Java Console is a window which displays the Java version and
text messages that a GUI programs or applets might output.
It is often used for debugging.
To save the console content (if necessary) press Ctrl-A or select the content with the mouse
and then press the button Copy.
That brings the content to the system clipboard. Then you can Paste it in text editing program.
Applications/Utilities/Java/ and run the program Java Preferencies.Advanced and set option Java Console to Show Console.
Now if you open a web page containing an applet the console windows will be opened too.
If you choose Hide Console the console window will be opened too
but minimized on the Dock.