TreeSet之定制排序和自然排序

TreeSet的几大特点：
 1、TreeSet中存储的类型必须是一致的，不能一下存int，一下又存string
 2、TreeSet在遍历集合元素时，是有顺序的【从小到大】（我的理解，如果存的字母，按字典序排列）
 3、排序：当向TreeSet中添加自定义对象时，有2种排序方法，1：自然排序  2、定制排序
        自然排序：要求自定义类实现java.lang.Comparable接口并重写compareTo(Object obj)方法。在此方法中，指明按照自定义类的哪个属性进行排序

一、自然排序示例：
             1、定义一个类（文章中为Employee）实现Comparable接口
             2、重写Comparable接口中的compareTo()方法
             3、在compareTo()中按指定属性进行排序（文章按name进行排序）
代码示例：

Employee类

public class Employee implements Comparable{

     public int compareTo(Object o) {
        if (o instanceof Employee) {
            Employee e = (Employee) o;
            return this.name.compareTo(e.name);
        }
        return 0;
    }

    private String name;
    private int age;
    private MyDate birthday;

    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;
    }

    public MyDate getBirthday() {
        return birthday;
    }

    public void setBirthday(MyDate birthday) {
        this.birthday = birthday;
    }

    public Employee(String name, int age, MyDate birthday) {
        this.name = name;
        this.age = age;
        this.birthday = birthday;
    }

    @Override
    public String toString() {
        return "Employee{" +
                "name='" + name + '\'' +
                ", age=" + age +
                ", birthday=" + birthday +
                '}';
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;

        Employee employee = (Employee) o;

        if (age != employee.age) return false;
        if (name != null ? !name.equals(employee.name) : employee.name != null) return false;
        return birthday != null ? birthday.equals(employee.birthday) : employee.birthday == null;
    }

    @Override
    public int hashCode() {
        int result = name != null ? name.hashCode() : 0;
        result = 31 * result + age;
        result = 31 * result + (birthday != null ? birthday.hashCode() : 0);
        return result;
    }
}

测试类

public class TreeSetTest {
    public static void main(String[] args) {
      test1();
    }
    /**
     * 自然排序  按name排序 所以你的Employee必须实现Comparable接口
     */
    public static void test1() {
        Employee e1 = new Employee("liudehua",55,new MyDate(4,12,1997));
        Employee e2 = new Employee("11",55,new MyDate(5,12,1997));
        Employee e3 = new Employee("22",55,new MyDate(6,12,1997));
        Employee e4 = new Employee("33",55,new MyDate(7,12,1997));
        Employee e5 = new Employee("44",55,new MyDate(8,12,1997));
        TreeSet set = new TreeSet();
        set.add(e1);
        set.add(e2);
        set.add(e3);
        set.add(e4);
        set.add(e5);

        Iterator i = set.iterator();
        while (i.hasNext()) {
            System.out.println(i.next());
        }
    }
}
输出结果按name进行排序。按照汉语拼音的顺序

二、定制排序示例

Employee1 类

public class Employee1 {

    private String name;
    private int age;
    private MyDate birthday;

    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;
    }

    public MyDate getBirthday() {
        return birthday;
    }

    public void setBirthday(MyDate birthday) {
        this.birthday = birthday;
    }

    public Employee1(String name, int age, MyDate birthday) {
        this.name = name;
        this.age = age;
        this.birthday = birthday;
    }

    @Override
    public String toString() {
        return "Employee{" +
                "name='" + name + '\'' +
                ", age=" + age +
                ", birthday=" + birthday +
                '}';
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;

        Employee1 employee = (Employee1) o;

        if (age != employee.age) return false;
        if (name != null ? !name.equals(employee.name) : employee.name != null) return false;
        return birthday != null ? birthday.equals(employee.birthday) : employee.birthday == null;
    }

    @Override
    public int hashCode() {
        int result = name != null ? name.hashCode() : 0;
        result = 31 * result + age;
        result = 31 * result + (birthday != null ? birthday.hashCode() : 0);
        return result;
    }
}

MyDate类

public class MyDate {
    private int day;
    private int month;
    private int year;

    public int getDay() {
        return day;
    }

    public void setDay(int day) {
        this.day = day;
    }

    public int getMonth() {
        return month;
    }

    public void setMonth(int month) {
        this.month = month;
    }

    public int getYear() {
        return year;
    }

    public void setYear(int year) {
        this.year = year;
    }

    public MyDate(int day, int month, int year) {
        this.day = day;
        this.month = month;
        this.year = year;
    }

    @Override
    public String toString() {
        return "MyDate{" +
                "day=" + day +
                ", month=" + month +
                ", year=" + year +
                '}';
    }

    //先重写MyDate的equals()和hashCode()方法，在重写Employee中的

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;

        MyDate myDate = (MyDate) o;

        if (day != myDate.day) return false;
        if (month != myDate.month) return false;
        return year == myDate.year;
    }

    @Override
    public int hashCode() {
        int result = day;
        result = 31 * result + month;
        result = 31 * result + year;
        return result;
    }
}

测试类

public class TreeSetTest1 {
    public static void main(String[] args) {
          test2();
    }
    /**
     * 定制排序  按指定生日来排
     */
    public static void test2() {
        Comparator comparator = new Comparator() {
            public int compare(Object o1, Object o2) {
                if (o1 instanceof Employee1 && o2 instanceof Employee1) {
                    Employee1 e1 = (Employee1)o1;
                    Employee1 e2 = (Employee1)o2;

                    MyDate birth1 = e1.getBirthday();
                    MyDate birth2 = e2.getBirthday();

                    if(birth1.getYear() != birth2.getYear()) {
                        //定义的类型是 int  所以使用“-”减号代替compareTo()
                        return birth1.getYear() - birth2.getYear();
                    } else {
                        if (birth1.getMonth() != birth2.getMonth()) {
                            return birth1.getMonth() - birth2.getMonth();
                        } else {
                            return birth1.getDay() - birth2.getDay();
                        }
                    }
                }
                return 0;
            }
        };

        // “一定要指明按特定对象进行比较  comparator参数一定要加”
        TreeSet set = new TreeSet(comparator);
        Employee1 e1 = new Employee1("liudehua",55,new MyDate(5,8,1990));
        Employee1 e2 = new Employee1("11",55,new MyDate(5,11,1997));
        Employee1 e3 = new Employee1("22",55,new MyDate(6,10,1997));
        Employee1 e4 = new Employee1("33",55,new MyDate(7,9,1997));
        Employee1 e5 = new Employee1("44",55,new MyDate(8,8,1997));
        set.add(e1);
        set.add(e2);
        set.add(e3);
        set.add(e4);
        set.add(e5);

        Iterator i = set.iterator();
        while (i.hasNext()) {
            System.out.println(i.next());
        }
    }
}

总结：

自然排序实现的是comparable接口。其在类可以修改时使用。

定制排序实现的是comparator接口。其在类不可以修改时使用

在使用定制排序或是自然排序时，在其用到的类中都要重写hashCode()与equals()方法

comparable和comparator的区别:[参考博文：http://blog.csdn.net/excellentyuxiao/article/details/52344594]
Comparator在util包下，Comparable在lang包下。java中的对象排序都是以comparable接口为标准的。comparator是在对象外部实现排序。