For all interessted people here, i have found a very simple and nice method to implement the jump function:

So, have fun with it and try the differet jump affects by changing the
Ease(...)getInstance()
.

public static void jump()
{
        final float jumpDuration = 0.8f;
        final int jumpHeight = 80;
        final float startY = Main.player.getY();
        final float peakY = startY - jumpHeight;

        SequenceEntityModifier jumpModifier = new SequenceEntityModifier(
                new MoveYModifier(jumpDuration, startY, peakY, EaseQuadOut.getInstance()),
                new MoveYModifier(jumpDuration, peakY, startY, EaseBounceOut.getInstance()));

Main.player.registerEntityModifier(jumpModifier);
}