2022.1.9Java Basics - General Classes

time related classes

Date class

Two key methods: construction method, getTime(), one is to write, and the other is to read a long string of numbers

        Date date1 = new Date(121312020222l);
        System.out.println(date1);
        System.out.println(date1.getTime());

        Date date2 = new Date(-21L*365*24*3600*1000);
        System.out.println(date2);
        System.out.println(date2.equals(date1));
        System.out.println(date2.before(date1));
        System.out.println(date2.after(date1));

DateForme

Two things are done, the str date is converted into a date object, and the date object is converted into a str date

        DateFormat format = new SimpleDateFormat("yyyy-MM-dd hh:mm:ss");
        String str= "1998-11-19 11:15:36";
        Date guoqing100 = format.parse(str);
        System.out.println(guoqing100.getTime());
        System.out.println(guoqing100);

        DateFormat format2= new SimpleDateFormat("yyyy年MM月dd日 hh时mm分ss秒");
        Date date2 = new Date(22223231545l);
        String date2Str=format2.format(date2);
        System.out.println(date2Str);

It is the day of the year, weeks

//技巧：现在是今年的第几天，几周
        Date now = new Date();
        DateFormat f1 = new SimpleDateFormat("今年的第D天，第w周");
        String str3 = f1.format(now);
        System.out.println(str3);

Math, Random classes

math

random

 

 File class

It is both a file and a directory, it can store directories or files

        File f= new File("a.txt");
        System.out.println("File是否存在："+f.exists());
        System.out.println("File是否是目录："+f.isDirectory());
        System.out.println("File是否是文件："+f.isFile());
        System.out.println("File最后修改时间："+new Date(f.lastModified()));
        System.out.println("File的大小："+f.length());
        System.out.println("File的文件名："+f.getName());
        System.out.println("File的目录路径"+f.getAbsolutePath());

 create delete file

        File f2= new File("D:/Users/JavaWebWork/javaSE/b.txt");//绝对路径
        f2.createNewFile();
        //f2.delete();
        
        /**
         * 对File对象创建空文件或目录或者删除
         */
        File f2 = new File("D:\\Users\\JavaWebWork\\javaSE\\电影\\华语");//目录结构中有一个不存在，不会创建整个目录树
        boolean flag = f2.mkdir();
        System.out.println(flag);

        flag = f2.mkdirs();//目录结构中有一个不存在，就会创建整个目录树
        System.out.println(flag);

enumerate

 

enum Season{
    SPRING,SUMMER,AUTUMN,WINTER
}
    public static void main(String[] args) {

        for(Season s:Season.values()){
            System.out.println(s);
        }

        int a = new Random().nextInt(4);//生成0，1，2,3
        switch (Season.values()[a]){
            case SPRING:
                System.out.println("春天");
                break;
            case SUMMER:
                System.out.println("夏天");
                break;
            case AUTUMN:
                System.out.println("秋天");
                break;
            case WINTER:
                System.out.println("冬天");
                break;
        }
    }

Print directory by recursion and File class

package com.demo;

import java.io.File;

//打印目录树、结合递归
public class PrintFileTree {
    public static void main(String[] args) {
        File f1 = new File(System.getProperty("user.dir"));
        printFile(f1,0);
    }

    static void printFile(File file,int level){
        for(int i=0;i<level;i++){
            System.out.print("-");
        }

        //输出文件名
        System.out.println(file.getName());

        if(file.isDirectory()){
            File[] files = file.listFiles();
            for(File temp:files){
                printFile(temp,level+1);
            }
        }
    }
}