无法从JTextField获取整数

Question

I'm trying to get an integer from a JTextField , but I keep getting a NumberFormatException . I used the code below:

JTextField price = new JTextField();
price.addActionListener(new ComboListener());
String inputText = price.getText();
int inputPrice = Integer.parseInt(inputText);

Every site says this is the proper way to do it, so I don't understand what I'm doing wrong.

edit: The full code is here:

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class RatePanel extends JPanel {

    private double[] rate; // exchange rates
    private String[] currencyName;
    private JLabel result;

    public RatePanel() {
        currencyName = new String[]{"Select the currency..",
            "European Euro", "Canadian Dollar",
            "Japanese Yen", "Australian Dollar",
            "Indian Rupee", "Mexican Peso"};

        rate = new double[]{0.0, 1.2103, 0.7351,
            0.0091, 0.6969, 0.0222, 0.0880};

        JLabel title = new JLabel("How much is that in dollars?");
        title.setAlignmentX(Component.CENTER_ALIGNMENT);
        title.setFont(new Font("Helvetica", Font.BOLD, 20));
        add(title);
        add(Box.createRigidArea(new Dimension(0, 100)));
        JLabel enter = new JLabel("Enter cost of item");
        enter.setAlignmentX(Component.LEFT_ALIGNMENT);
        enter.setFont(new Font("Helvetica", Font.BOLD, 20));
        add(enter);
        JTextField price = new JTextField();
        price.addActionListener(new BoxListener());
        price.setAlignmentX(Component.RIGHT_ALIGNMENT);
        add(price);
        add(Box.createRigidArea(new Dimension(0, 100)));
        JLabel select = new JLabel("Select a currency: ");
        select.setAlignmentX(Component.LEFT_ALIGNMENT);
        select.setFont(new Font("Helvetica", Font.BOLD, 20));
        add(select);
        JComboBox Cbox = new JComboBox(currencyName);
        Cbox.addActionListener(new ComboListener());
        Cbox.setAlignmentX(Component.RIGHT_ALIGNMENT);
        add(Cbox);
        String index = Cbox.getSelectedItem().toString();
    }

    public class BoxListener implements ActionListener {

        public void actionPerformed(ActionEvent event) {
            String inputText = price.getText();
            int inputPrice = Integer.parseInt(inputText);
        }
    }

    public class ComboListener implements ActionListener {

        public void actionPerformed(ActionEvent event, String index, double inputPrice, double[] rate) {
            double finalPrice = 0;
            switch (index) {
                case "European Euro":
                    finalPrice = inputPrice * rate[1];
                    break;
                case "Canadian Dollar":
                    finalPrice = inputPrice * rate[2];
                    break;
                case "Japanese Yen":
                    finalPrice = inputPrice * rate[3];
                    break;
                case "Australian Dollar":
                    finalPrice = inputPrice * rate[4];
                    break;
                case "Indian Rupee":
                    finalPrice = inputPrice * rate[5];
                    break;
                case "Mexican Peso":
                    finalPrice = inputPrice * rate[6];
                    break;
            }

            result = new JLabel(inputPrice + "USD equals " + finalPrice
                    + index);
            add(result);
        }

        @Override
        public void actionPerformed(ActionEvent e) {
            throw new UnsupportedOperationException("Not supported yet.");
        }
    }
}

Answer 1

I ran your code, it actually works fine (without giving me NumberFormatException ).

You get NumberFormatException probably because you attempted the following:

1. Press Enter on the textfield when the field is empty
2. Press Enter on the textfield when the field contains non-numeric input

You could add validations to your input before attempting to parse the textfield's content into integer:

public class BoxListener implements ActionListener {

    public void actionPerformed(ActionEvent event) {
        String s= price.getText();
        if(s.matches("[0-9]+"))               //Perform validation before parsing string
            inputPrice = Integer.parseInt(s);
    }
}

Also note that, instead of declaring inputPrice and your other components such as your textfields as local variables, I declared them as instance variables of RatePanel .

---

To declare your variables as instance variable instead of local variables:

class RatePanel extends JPanel{
    private JTextfield txtPrice;   // <--- declare here as instance variable
    private int inputPrice;        // <--- declare here as instance variable

    public RatePanel(){
        //If you declare in the constructor or other methods, they become local variables.
    }
}

Answer 2

When I try to define it in the actionListener method, it tells me it can't find the textbook

That is probably because you define 'price' as a local variable in the method that you create it. Your 'price' variable should be an instance variable that is visible to all methods in your class.

Answer 3

First of all, the code as written tries to access the text right after the text field is created, before the user has typed anything into it. The code accessing the text should be inside the action listener. If ComboListener is a class that you created, then the last two lines should be in its actionPerformed method; if you do it that way, make sure that the price variable is an instance variable or static variable (ie, outside a method definition—whether to make it static and which class to put it in depends on how the rest of your code is structured).

An alternative way is to use an anonymous inner class, like this:

// the 'final' modifier is necessary for anonymous inner classes
// access local variables
final JTextField price = new JTextField();
price.addActionListener(new ActionListener() {
    public void actionPerformed(ActionEvent e) {
        String inputText = price.getText();
        // do the conversion and other processing here...
    }
});

(I believe in newer versions of Java this can be abbreviated as

price.addActionListener((ActionEvent e) -> {
    String inputText = price.getText();
    // do the conversion and other processing here...
});

but I'm not 100% sure about that.)

Second, keep in mind that if you use parseInt , it'll only accept whole number values; things like 12.34 (with a decimal point) will cause an error. The simplest way to fix this is to use Double.parseDouble , although using double s with prices can cause rounding errors, so it's not the best way to do it. Also, make sure you don't put a $ or £ or € or whatever in the box; that could also cause an error.

Answer 4

You are reading a JTextField immediately after creating it. At that point, it's empty empty string, which is not a valid number.

You should put that code in the listener where it will fire when the user presses Enter or something. And, additionaly, try to be nice to the user and show an error message instead of throwing an exception.