我的JFrame上不显示JPanels

Question

public class Fantasy extends JFrame {
int di;
int mi;
int si;

public Fantasy() {
    setTitle("Fantasy Football");
    setDefaultCloseOperation(EXIT_ON_CLOSE);
    setLayout(new BorderLayout());
    setSize(700,700);
    setResizable(false);

    String[] positions = {"4-4-2", "4-3-3", "3-5-2", "5-3-2", "3-4-3", "4-5-1"};

    JComboBox select = new JComboBox(positions);
    add(select, BorderLayout.NORTH);
    select.addActionListener(new ActionListener() {
        @Override
        public void actionPerformed(ActionEvent e) {
            select.getItemAt(select.getSelectedIndex());
            Pattern p = Pattern.compile("(\\d)\\-(\\d)\\-(\\d)");
            Matcher m = p.matcher(select.toString());
            m.find();
            di = Integer.parseInt(m.group(1));
            mi = Integer.parseInt(m.group(2));
            si = Integer.parseInt(m.group(3));
        }
    });

    JPanel ptch = new JPanel();
    add(ptch, BorderLayout.CENTER);

    JPanel goal = new JPanel();
    goal.setLayout(new FlowLayout());
    goal.add(createGoalieHolder());

    JPanel dfnc = new JPanel();
    dfnc.setLayout(new FlowLayout());
    dfnc.add(createDefenceHolder());

    JPanel midf = new JPanel();
    midf.setLayout(new FlowLayout());
    midf.add(createMidfHolder());

    JPanel strk = new JPanel();
    strk.setLayout(new FlowLayout());
    strk.add(createStrkHolder());

    JPanel bnch = new JPanel();
    bnch.setLayout(new FlowLayout());
    bnch.add(createBenchHolder());

    ptch.add(goal);
    ptch.add(dfnc);
    ptch.add(midf);
    ptch.add(strk);
    add(bnch);
    setVisible(true);
}

private JPanel createGoalieHolder() {
    JPanel gh = new JPanel(new GridLayout(2,1));
    JTextField gk = new JTextField("Goalkeeper");
    JButton g = new JButton("+");
    gh.add(gk,g);
    return gh;
}

private JPanel createDefenceHolder() {
    JPanel dh = new JPanel(new GridLayout(2, 1));
    JTextField df = new JTextField("Defender");
    JButton d = new JButton("+");
    for (int a = 0; a < di; a++) {
        dh.add(df,d);
    }
    return dh;
}

private JPanel createMidfHolder() {
    JPanel mh = new JPanel(new GridLayout(2, 1));
    JTextField md = new JTextField("Midfielder");
    JButton m = new JButton("+");
    for (int b = 0; b < mi; b++) {
        mh.add(md,m);
    }
    return mh;
}


private JPanel createStrkHolder() {
    JPanel sh = new JPanel(new GridLayout(2, 1));
    JTextField sk = new JTextField("Striker");
    JButton s = new JButton("+");
    for (int c = 0; c < si; c++) {
        sh.add(sk,s);
    }
    return sh;
}

private JPanel createBenchHolder() {
        JPanel gkh = new JPanel(new GridLayout(2,1));
        JTextField gkt = new JTextField("Goalkeeper");
        JButton gkb = new JButton("+");
        gkh.add(gkt,gkb);

    for(int ad = 0; ad < (5-di); ad++) {
        JPanel dfh = new JPanel(new GridLayout(2,1));
        JTextField dft = new JTextField("Defender");
        JButton dfb = new JButton("+");
        dfh.add(dft,dfb);
        return dfh;
    }

    for(int bd = 0; bd < (5-mi); bd++) {
        JPanel mfh = new JPanel(new GridLayout(2,1));
        JTextField mft = new JTextField("Midfielder");
        JButton mfb = new JButton("+");
        mfh.add(mft,mfb);
        return mfh;
    }

    for(int cd = 0; cd < (3-si); cd++) {
        JPanel sih = new JPanel(new GridLayout(2,1));
        JTextField sit = new JTextField("Striker");
        JButton sib = new JButton("+");
        sih.add(sit,sib);
        return sih;
    }
    return gkh;
}
}

因此，createHolder方法应该使用JButtons和JTextFields创建JPanels。功能将在稍后添加。然而，在这一点上，我不能让任何东西真正出现在我的JFrame上，除了一个带有"Defender“字样的JTextField。我不确定还能怎么做。

Answer 1

所以，基本上，你这样做...

add(ptch, BorderLayout.CENTER);
//...
add(bnch);

这和做这个差不多。

add(ptch, BorderLayout.CENTER);
//...
add(bnch, BorderLayout.CENTER);

或者这个..。

//add(ptch, BorderLayout.CENTER);
//...
add(bnch, BorderLayout.CENTER);

为了所有强烈的目的。

BorderLayout将只管理任何给定位置的单个组件，这意味着实际上只有bnch被布局。

如果不知道自己到底想要做什么，就很难知道具体要建议什么，但可以从尝试开始

add(ptch, BorderLayout.CENTER);
//...
add(bnch, BorderLayout.SOUTH);

这应该会有帮助。

接下来..。

private JPanel createGoalieHolder() {
    JPanel gh = new JPanel(new GridLayout(2, 1));
    JTextField gk = new JTextField("Goalkeeper");
    JButton g = new JButton("+");
    gh.add(gk, g);
    return gh;
}

这里的问题是gh.add(gk, g);，第二个参数通常是布局的约束属性，相反，您应该执行更多类似于...

private JPanel createGoalieHolder() {
    JPanel gh = new JPanel(new GridLayout(2, 1));
    JTextField gk = new JTextField("Goalkeeper");
    JButton g = new JButton("+");
    gh.add(gk);
    gh.add(g); 
    return gh;
}

这几乎适用于您的所有其他create方法

您可能希望花更多时间了解Laying Out Components Within a Container、How to Use BorderLayout和How to Use GridLayout以了解更多详细信息

你会遇到的另一个问题是...

private JPanel createDefenceHolder() {
    JPanel dh = new JPanel(new GridLayout(2, 1));
    JTextField df = new JTextField("Defender");
    JButton d = new JButton("+");
    for (int a = 0; a < di; a++) {
        dh.add(df);
        dh.add(d);
    }
    return dh;
}

(我已经更正了add方法)组件只能驻留在单个父级中，因此上面的内容应该更类似于

private JPanel createDefenceHolder() {
    JPanel dh = new JPanel(new GridLayout(2, 1));
    for (int a = 0; a < di; a++) {
        JTextField df = new JTextField("Defender");
        JButton d = new JButton("+");
        dh.add(df);
        dh.add(d);
    }
    return dh;
}

但是，当您调用此方法时，di为0，并且您对di所做的任何更改都不会影响UI，除非您重新调用此方法并使用新面板更新UI