of their variables, and how to call theirmethods.You also learned how to copy and compare them, and how to convert them into otherobjects.Finally, you learned a bit about the Java class libraries which give you a whole slew ofclasses to play with in your own programs.You now have the fundamentals of how to deal with most simple things in the Java language.All you have left are arrays, conditionals, and loops, which you ll learn about tomorrow.Thenyou ll learn how to define and use classes in Java applications on Day 6, and launch directly intoapplets next week.With just about everything you do in your Java programs, you ll always comeback to objects.77LearningSams.netCenterDAYDAYWorking with Objects4Q&AQ I m confused about the differences between objects and the primitive data types,such as int and boolean.A The primitive types in the language (byte, short, int, long, float, double, and char)represent the smallest things in the language.They are not objects, although in manyways they can be handled like objects they can be assigned to variables and passed inand out of methods.Most of the operations that work exclusively on objects, however,will not.Objects usually represent instances of classes and as such, are much more complexdata types than simple numbers and characters, often containing numbers andcharacters as instance or class variables.Q In the section on calling methods, you had examples of calling a method with adifferent number of arguments each time and it gave a different kind of result.How is that possible?A That s called method overloading.Overloading enables the same function name to havedifferent behavior based on the arguments it s called with and the number and typeof arguments can vary.When you define methods in your own classes, you defineseparate method signatures with different sets or arguments and different definitions.When that method is called, Java figures out which definition to execute based on thenumber and type of arguments with which you called it.You ll learn all about this on Day 6.Q No operator overloading in Java? Why not? I thought Java was based on C++,and C++ has operator overloading.A Java was indeed based on C++, but it was also designed to be simple, so many ofC++ s features have been removed.The argument against operator overloading is thatbecause the operator can be defined to mean anything, it makes it very difficult tofigure out what any given operator is doing at any one time.This can result in entirelyunreadable code.Given the potential for abuse, the designers of Java felt it was one ofthe C++ features that was best left out.78SSFRWTMabcd55WEEK15Arrays,Conditionals,and Loopsby Laura Lemay79LearningSams.netCenterSSFRWTMDAYDAYArrays, Conditionals, and Loops5Although you could write Java programs using what you ve learned so far, those programs wouldbe pretty dull.Much of the good stuff in Java or in any programming language results when youhave arrays to store values in and control-flow constructs (loops and conditionals) to executedifferent bits of a program based on tests.Today, you ll find out about the following:Arrays, one of the most useful objects in Java, which enable you to collect objects intoan easy-to-manage listBlock statements, for grouping together related statementsif and switch, for conditional testsfor and while loops, for iteration or repeating a statement or statements multipletimesArraysArrays in Java are different than they are in other languages.Arrays in Java are actual objects thatcan be passed around and treated just like other objects.Arrays are a way to store a list of items.Each element of the array holds an individual item,NEWand you can place items into and remove items from those slots as you need to.TERMArrays can contain any type of value (base types or objects), but you can t store different typesin a single array.You can have an array of integers, or an array of strings, or an array of arrays,but you can t have an array that contains, for example, both strings and integers.To create an array in Java, you use three steps:1.Declare a variable to hold the array.2.Create a new array object and assign it to the array variable.3.Store things in that array.Declaring Array VariablesThe first step to creating an array is creating a variable that will hold the array, just as you wouldany other variable.Array variables indicate the type of object the array will hold (just as they dofor any variable) and the name of the array, followed by empty brackets ([]).The following areall typical array variable declarations:String difficultWords[];Point hits[];int temps[];80SSFRWTMabcdAn alternate method of defining an array variable is to put the brackets after the type instead ofafter the variable.They are equivalent, but this latter form is often much more readable.So, forexample, these three declarations could be written like this:String[] difficultWords;Point[] hits;int[] temps;Creating Array ObjectsThe second step is to create an array object and assign it to that variable [ Pobierz całość w formacie PDF ]