java - 如何封装一个集合？

Question

import java.util.ArrayList;
import java.util.HashSet;
import java.util.Set;

public class A
{
    private Set<String> users = new HashSet<String>();

    public Set<String> getUsers()
    {
        return users;
    }

    public void setUsers( Set<String> users )
    {
        this.users.clear();
        if( users != null )
        {
            this.users.addAll( users );
        }
    }

    public static void main( String[] args )
    {
        A a = new A();
        Set<String> temp = new HashSet<String>();
        temp.add( "a1" );
        temp.add( "a2" );
        a.setUsers( temp );
        Set<String> previousUsers = a.getUsers();
        System.out.println( previousUsers.size() ); //output 2

        temp.add( "a3" );
        a.setUsers( temp );
        System.out.println( previousUsers.size() );//output 3, how to make it still 2?
    }
}

问题:如果我希望 previuosUsers 在调用新的 setUsers 后仍然保留旧值，我应该如何修改这个程序？

Answer 1

您可以改用此构造函数 public HashSet(Collection<? extends E> c) ，而不是直接将返回值分配给 previousUsers .