InputEvent spec :
"public static final int BUTTON1_MASK
    The Mouse Button1 modifier constant. It is recommended that BUTTON1_DOWN_MASK be used instead."
The same for other masks.

That time java.awt.Robot.mousePress/Release() says:
"public void mousePress(int buttons)
    Presses one or more mouse buttons. The mouse buttons should be released using the mouseRelease method.
    Parameters:
        buttons - the Button mask; a combination of one or more of these flags:
>> * InputEvent.BUTTON1_MASK
>> * InputEvent.BUTTON2_MASK
>> * InputEvent.BUTTON3_MASK
[skip]"