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The only thing I can think of is to uninstall java and reinstall it.

The java program could not find the HelloDialog.class file.
Check that the file exists. It is created by the javac program.
Look at the tutorial for help: "Hello World!" for Microsoft Windows (The...

Open a command prompt window, change to the directory with the HelloDialog.class file and enter:
java HelloDialog

copy the full contents of the command prompt window and paste it here. If there...