Questions

Question 1.A programmer installs a new library contained in a .jar file. In order to access the library from his code, he sets the CLASSPATH environment variable to point to the new .jar file. Now he finds that he gets an error message when he tries to launch simple applications:

java Hello
Exception in thread "main" java.lang.NoClassDefFoundError: Hello

In this case, the Hello class is compiled into a .class file in the current directory — yet the java command can't seem to find it. What's going wrong?

Answer 1. A class is only found if it appears in the class path. By default, the class path consists of the current directory. If the CLASSPATH environment variable is set, and doesn't include the current directory, the launcher can no longer find classes in the current directory. The solution is to change the CLASSPATH variable to include the current directory. For example, if the CLASSPATH value is c:\java\newLibrary.jar (Windows) or /home/me/newLibrary.jar (UNIX or Linux) it needs to be changed to .;c:\java\newLibrary.jar or .:/home/me/newLibrary.jar.

Exercises

Exercise 1. Write an application, PersistentEcho, with the following features:

• If PersistentEcho is run with command line arguments, it prints out those arguments. It also saves the string printed out to a property, and saves the property to a file called PersistentEcho.txt
• If PersistentEcho is run with no command line arguments, it looks for an environment variable called PERSISTENTECHO. If that variable exists, PersistentEcho prints out its value, and also saves the value in the same way it does for command line arguments.
• If PersistentEcho is run with no command line arguments, and the PERSISTENTECHO environment variable is not defined, it retrieves the property value from PersistentEcho.txt and prints that out.

Answer 1.

/*
 * Copyright (c) 1995 - 2008 Sun Microsystems, Inc.  All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 *   - Redistributions of source code must retain the above copyright
 *     notice, this list of conditions and the following disclaimer.
 *
 *   - Redistributions in binary form must reproduce the above copyright
 *     notice, this list of conditions and the following disclaimer in the
 *     documentation and/or other materials provided with the distribution.
 *
 *   - Neither the name of Sun Microsystems nor the names of its
 *     contributors may be used to endorse or promote products derived
 *     from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS
 * IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
 * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
 * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT OWNER OR
 * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
 * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
 * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
 * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
 * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
 * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */ 

import java.util.Properties;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;

public class PersistentEcho {
    public static void main (String[] args) {
        String argString = "";
        boolean notProperty = true;

        //Are there arguments? If so retrieve them.
        if (args.length > 0) {
            for (String arg: args) {
                argString += arg + " ";
            }
            argString = argString.trim();
        }
        //No arguments, is there an environment variable? If so,
        //retrieve it.
        else if ((argString = System.getenv("PERSISTENTECHO")) != null) {}
        //No environment variable either. Retrieve property value.
        else {
            notProperty = false;
            //Set argString to null. If it's still null after we exit
            //the try block, we've failed to retrieve the property
            //value.
            argString = null;
            FileInputStream fileInputStream = null;
            try {
                fileInputStream = new FileInputStream("PersistentEcho.txt");
                Properties inProperties = new Properties();
                inProperties.load(fileInputStream);
                argString = inProperties.getProperty("argString");
            } catch (IOException e) {
                System.err.println("Can't read property file.");
                System.exit(1);
            } finally {
                if (fileInputStream != null) {
                    try {
                        fileInputStream.close();
                    } catch(IOException e) {};
                }
            }
        }
        if (argString == null) {
            System.err.println("Couldn't find argString property");
            System.exit(1);
        }

        //Somehow, we got the value. Echo it already!
        System.out.println(argString);

        //If we didn't retrieve the value from the property, save it
        //in the property.
        if (notProperty) {
            Properties outProperties = new Properties();
            outProperties.setProperty("argString", argString);
            FileOutputStream fileOutputStream = null;
            try {
                fileOutputStream = new FileOutputStream("PersistentEcho.txt");
                outProperties.store(fileOutputStream,
                        "PersistentEcho properties");
            } catch (IOException e) {}
            finally {
                if (fileOutputStream != null) {
                    try {
                        fileOutputStream.close();
                    } catch(IOException e) {};
                }
            }
        }
    }
}