//方法1(最快)
System.currentTimeMillis();   
//方法2  
Calendar.getInstance().getTimeInMillis();  
//方法3  
new Date().getTime(); 

public class TimeTest {
    public static void main(String[] args) {
        Long timeStamp = System.currentTimeMillis();
        SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
        SimpleDateFormat sdf2 = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss:SSS");
        System.out.println("Long類型的時(shí)間戳："+timeStamp);
        System.out.println("格式化后的時(shí)間："+sdf.format(timeStamp));
        System.out.println("格式化后的時(shí)間帶毫秒："+sdf2.format(timeStamp));
    }
}

import java.text.SimpleDateFormat;
import java.util.Date;

public class TimestampConverter {
    public static void main(String[] args) {
        long timestamp = 1599475200000L; // 時(shí)間戳，以毫秒為單位

        // 將時(shí)間戳轉(zhuǎn)換為日期時(shí)間字符串
        String dateTimeString = convertToDateTimeString(timestamp);
        System.out.println("轉(zhuǎn)換后的日期時(shí)間字符串：" + dateTimeString);
    }

    public static String convertToDateTimeString(long timestamp) {
        // 創(chuàng)建一個(gè)Date對(duì)象，將時(shí)間戳作為參數(shù)傳遞給構(gòu)造函數(shù)
        Date date = new Date(timestamp);

        // 創(chuàng)建SimpleDateFormat對(duì)象，定義日期時(shí)間的格式
        SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");

        // 使用SimpleDateFormat對(duì)象的format()方法將Date對(duì)象格式化為字符串
        String dateTimeString = sdf.format(date);

        return dateTimeString;
    }
}