虽然只适用于后面的println行

Question

我刚刚挖掘了几年前我做过的一个java项目，它是关于一个国际象棋游戏，我发现了一个奇怪的行为：

程序启动，窗口打开，所有按钮都放好，但游戏无法启动。我不明白的是，添加一个简单（无用）的打印将使游戏开始。

因此，只有下一条打印线时，程序才会进入while。

public static void main(String[] args){
    //The white pieces are at bottom

    int gioco = 0;

    while (!(gioco == 1)){

        new Window();

        //creating, initializing and first printing of the board
        final Board scacchiera = new Board();
        Configuration.initialization(scacchiera);
        //System.out.println(scacchiera.toString());

        while (!scacchiera.getCheckmate()){

            //THE NEXT LINE IS THE ONE
            //if it is not there, the program doesn't work
            //everything inside the println is ok
            System.out.println(sem);

            if (sem){//Waiting the inputs from the window
                sem = false;

                if(scacchiera.getChessman(x, y).getColour() == turn){

                    if (scacchiera.move(scacchiera, scacchiera.getChessman(x, y), newX, newY)){
                        turn = (turn == Colour.bianco) ? Colour.nero : Colour.bianco;
                        MyFrame.generateBoard(scacchiera);
                    }
                    //System.out.println(scacchiera.toString());
                }
                else{
                    JOptionPane.showMessageDialog(null, "It's not your turn.", "Turn error", JOptionPane.WARNING_MESSAGE);
                }
            }
        }

        JOptionPane option = new JOptionPane ("CheckMate!\n"
                + "Do you want to play another game?", JOptionPane.QUESTION_MESSAGE, JOptionPane.YES_NO_OPTION, null);

        JDialog dialog = null;
        if (turn == Colour.bianco){
            dialog = option.createDialog(null,"Black side won!");
        }
        else{
            dialog = option.createDialog(null,"White side won!");
        }
        dialog.pack();
        dialog.setVisible(true);
        gioco = ((Integer)option.getValue()).intValue(); //saving the choice: 0: yes 1: no

        if(gioco == 1){
            JOptionPane.showMessageDialog(null, "See you soon!");
            System.exit(0);
        }

        //TODO
        //se voglio fare un'altra partita devo prima chiudere la finestra della partita precedente?

    }   
}

为什么？