运行程序时出现异常

Question

尝试运行程序MOBGame时出现此错误。

Exception in thread "main" java.lang.NoSuchMethodError:      org.newdawn.slick.Font.drawString(FFLjava/lang/CharSequence;)V
at org.newdawn.slick.GameContainer.updateAndRender(GameContainer.java:711)
at org.newdawn.slick.AppGameContainer.gameLoop(AppGameContainer.java:456)
at org.newdawn.slick.AppGameContainer.start(AppGameContainer.java:361)
at org.graysontsa.vgdesign.MOBGame.main(MOBGame.java:27)

该文件的来源是：

import org.newdawn.slick.state.StateBasedGame;
import org.newdawn.slick.AppGameContainer;
import org.newdawn.slick.GameContainer; 
import org.newdawn.slick.SlickException;

public class MOBGame extends StateBasedGame{

    public static final int STARTSCREENSTATE = 0;
    public static final int GAMESTATE = 1;

    public MOBGame(){
        super("MOB");

        //this.addState(new StartScreenState(STARTSCREENSTATE));
        this.addState(new GameState(GAMESTATE));
        this.enterState(GAMESTATE);
        //this.enterState(STARTSCREENSTATE);
    }

    public static void main(String[] args){
        try{
           AppGameContainer app = new AppGameContainer(new MOBGame());

            app.setDisplayMode(640, 480, false);
            app.start();
        }catch (SlickException e){
            e.printStackTrace();
        }
    }

    @Override
    public void initStatesList(GameContainer gameContainer) throws SlickException {
        // this.getState(STARTSCREENSTATE).init(gameContainer, this);
        this.getState(GAMESTATE).init(gameContainer, this);
    }
}

我已经用另一个程序对此进行了测试，并且它也适用于此。但是，它不想使用它，这两个文件非常相似，只是我在这个程序中使用状态。