Question

我是新手，并且想知道为什么有时我的应用程序会显示为空白，有时它会显示组件。这似乎是零星的。没有抛出异常或类似的东西。它经常出现一个空白的JFrame。有时当我关闭应用程序并再次运行它时，它会正确显示组件，但它主要显示为空白。我在代码中做错了吗？我正在使用Eclipse IDE，如果重要的话。这是代码：

import java.applet.Applet;
import java.awt.BorderLayout;
import java.awt.FlowLayout;
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;
import java.io.File;
import java.io.FileNotFoundException;
import java.util.ArrayList;
import java.util.Scanner;

import javax.swing.*;


public class Main extends JFrame implements ActionListener {
    private static final long serialVersionUID = 1L;
    JRadioButton randomRadioButton;
    JRadioButton uniqueRadioButton;
    JRadioButton participationRadioButton;
    ArrayList<Student> allStudents;
    JFrame mainFrame;

    public Main(){
        allStudents = new ArrayList<Student>();
        processAllStudents();
        mainFrame = new JFrame();
        mainFrame.setVisible(true);
        mainFrame.setSize(250, 400);
        mainFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        JPanel componentHolder = new JPanel();
        componentHolder.setLayout(new GridLayout(5,1));

        JLabel titleText = new JLabel("                     Randomizer");
        componentHolder.add(titleText);
        JButton picker = new JButton("Pick a Student");
        JFileChooser filePick = new JFileChooser();
        filePick.addActionListener(this);

        ButtonGroup allRadioButtons = new ButtonGroup();
        randomRadioButton = new JRadioButton("Completely Random");
        uniqueRadioButton = new JRadioButton("Unique");
        participationRadioButton = new JRadioButton("Complete Participation");

        allRadioButtons.add(randomRadioButton);
        allRadioButtons.add(uniqueRadioButton);
        allRadioButtons.add(participationRadioButton);

        componentHolder.add(randomRadioButton);
        componentHolder.add(uniqueRadioButton);
        componentHolder.add(participationRadioButton);

        picker.addActionListener(this);

        componentHolder.add(picker);
        componentHolder.add(filePick);
        mainFrame.add(componentHolder);
    }

    public void actionPerformed(ActionEvent e){
        if(e.getActionCommand().equals("Pick a Student")){
            if(randomRadioButton.isSelected()){
                Student result = getStudentRandom();
                result.increment();
                String resultString = new String(result.getName() + ", " + result.getFrequency());
                System.out.println(resultString);
                JLabel resultLabel = new JLabel(resultString);
                JOptionPane.showMessageDialog(mainFrame, resultLabel);
            }
            else if(uniqueRadioButton.isSelected()){
                Student firstStudent = getStudentRandom();
                Student secondStudent = getStudentRandom();
                Student result;
                if(firstStudent.getName().equals(secondStudent.getName())){
                    result = secondStudent;
                }
                else{
                    result = firstStudent;
                }
                result.increment();
                String resultString = new String(result.getName() + ", " + result.getFrequency());
                System.out.println(resultString);
                JLabel resultLabel = new JLabel(resultString);
                JOptionPane.showMessageDialog(mainFrame, resultLabel);
            }
            else if(participationRadioButton.isSelected()){
                Student result = selectStudentParticipant();
                result.increment();
                JOptionPane.showMessageDialog(mainFrame, result.getName() + ", " + result.getFrequency());
            }

        } else System.out.println("Error.");


    }
    public void processAllStudents(){
        File f = new File("Test.txt");
        Scanner scanFile = null;
        try {
            scanFile = new Scanner(f);
        } catch (FileNotFoundException e) {
            System.out.println("File Not Found");
        }
        while(scanFile.hasNext()){
            String name = scanFile.next();
            int frequency = scanFile.nextInt();
            Student s = new Student(name, frequency);
            allStudents.add(s);
        }
    }
    public Student getStudentRandom(){
        int result = (int) (Math.random() * allStudents.size());
        return allStudents.get(result);
    }
    public Student selectStudentParticipant(){
        Student temp = null;    //Start of bubble sort algorithm

        for(int i = 0; i < allStudents.size() - 1; i++){
            Student firstStudent = allStudents.get(i);
            Student secondStudent = allStudents.get(i+1);
            if(firstStudent.getFrequency() > secondStudent.getFrequency()){
                temp = firstStudent;
                firstStudent = secondStudent;
                secondStudent = temp;
            }
        }

        //End of bubble sort algorithm. Data structure now sorted increasing

        int firstRandom = (int) (Math.random() * (allStudents.size()/2) + 0.2 * allStudents.size());    //Likely to be bigger
        int secondRandom = (int) (Math.random() * (allStudents.size()/2));
        int randomIndex = 0;    //Used to represent a random index

        if(firstRandom > secondRandom){ //More likely. Selects from first half of list
            randomIndex = (int) (Math.random() * allStudents.size()/2);
        }

        else if(firstRandom < secondRandom){    //Possible, but less likely
            randomIndex = (int) ((Math.random() * allStudents.size()/2) + allStudents.size()/2);
        }

        else{   //If the two random numbers are the same
            randomIndex = (int) (Math.random() * allStudents.size()/2);
        }

        return allStudents.get(randomIndex);

    }
    public static void main(String[] args){
        new Main();
    }


}

Answer 1

setVisible(true)必须是Main方法中调用的最后一个方法。否则，JFrame上不会呈现任何内容

Answer 2

为什么有时候我的应用程序会出现空白，有时甚至是空白显示组件。这似乎是零星的。

这不是零星的，当你错过frame.pack()来告诉框架包装其所有组件时会发生这种情况

执行以下操作，框架应显示正常：

mainFrame.add(componentHolder);
mainFrame.pack();
mainFrame.setVisible(true);

Java Swing Blank JFrame即将推出？

2 个答案: