New to Java

Do you want to run it on the user-tier or the server? User-tier code is typically for doing things like interacting with the user's machine (e.g. opening Acrobat Reader, accessing hardware attached to user's machine, etc). Server side code is usually for doing things that do not use a UI and suppliment what Forms can process (e.g. retrieving data from a REST service, performing complex calculations where the result will be used by the application's PLSQL, etc).

Yes, i want to run it on user's machine.

So what you want is called a Forms Java Bean or Plugable Java Components (PJC). A Java Bean is typically used when you want to create your own widget or introduce something unrelated to Forms (e.g. HTML viewer). A PJC is typically used when you want to extend behavior that Forms already provides. For example if you didn't like the way the cursor appears in a Forms text item, you can use a PJC to change that behavior and/or appearance.
There are many example that can be found on the Internet.
https://www.google.com/search?q=oracle+forms+beans
Some information about creating a Java Bean and PJCs can be found in the Form Builder Help.

Thank you for giving me a answer,

After some research and studying oracle forms bean, a jar file need to create to implement java code in oracle bean.

I had create a my custom jar file using netbean tool (which i want to used on my forms) but it show that class not fond error when executing form.

A jar file is also placed in oracle/forms/java/ folder and also add configuration in frmcfg. File as per research .
But i can't find the way to make .jar which is accessible in oracle forms 12c version.

I execute examples files which is download from custom pjc demo. It is working well. But file not working.

You must stop the managed server before copying new files into /forms/java. If the file is already in the directory, stop the server and restart it. Note that the file must be declared in the Web Configuration (i.e. formsweb.cfg) but this must be edited using Fusion Middleware Control. Do not edit the file in a text editor. Also, if you are using Java Web Start, the entry must be made in extensions.jnlp which is found in /forms/java. Again, any changes to this directory will require server restart.

Yes, i followed all this steps but still it showing class not found error.

Verify that your JAR is being downloaded - look at the Java Console. If it is, consider the contents of your JAR. Are there sub-directories within it? If so, be sure you reference that path correctly in your form.

Thank you,
After adding jar file path into the classpath parameter in the default configuration file and restart the weblogic server it it now accessible and working properly.

I will guess and say that you did not mean "adding jar file path to classpath", but rather adding to ARCHIVE. In most cases, the server classpath has nothing to do with JARs used on the user-tier.