Java工具类

System.out.println("PI的值："+Math.PI);
        System.out.println("-10的绝对值："+Math.abs(-10));
        System.out.println("Math.random()="+Math.random());
        System.out.println("Math.round(98.56)="+Math.round(98.568977d));
        System.out.println("Math.round(88.56789)="+Math.round(88.56789*100)/100.0);
        System.out.println("Math.sqrt(2d)="+Math.sqrt(2d));

//随机数
        Random r = new Random();
        int x = r.nextInt(100);
        System.out.println("x="+x);

//创建了一个日期对象
        Date date = new Date();
        System.out.println("当前时间的毫秒："+date.getTime());
        System.out.println(date);

Calendar c = Calendar.getInstance();
//        Calendar c = new GregorianCalendar();
        
        int year = c.get(Calendar.YEAR);//年
        int month = c.get(Calendar.MONTH);//月
        int day = c.get(Calendar.DAY_OF_MONTH);//日
        int hour = c.get(Calendar.HOUR_OF_DAY);//时
        int minute = c.get(Calendar.MINUTE);//分
        int second = c.get(Calendar.SECOND);//秒
        int millisecond = c.get(Calendar.MILLISECOND);//毫秒
        //星期
        String[] weeks = {"星期日","星期一","星期二","星期三","星期四","星期五","星期六"};
        int week = c.get(Calendar.DAY_OF_WEEK);
        
        StringBuilder sb = new StringBuilder();
      sb.append(year).append("年").append(month).append("月").append(day).append("日  ")
        .append(hour).append(":").append(minute).append(":").append(second).append(" ")
        .append(millisecond).append(" ").append(weeks[week]);
        
        System.out.println(sb.toString());

//        DateFormat df = DateFormat.getDateInstance();
        //参数（指定日期风格，指定语言环境）
//        DateFormat df = DateFormat.getDateInstance(DateFormat.LONG, Locale.CHINA);
//        DateFormat df = DateFormat.getTimeInstance();
        DateFormat df = DateFormat.getTimeInstance(DateFormat.LONG, Locale.getDefault());
        
        String nowDate = df.format(new Date());
        System.out.println("当前日期为："+nowDate);

//用默认的模式和默认语言环境的日期格式符号构造 SimpleDateFormat。
//        SimpleDateFormat sdf = new SimpleDateFormat();
        SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss SSS");
        
        String s = sdf.format(new Date());
        System.out.println(s);

/**
 * 人类
 * 如果自定义类的对象要实现排序（比较）,那么就要实现Comparable接口，并实现接口的
 * compareTo方法
 */
public class Person implements Comparable<Person>{
    private String name;
    private int age;    
    public Person() {
    }
    public Person(String name, int age) {
        this.name = name;
        this.age = age;
    }
    
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
    @Override
    public String toString() {
        return "Person [name=" + name + ", age=" + age + "]";
    }
    //比较器的比较方法
    //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
    @Override
    public int compareTo(Person o) {
        //实现：定义对象之间的比较规则
        if(o==null){
            throw new NullPointerException();
        }
        if(this.age<o.age){
            return -1;
        }else if(this.age>o.age){
            return 1;
        }        
        return 0;
    }
}

/**
 * 人类*/
public class Person2{
    private String name;
    private int age;
        
    public Person2() {
    }
    public Person2(String name, int age) {
        this.name = name;
        this.age = age;
    }
    
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
    @Override
    public String toString() {
        return "Person [name=" + name + ", age=" + age + "]";
    }
}

/**
 * 自定义的类比较器
 */
public class Person2Comparator implements Comparator<Person2>{

    @Override
    public int compare(Person2 o1, Person2 o2) {
        if(o1==null || o2==null){
            throw new NullPointerException();
        }
        if(o1.getAge()<o2.getAge()){
            return -1;
        }else if(o1.getAge()>o2.getAge()){
            return 1;
        }
        return 0;
    }

}

//对象的排序
public class PersonSortDemo {
    public static void main(String[] args) {
        String[] names = {"张三","李四","王五","刘备","张飞","关羽"};
        
        Person[] persons = { 
                new Person("张三", 27), new Person("李四", 29),
                new Person("王五", 41), new Person("刘备", 33),
                new Person("张飞", 25), new Person("关羽", 30) };
        
        Person2[] persons2 = { 
                new Person2("张三", 27), new Person2("李四", 29),
                new Person2("王五", 41), new Person2("刘备", 33),
                new Person2("张飞", 25), new Person2("关羽", 30) };    
        
        Arrays.sort(names);
        System.out.println(Arrays.toString(names));
        
        //自定义类对象数组排序
        Arrays.sort(persons);
        System.out.println(Arrays.toString(persons));
        
        //使用自定义的比较器
        Arrays.sort(persons2, new Person2Comparator());
        System.out.println(Arrays.toString(persons2));
                
    }
}

/**
 * 一个类要想实现克隆功能，必须：
 * 1、实现Cloneable接口，该接口是一个标记接口
 * 2、重写Object类中的clone方法
 */
public class Cat implements Cloneable{
    private String name;
    private int age;
    
    public Cat(String name, int age) {
        super();
        this.name = name;
        this.age = age;
    }
    
    public Cat() {
        super();
    }

    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
    @Override
    public String toString() {
        return "Cat [name=" + name + ", age=" + age + "]";
    }
    //重写Object的clone方法
    @Override
    protected Object clone() throws CloneNotSupportedException {
        return super.clone();
    }    
}

//要创建一组类似的对象，那么从性能上考虑，就可以使用克隆技术
public class Test {
    public static void main(String[] args) {
        Cat c = new Cat("张三",3);
        System.out.println("c="+c);
        try {
            Cat c1 = (Cat)c.clone();
            System.out.println("c1="+c1);
        } catch (CloneNotSupportedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

}