/** * Math类、Random类、Arrays类：具体查JAVA手册。。。。。。 */public class Main {    public static void main(String[] args) {        String[] s1 = {"a","b","c","d","e"};        String[] s2 = {"a","b","c","d","e"};        boolean b = Arrays.equals(s1,s2); //比较两数组内容        System.out.println(b); //输出true                String[] s3 = new String[6];        Arrays.fill(s3,"井空");        System.out.println(s3.length);        for(String i:s3) System.out.print(i+" "); System.out.println();                String[] s4 = Arrays.copyOfRange(s1, 2, 3); //[2,3)        for(String i:s4) System.out.print(i+" "); System.out.println();                int index = Arrays.binarySearch(s1, "c");        System.out.println(index);    }}

/** * 日期操作类 * Date类：表示特定的瞬间，精确到毫秒，也是程序运行时的当前时间 * Calendar类：可以精确到毫秒 * DateFormat类 * SimpleDateFormat类：日期格式化类 */public class Main {    public static void main(String[] args) {        Date d1 = new Date();        Date c = new Date();        System.out.println(d1.getTime()); //毫秒表示        System.out.println(d1); //格式表示                //两种实例化方式        Calendar ca1 = Calendar.getInstance();        Calendar ca2 = new GregorianCalendar(); //阳历        int year = ca1.get(Calendar.YEAR);        int month = ca1.get(Calendar.MONTH)+1;        int day = ca1.get(Calendar.DAY_OF_MONTH);        int hour = ca1.get(Calendar.HOUR);        int minute = ca1.get(Calendar.MINUTE);        int second = ca1.get(Calendar.SECOND);        System.out.println(year+" "+month+" "+day);        System.out.println(hour+" "+minute+" "+second);                DateFormat df1 = DateFormat.getDateInstance();        DateFormat df2 = DateFormat.getTimeInstance();        String nowDate = df1.format(new Date());        String nowTime = df2.format(new Date());        System.out.println(nowDate);        System.out.println(nowTime);                SimpleDateFormat sdf1 = new SimpleDateFormat("yyyy^MM^dd HH:mm:ss SSS毫秒");        String swa = sdf1.format(new Date());        System.out.println(swa);    }}

/** * 对象比较器：对两个或多个数据项进行比较，以确定它们是否相等，或确定它们之间的大小关系 * Comparable接口：强行对实现它的每个类的对象进行整体排序，类的compareTo方法被称为它的自然比较方法 * 如果自定义类的对象要实现排序，那么就要实现Comparable接口，并实现接口的CompareTo方法 *  * Comparable接口要求自定义类去实现，按照OO原则：对修改关闭，对扩展开放。 * 如果这个类已经定义好，不想再去修改它，如何实现？ 答：Arrays.sort有一个重载方法： */public class Main {    public static void main(String[] args) {        String[] names = {"a","b","c","d","e","f"};        Person[] persons = {            new Person("a",10), new Person("b",9),            new Person("c",8),  new Person("d",7),            new Person("e",6),  new Person("f",5),        };        //自定义类对对象数组排序        Arrays.sort(persons);        for(Person i:persons) System.out.println(i.name+" "+i.age);                        Person2[] persons2 = {            new Person2("a",10), new Person2("b",9),            new Person2("c",8),  new Person2("d",7),            new Person2("e",6),  new Person2("f",5),        };        Arrays.sort(persons2,new Person2Comparator()); //使用自定义的比较器        for(Person2 i:persons2) System.out.println(i.name+" "+i.age);        System.out.println(Arrays.toString(persons2));//返回数组内容的字符串表示形式    }}class Person implements Comparable<Person>{    public String name;    public int age;    Person(String name,int age){        this.name = name;        this.age = age;    }    @Override    public int compareTo(Person o) { //比较器的比较方法        //实现：定义对象之间的比较规则        if(o==null){            throw new NullPointerException();        }        //如果当前对象小于参数对象，返回-1        //如果当前对象等于参数对象，返回0        //如果当前对象大于参数对象，返回1        if(this.age < o.age)             return -1;        else if(this.age < o.age)            return 1;        return 0;    }}class Person2{    public String name;    public int age;    Person2(String name,int age){        this.name = name;        this.age = age;    }}class Person2Comparator implements Comparator<Person2>{    @Override    public int compare(Person2 o1, Person2 o2) {       if(o1==null || o2==null)           throw new NullPointerException();       if(o1.age < o2.age)           return -1;       else if(o1.age > o2.age)           return 1;       return 0;    }}