Can't use "java" command

Question

I just can't use the java command...
The paths have been set and everything should work...
If I just type "java" (without the "") I get:
Usage: java [-options] class [args...]
(to execute a class)
or java -jar [-options] jarfile [args...]
(to execute a jar file)
where options include:
-client to select the "client" VM
-server to select the "server" VM
-hotspot is a synonym for the "client" VM [deprecated]
The default VM is client.
-cp -classpath <directories and zip/jar files separated by ;>
set search path for application classes and resources
-D<name>=<value>
set a system property
-verbose[:class|gc|jni]
enable verbose output
-version print product version and exit
-showversion print product version and continue
-? -help print this help message
-X print help on non-standard options
-ea[:<packagename>...|:<classname>]
-enableassertions[:<packagename>...|:<classname>]
enable assertions
-da[:<packagename>...|:<classname>]
-disableassertions[:<packagename>...|:<classname>]
disable assertions
-esa | -enablesystemassertions
enable system assertions
-dsa | -disablesystemassertions
disable system assertions
And if I type "java Oper" (It compiled normally and worked on my school computer) it gives me this error:
Exception in thread "main" java.lang.NoClassDefFoundError: Oper
It works everywhere but on my computer...
Oh, and I have Java Builder 3 University Edition installed. When I use that to compile + run my program, it works perfectely... I just can't use "java" in command prompt. I have a Windows ME.
Another note, this may be related: I can't use certain Java applets on my computer, like the IRC applets for example. I tried it on both Netscape 7.01 and Internet Explorer 6.0.2800.1106. It displays the same kind of error, the Exception in thread "..." java.lang.NoClassDefFoundError: ... .
And I've already tried uninstalling + reinstalling. Any other suggestions?

Answer 1

start -> run -> cmd
(if it doesn't open command prompt, then try command instead of cmd)
once you have reached command prompt, then you change to some empty directory where you would like to create java or class files. (use cd command for that)
once you have reached that directory, type: notepad Hello.java
(it should open notepad, and it should prompt you a question if you want to create file Hello.java (since it didn't excist) ... click yes)
in notepad type the following:class Hello {
public static void main(String[] args) {
  System.out.println("Hello");
}once you have written that into notepad, then click File and choose Save from there.
now you may close your notepad.
now go back to that command prompt and type in command dir Hello.*
it should lis you all Hello.* files, and Hello.java should be one of these.
now, when you have seen that in your current directory a Hello.java does excist, you may type javac Hello.java
if it does compile whitout any errors then everything is OK so far.
type again dir Hello.*
now you should see two files, Hello.java and Hello.class
if you now saw that Hello.class was in the current directory, you may type the following command: java Hello
it should display a String "Hello" on your screen.
if it doesn't work, then you have problem with java not working from commandline (some classpaths are wrong or unset or smthng similar)
if you suspect that something doesn't work, then always test it with simplest task.