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。