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本文发表在 rolia.net 枫下论坛Q: Jabber, I frequently get a NullPoiterException but I did not understand why. Can you say something about it?

A: Yes. Let me first give you an example in the real life. John and Mary decided to have a baby next year. And they have decided to give it a name "Mark" if it turns out to be a boy. We know a baby can smill and cry. Because Mary is so eager to have a baby, she often says:

Mark, smile!
Mark, smile!
Mark, smile!

Let me ask you a stupid question: As Mary says "Mark, smile!", will "that baby" smile???

No! The reason is that the baby have not yet been borne. This is exactly a NullPoiterException---Mark is not related to some concrete, existing baby. Before the baby is born, you cannot ask it smile.

In Java, programmers uaually first declare a varable name, then create an object instance using "new", next assign this instance to the variable. After that, you can call mathods through variable name. This sounds like:

//other code
Child child= new Child("Mark");
child.smile();
// other code, we assume we have a class Child defined somewhere else

The above cold is OK. However, the following code will throw NullPointerExpecetion:

//other code
Child child =null;
child.smile();
//other code

The reason is that you just hold a variable name "child" for the futire use and have not yet
related to any existing Child.更多精彩文章及讨论，请光临枫下论坛 rolia.net

本文发表在 rolia.net 枫下论坛A: Yes. After we declare a varable, we don't alway create a new object and assign it to the variable. Often, we get a referece to another Object by calling some other methed of some other Object, and then assign this refernce to our newly declared variable name.

//other code
Hashtable ht = new Hashtable();
ht.put("name", "John");
String s1= ( (String)ht.get("name") ).toUpperCase();
System.out.println(s1);
String s2= ( (String)ht.get("age") ).toUpperCase();
System.out.println(s2);
//other

If you test this code, you will see "JOHN" being printed out and then an NullPointerException being thrown.

The reason is that you get a null as you try to get "age" from the Hashtable ht. Since it is a null, how can you call its toUpperCase() method? So the code fails at the last but two line.

In Java library, many motheds return an Object referece in the normal case and a null in the abnormal case. If the execution your code run into the abnormal case, the
NullPointerException brings you some trouble. This is where complexities arise.更多精彩文章及讨论，请光临枫下论坛 rolia.net

本文发表在 rolia.net 枫下论坛Q: I am trying to learn Java. I copied a Hello.java example to C:\sources and then succeeded in compiling the course file into the class file. Then, I tried to run this example by entering "java Hello". However, I got an effor message saying "....java.lang.NoClassDefFoundError:...". What's wrong?

A: Very probably, your Windows envirenment variable CLASSPATH does not include ".", --- the current directory. To execute your command "java Hello", Java Virtual Machine needs to search the class file Hello.class according to CLASSPATH. If it fails to find the class file, it will complain by throwing the message you cited. For a Java programmer, it is a good practice to include the current directory in the CLASSPATH.

To make your example work, you can reset the CLASSPATH by issuing

"set CLASSPATH=.;%CLASSPATH%"

in the MS-DOS. The above command means to append . (the current directory) to the CLASSPATH.

Note: Windowes envirment variable should be sandwished by a pair of percentage signs (%). After you reset the CLASSPATH, you will have luck to run your example.

PATH and CLASSPATH are two most basic environment variables for Java programmers. Even seasoned Java developers can get into dark with them. A Java novice needs to make many mistakes with them before mastering them.更多精彩文章及讨论，请光临枫下论坛 rolia.net

本文发表在 rolia.net 枫下论坛Q: I see that some Java library classes define some final constants. By curiosity, I printed out some of these final constants using "System.out.println(...)". I observed that most of these constants are just simple numbers such 0, 1, 2. Why other to define them?

A: If you use 0, 1 , 2 ... directly in the code, you may forget what they mean in a few days. In addition, the same number can stand for different things in a different contexts.

Hence, people usuall call these numbers "magic numbers". Remember this world, it will help to talk Java as a native Canadian does.

Ia a real project, it is a common practice to define an interface to hold all the final constants.

For instance, if you buikd a Web site using Java servlet and JSP, you can define an interface called PageNames to collect all the page names. By doing so, you can avoid hard-code pages in your source files so that they can be easily maintained. In a job interview, I have been asked: Where is the good place to define constants for a project?更多精彩文章及讨论，请光临枫下论坛 rolia.net