java JOptionPane取消按钮和获取输入

Question

我试图让用户在那里输入名称，如果它留空，它会再次询问，如果他们填写它，它会设置一个JLabel或点击取消退出
我的最后一个if语句是错误的，它不像nameEnt

public Player() {
    //setBackground(Color.green);
    setSize(600, 400);
    name = new JLabel();//Input hint
    JOptionPane nameOption = new JOptionPane();
    String nameEnt = nameOption.showInputDialog("First Name: ");
    if (!nameEnt.matches("[a-zA-Z]+")) {
        name.setText(nameEnt);
    }
    if (nameEnt.length() == 0) {
        //if this condition is true JOption stays until name is entered or canceled 
    }
    if (nameEnt == nameOption.CANCEL_OPTION) {
        System.exit(0);
    }
}

Answer 1

JOptionPane.CANCEL_OPTION是一个静态int字段，不能将String与int与==进行比较

---

良好实践

如果您想一次性使用确定和取消按钮JOptionPane.showConfirmDialog和JOptionPane.showInputDialog()，但这是不可能的，我建议您改为使用以下按钮：

JTextField nameF = new JTextField(20);//create TextField

JPanel myPanel = new JPanel();//cerate JPanel
myPanel.add(new JLabel("Name"));
myPanel.add(nameF);//add your JTextField to your panel

int result;
do {
    result = JOptionPane.showConfirmDialog(null, myPanel,
            "Title of Panel", JOptionPane.OK_CANCEL_OPTION);//add your panel to JOptionPane
    if (result == JOptionPane.OK_OPTION) {//if the user press OK then
        if (nameF.getText().isEmpty()) {//check if the input is empty
            //if this condition is true JOption stays until name is entered or canceled 
        } else if (!nameF.getText().matches("[a-zA-Z]+")) {//check if the input match with your regex
            //name match exactly
            //name.setText(nameF.getText());
        }
    }
} while (result != JOptionPane.CANCEL_OPTION);//If the user hit cancel then exit

Answer 2

根据JOptionPane API，如果用户取消对话框，则返回null

所以正确的解决方案是不使用equals，而是先检查返回值是否为null，然后再检查其长度

public Player() {
    //setBackground(Color.green);
    setSize(600, 400);
    name = new JLabel();//Input hint
    JOptionPane nameOption = new JOptionPane();
    String nameEnt = nameOption.showInputDialog("First Name: ");
    if (nameEnt == null) {
        // user canceled. get out of here. 
        System.exit(0);

        // or return;  
        // or throw some exception
    }
    if (!nameEnt.matches("[a-zA-Z]+")) {
        name.setText(nameEnt);
    }
    if (nameEnt.length() == 0) {
        //if this condition is true JOption stays until name is entered or canceled 
    }
    // if (nameEnt == nameOption.CANCEL_OPTION) {
       //  System.exit(0);
    // }
}

但是为什么要用这种方式创建JOptionPane呢？最好使用静态的创建方法

// don't use null as the first parameter if the GUI is already showing
String nameEnt = JOptionPane.showInputDialog(null, "First Name: ");
if (nameEnt == null) {
    // user canceled. get out of here. 
    System.exit(0);
}

或者像这样，如果你试图循环获取输入：

public Player() {
    setSize(600, 400);  // This is not good to do. Ask for details and I'll tell.

    name = new JLabel();// Don't forget to add this to the GUI!

    String nameEnt = "";
    while (nameEnt.trim().isEmpty()) {
        // if the GUI is already showing, pass a component from it as the first param here, not null
        nameEnt = JOptionPane.showInputDialog(null, "First Name: ");
        if (nameEnt == null) {
            // user canceled. get out of here. 
            System.exit(0);

            // or return;  
            // or throw some exception
        } else if (!nameEnt.matches("[a-zA-Z]+")) {
            name.setText(nameEnt);
        } else {
            // set it to "" so that we keep looping
            nameEnt = "";
        }
    }
}