Figure 44 in Java

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CHAPTER 4: ACCESS CONTROL
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Searching for Classes
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/top src * Ajava Bjava bin bin pkg * current directory Bclass /top src * Ajava Bjava bin pkg Aclass Bclass (a) (b) (c) /top src * Ajava Bjava
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// File name: Ajava package pkg; class A { B b; } // A uses B
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// File name: Bjava package pkg; class B { }
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>javac -d /bin Bjava
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Next, we try to compile the file Ajava, and get the following results:
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>javac -d /bin Ajava Ajava:3: cannot find symbol symbol : class B location: class pkgA public class A { B b; } ^ 1 error
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The compiler cannot find the class B, ie, the file Bclass containing the Java byte code for the class B From Figure 44b we can see that it is in the package pkg under the directory bin, but the compiler cannot find it This is hardly surprising, as there is no byte code file for the class B in the current directory, which is the default value of the class path The command below sets the value of the class path to be /top/ bin, and compilation is successful (see Figure 44c):
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>javac -cp /top/bin -d /bin Ajava
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It is very important to understand that when we want the JDK tool to search in a named package, it is the location of the package that is specified, ie, the class path indicates the directory that contains the first element of the fully qualified package name In Figure 44c, the package pkg is contained under the directory whose absolute path is /top/bin The following command will not work, as the directory /top/ bin/pkg does not contain a package with the name pkg that has a class B:
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43: SEARCHING FOR CLASSES >javac -cp /top/bin/pkg -d /bin Ajava
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Also, the compiler is not using the class path to find the source file(s) that are specified in the command line In the command above, the source file has the relative pathname /Ajava So the compiler looks for the source file in the current directory The class path is used to find classes used by the class A Given the file hierarchy in Figure 43, the following -cp option sets the class path so that all packages (wizardpandorasBox, wizardpandorasBoxartifacts, wizardspells) in Figure 43 will be searched, as all packages are located under the specified directory:
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-cp /pgjc/work
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However, the following -cp option will not help in finding any of the packages in Figure 43, as none of the packages are located under the specified directory:
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>java -cp /pgjc/work/wizard pandorasBoxClown
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The command above also illustrates an important point about package names: the fully qualified package name should not be split The package name for the class wizardpandorasBoxClown is wizardpandorasBox, and must be specified fully The following command will search all packages in Figure 43 for classes that are used by the class wizardpandorasBoxClown:
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>java -cp /pgjc/work wizardpandorasBoxClown
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The class path can specify several entries, ie, several locations, and the JDK tool searches them in the order they are specified, from left to right
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-cp /pgjc/work:/top/bin/pkg:
We have used the path-separator character ':' for Unix platforms to separate the entries, and also included the current directory () as an entry There should be no white space on either side of the path-separator character The search in the class path entries stops once the required class file is found Therefore, the order in which entries are specified can be significant If a class B is found in a package pkg located under the directory /ext/lib1, and also in a package pkg located under the directory /ext/lib2, the order in which the entries are specified in the two -cp options shown below is significant They will result in the class pkgB being found under /ext/lib1 and /ext/lib2, respectively
-cp /ext/lib1:/ext/lib2 -cp /ext/lib2:/ext/lib1
The examples so far have used absolute pathnames for class path entries We can of course use relative pathnames as well If the current directory has the absolute pathname /pgjc/work in Figure 43, the following command will search the packages under the current directory: