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ShowJavaConsole

ShowJavaConsole[]
displays the Java console window and begins capturing output sent to the Java System.out and System.err streams.

ShowJavaConsole["stdout"]
captures only System.out.

ShowJavaConsole["stederr"]
captures only System.err.

Anything written to these streams before ShowJavaConsole is first called will not appear, but subsequent output will be captured even if the window is closed and reopened.