Java Slick2D how to draw text with a centered alingment

If I understand this question correctly you want to create a class that writes text on a button for you centered to a rect.

public class DrawCentered(){
    public DrawCentered(String chars, Rect r){
        TrueTypeFont font = new TrueTypeFont(new Font(...));
        int width = font.getWidth(chars);
        int height = font.getHeight(chars);

        g.drawString((r.x + r.width / 2) - (width / 2), 
                     (r.y + r.height / 2) - (height / 2), 
                     chars);
    }
}

Note Above code is untested

I hope this helps, and if you have any further questions, don't hesitate to ask.
JavaDocs such as:
Slick2D TrueTypeFont JavaDoc
JavaAwtFont JavaDoc are great tools to help you when you are stuck.

Cheers!