Introduction to several categories in java

1. Date class

(1) Overview of Date class: represents a specific moment expressed to milliseconds

(2) The main construction methods are Two types:

public Date()
public Date(long date)  //把一个long类型的毫秒值转换成一个日期对象

(3) Member method:

public long getTime():  获取一个日期对象对象毫秒值
public void setTime(long time): 给一个日期对象设置上指定的毫秒值 例：date.setTime(1000 * 60 * 60) ;

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2, Calendar class

(1) Overview of Calendar: Calendar is an abstract class and cannot create objects through new. You can obtain its objects through one of its static member methods getInstance(). It is a specific moment with a group such as YEAR, MONTH, DAY_OF_MONTH, HOUR

(2) Member methods:

public static Calendar getInstance()使用默认时区和语言环境获得一个日历对象
public int get(int field)获得给定日历字段对应的值 field通过Calendar提供的字段来拿

(3) add() and set() methods of the Calendar class

(4) Members Method: Member method

public void add(int field,int amount)   根据日历的规则，为给定的日历字段添加或减去指定的时间量
public final void set(int year,int month,int date)  设置日历时间 年月日

3, math class

(1) Overview of math: The Math class contains methods for performing basic mathematical operations, such as elementary exponents, logarithms, square roots, and trigonometric operations function.

(2) Member variables:

public static final double E :      自然底数
public static final double PI:      圆周率

(3) Member methods:

    public static int abs(int a)        取绝对值
    public static double ceil(double a) 向上取整
    public static double floor(double a)    向下取整
    public static int max(int a,int b)      获取最大值
    public static int min(int a, int b) 获取最小值
    public static double pow(double a,double b) 获取a的b次幂
    public static double random()   获取随机数 返回带正号的 double 值，该值大于等于 0.0 且小于 1.0。
    public static int round(float a) 四舍五入
    public static double sqrt(double a)获取正平方根

4. Random class

(1) Overview of the Random class: This class is used to generate random numbers. If two Random instances are created with the same seed, and the same sequence of method calls is made to each instance, they will generate and return the same sequence of numbers.

(2) Construction method:

public int nextInt()//没有参数 表示的随机数范围 是int类型的范围
public int nextInt(int n)//可以指定一个随机数范围

5. System class

(1) Overview of System: The System class contains some useful class fields and methods. It cannot be instantiated.

(2) Member method:

public static void gc()//调用垃圾回收器，finalize() 和System类中gc()有关联
public static void exit(int status)//退出java虚拟机 0 为正常退出 非0为 异常退出
public static long currentTimeMillis()//获取当前时间的毫秒值

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