• Select "Run -> Run Configurations" from the menu.
• Search for you project in the list on the left and select it.
• Select the "Arguments" tab on the right.
• Write the argument you want to pass to the programm in "Programm arguments".
• Click "Run"

Perform the following steps:

1. Create a new project in eclipse
2. Link your classes from the project properties' buildpath
3. Go to Run->Run Configurations
4. Create a new configration
5. Point your main class
6. And Run

It's important to note that you can't just run arbitrary Java code, but have to have some structure set up (for example, if you're going to call a method, what arguments are you passing in?) To run a specific piece of Java code, you should consider creating a main method in that class that just launches some specific piece of code under the constraints that you'd like. One way to do this would be to add a method public static void main(String[] args) containing the code you want to run. Once you've done this, you can right-click on it, choose the "Run as..." option, and select "Java application" to run the program you've written.

Alternatively, if you just want to check whether some specific piece of code is working, you could consider using JUnit to write unit tests for it. You could then execute the JUnit test suite from Eclipse to check whether that specific piece of code is functioning correctly. This is what I would suggest doing, as it makes it possible to test the software through every step of the development.

Hope this helps!