Java SE specification states for java.awt.Color.brighter()/darker() methods:

For instance "darker"
  "This method applies an arbitrary scale factor to each of the three RGB
   components of this Color to create a darker version of this Color."

This means that alpha component must remain unchanged, however it is not
the case for many Java implementations: 1.4.2, 5.0, 6.0, 7.

The code snippet helps to reproduce the problem:
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import java.awt.Color;

public class TestColor {

    private void test(boolean direction) {
        Color color = new Color(20, 20, 20, 125);
        System.out.println(color.getAlpha());
        final int initialAlpha = color.getAlpha();
        for (int i = 0; i < 3; i++) {
            if (direction) {
                color = color.brighter();
            } else {
                color = color.darker();
            }
            if (color.getAlpha() != initialAlpha) {
                System.out.println(direction ? "Color.brighter() failed: " + color.getAlpha() + " " + initialAlpha :
                                               "Color.darker() failed: " + color.getAlpha() + " " + initialAlpha);
            }
        }
    }

    public static void main(String argv[]) {
        new TestColor().test(true);
        new TestColor().test(false);
   }
}
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