例如：

Set  set1 = new HashSet();

set1.add( object1 );

set1.add( ... );

set1.add( objectn );

Set set2 = new HashSet(  set1 );

或者 

Set  set3 = new HashSet( set1 );

set3.addAll( set1 );

set2 与 set3 中存储的都是 set1 元素的 “引用”

代码如下：

/*** 测试 addAll( Collections )* 应该是保存的同一引用。* @author gqltt**/
public class SetAddAllTest {/*** @param args*/public static void main(String[] args) {testSetWithPerson();}private static <T> void print( String desc, Set<T> set ){System.out.println( desc );for( T s : set ){System.out.print( s + "  ");}System.out.println( "\n" );}/*** 测试元素为应用类型*/private static void testSetWithPerson(){Set<Person> set1 = new HashSet<Person>();set1.add( new Person("b", 25) );set1.add( new Person("a", 20) );set1.add( new Person("c", 26) );Set<Person> set2 = new TreeSet<Person>();set2.addAll( set1 );//print( "set1", set1 );print( "set2", set2 );changeName( getPersonByName( set1, "a" ), "abc" );print( "set1 after modify element Person {name a->abc }:", set1 );print( "set2 after set1 modify element:", set2 );}private static Person getPersonByName( Set<Person> set, String name ){for( Person p : set ){if( p.name.equals( name ) ){return p;}}return null;}private static void changeName( Person p, String newName ){p.setName( newName );}static class Person implements Comparable<Person>{private String name;private int age;public Person(String name, int age) {super();this.name = name;this.age = age;}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;}@Overridepublic String toString() {return "(" + name + ", " + age + ")";}@Overridepublic int compareTo(Person person) {return age - person.age;}@Overridepublic boolean equals(Object obj) {if( obj == null ){return false;}if( ! ( getClass() == obj.getClass() ) ){return false;}Person p = (Person) obj;return name != null && name.equals( p.name ) && age == p.age;}@Overridepublic int hashCode() {int result = 47;result += name.hashCode();result += new Integer( age ).hashCode();return result;}}}

测试结果输出：

set1
(b, 25) (c, 26) (a, 20)

set2
(a, 20) (b, 25) (c, 26)

set1 after modify element Person {name a->abc }:
(b, 25) (c, 26) (abc, 20)

set2 after set1 modify element:
(abc, 20) (b, 25) (c, 26)