JAVA类集----Set接口排序及重复元素说明

import java.util.Set;import java.util.TreeSet;public class Person{private String name;private int age;public Person(String name, int age){this.name = name;this.age = age;}public String toString(){return "姓名:" + this.name + "；年龄：" + this.age;}public static void main(String[] args){Set<Person> allSet = new TreeSet<Person>();allSet.add(new Person("张三", 30));allSet.add(new Person("李四", 31));allSet.add(new Person("王五", 32));allSet.add(new Person("王五", 32));allSet.add(new Person("王五", 33));allSet.add(new Person("赵六", 33));allSet.add(new Person("孙七", 33));System.out.println(allSet);}}

执行时出现以下错误：

Exception in thread "main" java.lang.ClassCastException: Person cannot be cast t
o java.lang.Comparable
        at java.util.TreeMap.compare(Unknown Source)
        at java.util.TreeMap.put(Unknown Source)
        at java.util.TreeSet.add(Unknown Source)
        at Person.main(Person.java:17)

修改如下：继承Comparable接口，覆写compareTo()方法

import java.util.Set;import java.util.TreeSet;public class Person implements Comparable<Person>{private String name;private int age;public Person(String name, int age){this.name = name;this.age = age;}public String toString(){return "姓名:" + this.name + "；年龄：" + this.age;}public int compareTo(Person per){if(this.age < per.age){return 1;}else if(this.age > per.age){return -1;}else{return 0;}}public static void main(String[] args){Set<Person> allSet = new TreeSet<Person>();allSet.add(new Person("张三", 30));allSet.add(new Person("李四", 31));allSet.add(new Person("王五", 32));allSet.add(new Person("王五", 32));allSet.add(new Person("王五", 33));allSet.add(new Person("赵六", 33));allSet.add(new Person("孙七", 33));System.out.println(allSet);}}

此时完成了排序，但是年龄相同的元素没有了，继续修改。

import java.util.Set;import java.util.TreeSet;public class Person implements Comparable<Person>{private String name;private int age;public Person(String name, int age){this.name = name;this.age = age;}public String toString(){return "姓名:" + this.name + "；年龄：" + this.age;}public int compareTo(Person per){if(this.age < per.age){return 1;}else if(this.age > per.age){return -1;}else{return this.name.compareTo(per.name);//调用String中的compareTo()方法}}public static void main(String[] args){Set<Person> allSet = new TreeSet<Person>();allSet.add(new Person("张三", 30));allSet.add(new Person("李四", 31));allSet.add(new Person("王五", 32));allSet.add(new Person("王五", 32));allSet.add(new Person("王五", 33));allSet.add(new Person("赵六", 33));allSet.add(new Person("孙七", 33));System.out.println(allSet);}}

此时并没有去掉重复的元素，要想去掉重复元素，则需要Object类中的两个方法帮助：

1.hashCode():表示一个唯一的编码，一般通过计算表示

2.equals():进行对象的比较操作

import java.util.Set;import java.util.HashSet;public class Person{private String name;private int age;public Person(String name, int age){this.name = name;this.age = age;}public String toString(){return "姓名:" + this.name + "；年龄：" + this.age;}public boolean equals(Object obj){//覆写equals对象完成比较if(this == obj){return true;}if(!(obj instanceof Person)){return false;}Person p = (Person)obj;//向下转型if(this.name.equals(p.name) && this.age == p.age){return true;}else{return false;}}public int hashCode(){return this.name.hashCode() * this.age;//定义一个公式}public static void main(String[] args){Set<Person> allSet = new HashSet<Person>();allSet.add(new Person("张三", 30));allSet.add(new Person("李四", 31));allSet.add(new Person("王五", 32));allSet.add(new Person("王五", 32));allSet.add(new Person("王五", 32));allSet.add(new Person("赵六", 33));allSet.add(new Person("孙七", 33));System.out.println(allSet);}}

总结：

1.一个好的类应该覆写Object类中的equals()、hashCode()、toString()方法，String类中已经覆写完成了。
2.Set接口依靠hashCode()和equals()完成重复元素的判断

3.TreeSet依靠Comparable接口完成排序的。