如何制作选择性 gettext()

How to make a selective gettext()

我正在尝试做一个货币兑换程序,但如果第二个输入(这里的 s1)为空,程序会给出 NumberFormatException:Empty String error。它在 first(s here) 为空时起作用。所以我想知道是否有另一种方法可以通过一个按钮使其具有选择性。为什么当第二个字段为空时它不工作?

public class kanvas1 implements ActionListener,WindowListener{


    private JTextField tf;
    private JTextField tf1;
    private JTextField tf2;
    private JTextField tf3;
    private JLabel lb;
    private JLabel lb1;

    private JButton bt;
    private JButton bt1;

    public kanvas1()
    {

        tf=new JTextField();
        tf1=new JTextField();
        tf2=new JTextField();
        tf3=new JTextField();
        lb=new JLabel("$");
        lb1=new JLabel("TL");
        bt=new JButton("Cevir");
        bt1=new JButton("Sıfırla");


        pencere();


    }

    public void pencere() {
        tf.setBounds(50,20,150,50);
        tf1.setBounds(50,80, 150, 50);
        tf2.setBounds(220,20,150,50);
        tf3.setBounds(220,80,150,50);
        lb.setBounds(30,20,20, 50);
        lb1.setBounds(30,80,20,50);
        bt.setBounds(400,20,100, 50);
        bt1.setBounds(400,80,100,50);
        bt.addActionListener(this);
        bt1.addActionListener(this);
        JFrame ab=new JFrame();
        ab.setVisible(true);
        ab.setSize(600,200);
        ab.setLayout(null);
        ab.add(tf);ab.add(tf1);ab.add(tf2);ab.add(tf3);ab.add(bt);ab.add(bt1);ab.add(lb);ab.add(lb1);
        bt.addActionListener(new ActionListener(){
            public void actionPerformed(ActionEvent arg0) {


                String s=tf.getText(); //problem is here


                double a=Double.parseDouble(s);

                double c=a*5.44;

                String result=String.valueOf(c);

                tf2.setText(result);




            }

        });
        bt.addActionListener(new ActionListener(){
            public void actionPerformed(ActionEvent arg0) {



                String s1=tf1.getText();
                                          //and here

                double b=Double.parseDouble(s1);

                double d=b*0.18;

                String result1=String.valueOf(d);

                tf3.setText(result1);


            }

        });
        bt1.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent arg0) {
                tf.setText("");
                tf1.setText("");
                tf2.setText("");
                tf3.setText("");

            }
        });


    }


    @Override
    public void windowActivated(WindowEvent arg0) {
        // TODO Auto-generated method stub

    }

    @Override
    public void windowClosed(WindowEvent arg0) {
        // TODO Auto-generated method stub

    }

    @Override
    public void windowClosing(WindowEvent arg0) {
        System.exit(0);

    }

    @Override
    public void windowDeactivated(WindowEvent arg0) {
        // TODO Auto-generated method stub

    }

    @Override
    public void windowDeiconified(WindowEvent arg0) {
        // TODO Auto-generated method stub

    }

    @Override
    public void windowIconified(WindowEvent arg0) {
        // TODO Auto-generated method stub

    }

    @Override
    public void windowOpened(WindowEvent arg0) {
        // TODO Auto-generated method stub

    }

    public static void main(String args[]) {

        new kanvas1();



    }

    @Override
    public void actionPerformed(ActionEvent arg0) {
        // TODO Auto-generated method stub

    }   
}

你可以像这样在解析之前添加一个检查:

if (s1.isEmpty())
     return;

它失败了,因为 Double.parseDouble(String arg) 试图将作为参数传递的字符串转换为该字符串表示的 Double。这意味着 "12"、"12.0" 等字符串 是有效输入,而 "12a"、"ac"、"" 等字符串[= =28=]。注定要失败;因为,毕竟 12a 不是双精度数(甚至不是数字!)。

如果文本字段为空并且您执行 getText(),您将检索到的是 ""(空字符串)而不是 null

因此,如果您想做出任何有条件的决定,请将您的条件基于检索到的文本 empty i.e. "" 而不是 null

不用为同一个按钮写两次 addActionListener,您可以写一个 addActionListener此外,避免将任何字段留空,您将得到 NumberFormatException,因为您的 JTextField 之一给出了空白值或任何其他非双精度值。使用 Double.parseDouble("12.34") 进行解析时,应始终包含字符串格式的双精度值。解析其他值会抛出 NumberFormatException

bt.addActionListener(new ActionListener(){
    public void actionPerformed(ActionEvent arg0) {


        String s=tf.getText(); //problem is here


        double a=Double.parseDouble(s);

        double c=a*5.44;

        String result=String.valueOf(c);

        tf2.setText(result);




    }

});
bt.addActionListener(new ActionListener(){
    public void actionPerformed(ActionEvent arg0) {



        String s1=tf1.getText();
                                  //and here

        double b=Double.parseDouble(s1);

        double d=b*0.18;

        String result1=String.valueOf(d);

        tf3.setText(result1);


    }

});

将上面的代码替换为:

bt.addActionListener(new ActionListener(){
    public void actionPerformed(ActionEvent arg0) {


        String s=tf.getText();


        double a=Double.parseDouble(s);

        double c=a*5.44;

        String result=String.valueOf(c);

        tf2.setText(result);



         String s1=tf1.getText();
         double b=Double.parseDouble(s1);

        double d=b*0.18;

        String result1=String.valueOf(d);

        tf3.setText(result1);



    }

});