boolean aboutFace;
    boolean onGround;
    boolean atWorldEdge;
    int ySpeed = 0, xSpeed = 0;
    final int jSpeed = 20




private void move()
    {
        ySpeed++; // adds gravity
        if (xSpeed != 0 && onGround) xSpeed +=aboutFace?1:-1; // add friction
        setLocation(getX()+xSpeed/10, getY()+ySpeed/2);
        // check for change in horizontal direction
        if((xSpeed>0 && aboutFace) || (xSpeed<0 && !aboutFace)) 
        {
            getImage().mirrorHorizontally();
            aboutFace = !aboutFace;
        }

        // check for obstacles
        onGround=false; // initialize value
        // check below the actor
        while(getOneObjectAtOffset(0, getImage().getHeight()/2+1, null)!=null)
        {
            setLocation(getX(), getY()-1); 
            onGround=true; 
            ySpeed=0;
        }
        // check above the actor
        while(getOneObjectAtOffset(0, -getImage().getHeight()/2-1, null)!=null) 
        {
            setLocation(getX(), getY()+1);
            ySpeed = 0;
        }
        // check to right of actor
        while(getOneObjectAtOffset(getImage().getWidth()/2+1, 0, null)!=null)
        {
            setLocation(getX()-1, getY());
            xSpeed = 0;
        }
        // check to left of actor
        while(getOneObjectAtOffset(-getImage().getWidth()/2-1, 0, null)!=null)
        {
            setLocation(getX()+1, getY());
            xSpeed = 0;
        }
    }

    public void movement()
    {
        if (Greenfoot.isKeyDown("left")) xSpeed-=2;
        if (Greenfoot.isKeyDown("right")) xSpeed+=2; 
        if (Greenfoot.isKeyDown("up") && onGround) 
    {
        ySpeed -= jSpeed; 
    }
}