AWT Robot has two constructors that initialize instance differently. Robot(GraphicsDevice) constructor does not invoke default constructor implementation and by this reason misses initialization of LEGAL_BUTTON_MASK variable.

Improper initialization of LEGAL_BUTTON_MASK could bring to an exception.

Exception in thread "main" java.lang.IllegalArgumentException: Invalid combination of button flags
at java.awt.Robot.checkButtonsArgument(Robot.java:325)
at java.awt.Robot.mousePress(Robot.java:257)
at FileListBetweenJVMsTest.<init>(FileListBetweenJVMsTest.java:119)
at FileListBetweenJVMsTest.main(FileListBetweenJVMsTest.java:150)

The exception is caused by the fact that LEGAL_BUTTON_MASK value is 0.