alignmentX
Sets the horizontal alignment along the x axis, where 0.0 aligns to the left, 0.5 centers, and 1.0 aligns to the right. A parent that honors alignment - a vertical BoxLayout - lines its children up by this value, so siblings given the same alignment stay in one column.
A component holding no alignment of its own reports the one its layout manager derives - a BoxLayout derives it from the children present - or the centered default where none is derived. Swing offers no way to hand an alignment back to the layout once one is set, so removing the modifier writes the alignment the layout derives at that moment. Swing also offers no way to ask whether an alignment was ever set, so an alignment a component does hold is restored only where it differs from the one its layout derives as the declaration attaches. One that matches it is read as none.
Return
this modifier with the horizontal alignment declared on it.
Parameters
the fraction of the component's width the parent lines its children up on; Swing clamps a value outside 0.0..1.0 into that range rather than rejecting it.