程序运行时不显示 JPanel 内容

Question

我使用swing创建了一个简单的日历。我在框架内使用了两个面板。两个面板都具有空布局和框架。程序运行正常，但在我最大化或最小化窗口或调整窗口大小之前不会出现任何内容窗口。

这是我的代码：

import java.awt.Color;
import java.time.DayOfWeek;
import java.time.LocalDate;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;

public class CalendarTest {
JFrame f;
JLabel monthYearLbl;
JLabel[] dayLbl,dateLbl;
JPanel monthAndYear,dayAndDate;
LocalDate ld;
String month,year,day;

//constructor
public CalendarTest(){
    f = new JFrame("Calender");
    f.setSize(365,250);
    f.setLayout(null);
    f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    f.setLocationRelativeTo(null);
    f.setVisible(true);
}
public void showMonthAndYear(){
    monthAndYear = new JPanel();
    monthAndYear.setBounds(0,0,350,25);
    f.add(monthAndYear);
    monthAndYear.setBackground(Color.MAGENTA);
    ld = LocalDate.now();
    month = String.valueOf(ld.getMonth());
    year = String.valueOf(ld.getYear());
    monthYearLbl = new JLabel(month+"-"+year);
    monthAndYear.add(monthYearLbl);
}
public void showDayAndDate(){
    dayAndDate = new JPanel();
    dayAndDate.setLayout(null);
    dayAndDate.setBounds(0,27,350,185);
    f.add(dayAndDate);
    dayAndDate.setBackground(Color.cyan);
    //getting the first day name of the current month
    DayOfWeek dow = LocalDate.of(ld.getYear(),ld.getMonth(),1).getDayOfWeek();
    int lengthOfMonth = LocalDate.of(ld.getYear(),ld.getMonth(),1).lengthOfMonth();
    int friday=0;
    dayLbl = new JLabel[8];
    int xCord = 10;
    for(int i=0;i<7;i++){
        String s = String.valueOf(dow);//converting dow object to string 
        String formatedDay = String.format("%.3s",s);//formating day in short form
        if(formatedDay== "FRI"){
            friday = i;
        }
        dayLbl[i] = new JLabel(formatedDay);//creating jlabel
        dow = dow.plus(1);//incrementing day name
        dayLbl[i].setBounds(xCord,10,30,20);
        dayAndDate.add(dayLbl[i]);
        xCord = xCord+50;//dynamic positioning of day name
    }
    dateLbl = new JLabel[35];
    int index = 1,ycord=35;
    //loop for showing the date from 1 to endofmonth
    for(int i=1;i<=5;i++){
        int xcord = 15;
        for(int j=0;j<7;j++){
            dateLbl[index] = new JLabel(String.valueOf(index));
            dateLbl[index].setBounds(xcord,ycord,20,20);
            if(j==friday){
                dateLbl[index].setBackground(Color.black);//indicating holiday
            }
            dayAndDate.add(dateLbl[index]);
            xcord = xcord+50;
            if(index==lengthOfMonth){
                break;
            }
            index++;
            f.add(dayAndDate);
        }
        ycord = ycord+25;
    }
}
public static void main(String[] args) {
    CalendarTest calendar = new CalendarTest();
    calendar.showMonthAndYear();
    calendar.showDayAndDate();

}

}

For better understanding click here to get the jar format of the program.

截图：

调整窗口大小之前

调整窗口大小后

Answer 1

回答我自己的问题：

在第一个面板上添加“重新验证”并在第二个面板上添加“重新绘制”后，它现在可以工作了..

Answer 2

你忘记在你的组件上调用setVisible(true)，就像提到的 Fast Snail 一样。您还应该调用 JPanel 的 repaint 方法。