问题
A layman's question on the definition and use of variables:
I need to make a Java GUI that gets user's input and stores it within a text file. However this writing has to be done inside an Actionlistener class (ie, user is clicking the button and text file is created and stored). This means that I have to define a variable in one class (public class) and use it in another (the one that defines the Actionlistener).
How can I do this? Are global variables the only way?
In my code I first define 'textfield' as JTextField and then I want it to be read (as 'text') and stored (in 'text.txt').
import javax.swing.*;
//...
import java.io.BufferedWriter;
public class Runcommand33
{
public static void main(String[] args)
{
final JFrame frame = new JFrame("Change Backlight");
// ...
// define frames, panels, buttons and positions
JTextField textfield = new JTextField();textfield.setBounds(35,20,160,30);
panel.add(textfield);
frame.setVisible(true);
button.addActionListener(new ButtonHandler());
}
}
class ButtonHandler implements ActionListener{
public void actionPerformed(ActionEvent event){
String text = textfield.getText();
textfield.setText("");
new BufferedWriter(new FileWriter("text.txt")).write(text).newLine().close();
// Afterwards 'text' is needed to run a command
}
}
When I compile I get
Runcommand33.java:45: error: cannot find symbol
String text = textfield.getText();
^
symbol: variable textfield
location: class ButtonHandler
Without lines String text = to new BufferedWriter the code compiles.
Note that I have tried the suggestions of this Get variable in other classes and this How do I access a variable of one class in the function of another class? but they didn't work.
Any suggestions?
回答1:
Let's look at this from a design perspective: ButtonHandler
sounds a little too generic. In what way is the button click "handled"? Ah, it saves the contents of the text field to a file, so it should be called "TextFieldSaver" (or preferably something less lame).
Now, a TextFieldSaver needs to have a text field to save, yes? So add a member variable to hold the text field, and pass the text field created in the main class through a constructor:
button.addActionListener(new TextFieldSaver(textfield));
....
class TextFieldSaver implements ActionListener {
JTextField textfield;
public TextFieldSaver(JTextField toBeSaved) {
textfield = toBeSaved;
}
public void actionPerformed(ActionEvent event) {
String text = textfield.getText();
textfield.setText("");
new BufferedWriter(new FileWriter("text.txt")).write(text).newLine().close();
}
}
This isn't the only way to do it, nor necessarily the best way, but I hope it shows how using proper names sometimes shows a way out.
回答2:
How about using an anonymous inner class, and make the textfield
variable final
:
button.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent event){
String text = textfield.getText();
textfield.setText("");
new BufferedWriter(new FileWriter("text.txt")).write(text).newLine().close();
// Afterwards 'text' is needed to run a command
}
});
Note, you would need to declare the textfield
as final
:
final JTextField textfield = new JTextField();
回答3:
There is no global variable in java. each class can have some public fields. and other class may access them
you can use them like this:
class A{
public String text;
}
class B{
public static void main(String []args){
A a= new A();
System.out.println(a.text);
}
}
来源:https://stackoverflow.com/questions/18090959/how-to-use-variables-defined-in-a-public-class-in-other-classes-in-java