程序运行时未显示JPanel Contents

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。两个面板的布局均为空，框架也都为零。程序运行良好，但在我最大化或最小化窗口或调整窗口大小之前，不会显示任何内容。

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();

}

}

为了更好地理解，请单击此处以获取程序的jar格式。

屏幕截图：

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“ revalidate”并在第二个面板上添加“ repaint”后，它现在可以工作了。

Answer 2

您忘记了在组件上调用setVisible(true) ，就像提到的Fast Snail。 您还应该调用JPanel的repaint方法。