Java(Math、Random, System,Date)

Math

        System.out.println(Math.PI);
        System.out.println(Math.abs(-10));
        
        //向上取整，结果为double
        System.out.println(Math.ceil(12.3));
        
        //向下取整，结果为double
        System.out.println(Math.floor(12.3));
        
        //获取两个值中的最大值
        System.out.println(Math.max(10, 30));
        
        //前面是底数后面是指数,2.0^3.0
        System.out.println(Math.pow(2, 3));
        
        //生成0.0-1.0之间的随机数，包括0.0
        System.out.println(Math.random());
        
        //四舍五入
        System.out.println(Math.round(12.3f));
        
        //开方
        System.out.println(Math.sqrt(3));

Random

        //1-100的随机数
        Random random = new Random(100);
        System.out.println(random.nextInt());

System

        //垃圾回收
        System.gc();
        //1970.1.1到现在的时间戳
        long start = System.currentTimeMillis();
        
        //终止程序
        System.exit(0);
        
        int[] src = {11,2,33,12};
        int[] dest = new int[8];
        
        //拷贝内容到 dest
        System.arraycopy(src, 0, dest, 0, src.length);
        

Date

      //从1970.1.1开始
        Date d1 = new Date(0);
        System.out.println(d1.getTime());
        DateFormat df1 = DateFormat.getDateInstance();
        
        //创建日期格式化类对象
        SimpleDateFormat sdf = new SimpleDateFormat("yyyy年mm月dd日 hh:mm:ss");
        System.out.println(sdf.format(d1));
        
        //将时间转为字符串
        String str = "1993年10月3日 01:02:03";
        try {
            Date date = sdf.parse(str);
            System.out.println(date);
        } catch (ParseException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }