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?
    }
}

Question: if i want previuosUsers still keep the old values after new setUsers been called, how should i modify this program?

Answer 1

You can use this constructor instead public HashSet(Collection<? extends E> c), than just directly assigning the return to previousUsers.