4，（了解）date、Comparable、comparator

一、日期格式化操作类：SimpleDateFormat

● 构造方法：public SimpleDateFormat(String pattern）
● 将日期格式化为字符串：public final String format(Date date)
● 将字符串格式化为日期：public Date parse(String source)

//将日期变为字符串，格式化显示
Date date = new Date(); // 当前日期
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss.SSS");
String str = sdf.format(date); // 将日期格式化为字符串

//将字符串变为日期
String str = "1980-09-19 12:12:12.122";
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss.SSS");
Date date = sdf.parse(str);

二、比较器

● comparator 是一个外比较器，适用于对象不支持自身比较的类，定义一个类实现comparator接口，覆写compara方法，对象比较的话可以使用comparaTo()方法。

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.Iterator;

class Person  {
    private String name ;
    private int age ;
        //省略set,get方法
    public Person(String name,int age) {
        this.name = name ;
        this.age = age ;
    }
    @Override
    public String toString() {
        return "Person [name=" + name + ", age=" + age + "]\n";
    }
    
}
class testComparatorMethod implements Comparator<Person>{
    @Override
    public int compare(Person o1, Person o2) {
        
        int num = o1.getName().compareTo(o2.getName());
        if (num != 0){
            return num;
        }
        int numage = o1.getAge() - o2.getAge();
        return numage;
        
    }
}
public class testComparator {
    public static void main(String[] args) throws Exception {
        ArrayList<Person> list = new ArrayList<Person>();
        Person per1 = new Person("aaa",25);
        Person per2 = new Person("bbb",21);
        Person per3 = new Person("ccc",27);
        Person per4 = new Person("ddd",23);
        list.add(per3);
        list.add(per2);
        list.add(per4);
        list.add(per1);
        Collections.sort(list, new testComparatorMethod());
        
        Iterator<Person> iter = list.iterator();
        while(iter.hasNext()){
            System.out.println(iter.next());
        }
    }

    
}

● comparable:内比较器，用于在类内部进行比较
实例：

public class TestComparable {
    public static void main(String[] args) {
        
        Person2 per = new Person2("aaa");
        Person2 per1 = new Person2("bbb");
        System.out.println(per.compareTo(per1));
    }
}

class Person2 implements Comparable<Person2> {
    private String name;

    public String getName() {
        return name;
    }

    public Person2(String name) {
        this.name = name;
    }

    @Override
    public int compareTo(Person2 per) {

        if (this.name.compareTo(per.name) > 0)
            return 1;
        else if (this.name.compareTo(per.name) == 0)
            return 0;
        else
            return -1;
    }

}