Set<> Collection<> {size();isEmpty();contains(Object o);Iterator<> iterator();Object[] toArray();<> [] toArray([] a);add(e);remove(Object o);containsAll(Collection<?> c);addAll(Collection<? > c);retainAll(Collection<?> c);removeAll(Collection<?> c);clear();equals(Object o);hashCode();
}

1.HashSet：

　　HashSet使用的是相当复杂的方式来存储元素的，使用HashSet能够最快的获取集合中的元素，效率非常高（以空间换时间）。会根据hashcode和equals来庞端是否是同一个对象，如果hashcode一样，并且equals返回true，则是同一个对象，不能重复存放。

package com.set;import java.util.HashSet;
import java.util.Set;class Student{int id;public Student(int id) {this.id = id;}@Overridepublic String toString() {return this.id+"";}@Overridepublic int hashCode() {return this.id;}@Overridepublic boolean equals(Object obj) {if (obj instanceof Student){Student  stu = (Student) obj;if (stu.id == this.id)return true;}return false;}
}
public class HashSetTest {public static void main(String[] args) {Set<Student> set = new HashSet<Student>();Student s1 = new Student(1);Student s2 = new Student(1);Student s3 = new Student(2);set.add(s1);set.add(s2);set.add(s3);for (Student s : set) {System.out.println(s);}}
}

正如上例所示，重写了hashCode()和equals()方法来区分同意对象后，就不能存放同以对象了。如果注释这两个方法，则所有Student对象视为不同对象，都可以存放。

　　2.TreeSet

　　TreeSet也不能存放重复对象，但是TreeSet会自动排序，如果存放的对象不能排序则会报错，所以存放的对象必须指定排序规则。排序规则包括自然排序和客户排序。

　　①自然排序：TreeSet要添加哪个对象就在哪个对象类上面实现java.lang.Comparable接口，并且重写comparaTo()方法，返回0则表示是同一个对象，否则为不同对象。

　　②客户排序：建立一个第三方类并实现java.util.Comparator接口。并重写方法。定义集合形式为TreeSet ts = new TreeSet(new 第三方类());

package com.set;import java.util.Set;
import java.util.TreeSet;class Student1 implements Comparable<Student1>{int id;public Student1(int id) {this.id = id;}@Overridepublic String toString() {return this.id+"";}@Overridepublic int hashCode() {return this.id;}@Overridepublic boolean equals(Object obj) {if (obj instanceof Student1){Student1  stu = (Student1) obj;if (stu.id == this.id)return true;}return false;}public int compareTo(Student1 o) {return (this.id-o.id);}
}public class TreeSetTest {public static void main(String[] args) {Set<Student1> set = new TreeSet<Student1>();Student1 s1 = new Student1(5);Student1 s2 = new Student1(1);Student1 s3 = new Student1(2);Student1 s4 = new Student1(4);Student1 s5 = new Student1(3);set.add(s1);set.add(s2);set.add(s3);set.add(s4);set.add(s5);for (Student1 s : set) {System.out.println(s);}}}

package com.set;import java.util.Set;
import java.util.TreeSet;class MySort implements java.util.Comparator<Student2>{public int compare(Student2 o1, Student2 o2) {return o2.id-o1.id;}
}
class Student2{int id;public Student2(int id) {this.id = id;}@Overridepublic String toString() {return this.id+"";}@Overridepublic int hashCode() {return this.id;}@Overridepublic boolean equals(Object obj) {if (obj instanceof Student2){Student2  stu = (Student2) obj;if (stu.id == this.id)return true;}return false;}
}
public class TreeSetTest2 {public static void main(String[] args) {Set<Student2> set = new TreeSet<Student2>(new MySort());Student2 s1 = new Student2(5);Student2 s2 = new Student2(1);Student2 s3 = new Student2(2);Student2 s4 = new Student2(4);Student2 s5 = new Student2(3);set.add(s1);set.add(s2);set.add(s3);set.add(s4);set.add(s5);for (Student2 s : set) {System.out.println(s);}}}

package com.set;import java.util.ArrayList;
import java.util.Collections;
import java.util.List;class MySort1 implements java.util.Comparator<Student3>{public int compare(Student3 o1, Student3 o2) {return o2.id-o1.id;}
}
class Student3 implements Comparable<Student3>{int id;public Student3(int id) {this.id = id;}@Overridepublic String toString() {return this.id+"";}public int compareTo(Student3 o) {return (this.id-o.id);}
}public class ListSort {public static void main(String[] args) {List<Student3> list = new ArrayList<Student3>();Student3 s1 = new Student3(5);Student3 s2 = new Student3(1);Student3 s3 = new Student3(2);Student3 s4 = new Student3(4);Student3 s5 = new Student3(3);list.add(s1);list.add(s2);list.add(s3);list.add(s4);list.add(s5);System.out.println(list);//自然排序：Collections.sort(list);System.out.println(list);//客户排序Collections.sort(list, new MySort1());System.out.println(list);}
}

　　3.LinkedHashSet

　　LinkedHashSet按照插入顺序保存对象，同时还保存了HashSet的查询速度。

对 set 的遍历

1.迭代遍历：

Set<String> set = new HashSet<String>();
Iterator<String> it = set.iterator();
while (it.hasNext()) {String str = it.next();System.out.println(str);
}

2.for循环遍历：

for (String str : set) {System.out.println(str);
}

Set<Object> set = new HashSet<Object>();
for (Object obj: set) {if(obj instanceof Integer){int aa= (Integer)obj;}else if(obj instanceof String){String aa = (String)obj}
}