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。