Java actionPerformed在GUI调整大小之前不会添加更改的标签

Question

I am a high school student trying to learn java and any help or advice is appreciated. The code does not set the text after I press the button. The actionPerformed does not do what it is supposed to until I change the size of the GUI for some reason. When I run it and press the button a . pops up but nothing else, if you were to run it then you would see what I am talking about. I think it stops during the action performer for some reason but I am really just guessing. Thank you.

import java.awt.*;
import java.text.DecimalFormat;
import java.text.NumberFormat;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JLabel;
import javax.swing.JTextField;
import javax.swing.JFrame;

public class GUI extends JFrame implements ActionListener{
public TextField mpgEnter;
public Label lblResult;
public Button btnEnter;
public TextField gasPriceEnter;
public TextField distanceEnter;
public double mpg;
public double distanceTravel;
double priceOfgas;

public GUI() {


setTitle("GUI Test");



setSize(280, 400);

setDefaultCloseOperation(EXIT_ON_CLOSE);
Label lbl2 = new Label("MPG ||");
add(lbl2);
mpgEnter = new TextField(5);
Label lbl3 = new Label("Price of gas ||");
add(lbl3);
gasPriceEnter = new TextField(5);
Label lbl4 = new Label("Distance");
add(lbl4);
distanceEnter = new TextField(5);
setLayout(new FlowLayout());
add(mpgEnter);
add(gasPriceEnter);
add(distanceEnter);

// add button
btnEnter = new Button("Enter");
btnEnter.addActionListener(this);
add(btnEnter);
//add label


//setResizable(false);
lblResult = new Label("");
add(lblResult);
setVisible(true);
}
public static double calculateGPM(double mpg, double gas,double distance){
    double gasPerMile = gas/mpg;
    double cost1 = gasPerMile * distance;
    NumberFormat formatter = new DecimalFormat("#0.00"); 
    double costFinal = Double.parseDouble(formatter.format(cost1));

    return costFinal;
}
@Override
public void actionPerformed(ActionEvent e) {

    //System.out.print(test.getText());
    mpg = Double.parseDouble(mpgEnter.getText());
    priceOfgas= Double.parseDouble(gasPriceEnter.getText());
    distanceTravel = Double.parseDouble(distanceEnter.getText()); 
    lblResult.setText("The price for your trip is: $" + calculateGPM(mpg, priceOfgas, distanceTravel));
    add(lblResult);

    }
    }

Answer 1

First, you're mixing heavy weight components with light weight components, this is going to cause you no end of issues and is best avoided.

Instead of:

• Label use JLabel
• TextField use JTextField
• Button use JButton

Your primary issue is the fact that your container isn't been updated when the text is changed, this is a limitation of java.awt.Label and simply changing it to javax.swing.JLabel will automatically cause the container to be revalidated once the text of the label has been changed.

See...

• Creating a GUI With JFC/Swing
• Laying Out Components Within a Container
• Should I avoid the use of set(Preferred|Maximum|Minimum)Size methods in Java Swing?

for more details

Answer 2

The lblResult label is initialized with "". When you set the value (a large text) there is no room for it inside the flow layout.

I think you must set a more elaborated layout or a minimum width for the result label:

Label width:

  setSize(400, 400);
  lblResult.setPreferredSize(new Dimension(200, 20));

Example Layout:

    final JPanel north = new JPanel(new FlowLayout());
    north.add(mpgEnter);
    north.add(gasPriceEnter);
    north.add(distanceEnter);
    // add button
    btnEnter = new Button("Enter");
    btnEnter.addActionListener(this);
    north.add(btnEnter);
    // add label
    add(north, BorderLayout.NORTH);
    // setResizable(false);
    lblResult = new Label("");
    lblResult.setPreferredSize(new Dimension(200, 20));
    add(lblResult, BorderLayout.CENTER);