代码：

import java.text.ParseException;
import java.text.SimpleDateFormat;

public class MainProcess {
    public static void main(String[] args) throws ParseException {
        // 1.set format
        SimpleDateFormat timeSmat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");

        // 2.date to timestamp
        String myDate = "2015-01-01 00:00:00";
        long parse = timeSmat.parse(myDate).getTime();
        System.out.println("date to ts:" + parse);

        // 3.timestamp to date
        long myTs = 1420041600000L;
        String format = timeSmat.format(myTs);
        System.out.println("ts to date:" + format);
    }
}

输出：

如果要封装成方法：

import java.text.ParseException;
import java.text.SimpleDateFormat;
 
 
 
public class dateFormatUtil {
    public static void main(String[] args) throws ParseException {
        long time = System.currentTimeMillis();
        System.out.println(time);
        System.out.println(timeToFormat(time));
        System.out.println(timeToSecond(timeToFormat(time)));
    }
	//timestamp to date
    public static String timeToFormat(long time) {
        SimpleDateFormat timeSmat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");//设置日期格式
        return timeSmat.format(time);
    }
    //date to timestamp
    public static long timeToSecond(String date) throws ParseException {
        SimpleDateFormat timeSmat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
        return timeSmat.parse(date).getTime();
    }
}