code: public Packer setFillX(final boolean how) {
          if (how == true) {
              gc.fill = GridBagConstraints.HORIZONTAL;
              gc.weightx = 1;
          } else {
              gc.weightx = 0;
              gc.fill = 0;
          }
          setConstraints(comp, gc);
          return this;
      }
docs: Add fill=HORIZONTAL, weightx=1 to the constraints for the current
  component if how == true. fill=0, weightx=0 if how is false.