Java coding challenge

DisappointingGroup.java

package com.examplefoobar.utils;

import javax.annotation.concurrent.ThreadSafe;

import java.io.Closeable;
import java.io.File;
import java.io.FileWriter;
import java.io.IOException;
import java.util.HashSet;

/**
 * FIXME: This class was written by a really junior software engineer.
 *        It seems there are a lot of rooms for improvement in this class.
 *        Please give us your feedback about this class (with suggestions if possible.)
 */

/**
 * A thread-safe container that stores a group ID and members.
 *
 * This class can store <tt>Member</tt> and/or <tt>AdminMember</tt>.
 * Also, it can start and stop a background task that writes a member list to specified 2 files.
 *
 * This class is called many times, so it should have a good performance.
 *
 * Example usage:
 * 
 *    DisappointingGroup group = new DisappointingGroup("group-001");
 *
 *    group.addMember(new DisappointingGroup.Member("member-100", 42));
 *    group.addMember(new DisappointingGroup.AdminMember("admin-999", 30));
 *    group.addMember(new DisappointingGroup.Member("member-321", 15));
 *
 *    group.startLoggingMemberList10Times("/tmp/output.primary", "/tmp/output.secondary");
 */
@ThreadSafe
public class DisappointingGroup
    implements Closeable
{
    String groupId;
    HashSet<Member> members;
    boolean isRunning;
    boolean shouldStop;

    class Member
    {
        String memberId;
        int age;

        Member(String memberId, int age)
        {
            this.memberId = memberId;
            this.age = age;
        }

        public String getMemberId()
        {
            return memberId;
        }

        public int getAge()
        {
            return age;
        }

        public boolean equals(Object o)
        {
            // If `memberId` matches the other's one, they should be treated as the same `Member` objects.
            Member member = (Member) o;
            return this.memberId == member.memberId;
        }
    }

    class AdminMember extends Member
    {
        AdminMember(String memberId, int age)
        {
            super(memberId, age);
        }
    }

    public DisappointingGroup(String groupId)
    {
        this.groupId = groupId;
        this.members = new HashSet<>();
    }

    public void addMember(Member member)
    {
        members.add(member);
    }

    private String getDecoratedMemberId(Member member)
    {
        if (member instanceof Member) {
            return member.getMemberId() + "(normal)";
        }
        else if (member instanceof AdminMember) {
            return member.getMemberId() + "(admin)";
        }
        return null;
    }

    private String getMembersAsStringFlooringAge()
    {
        String buf = "";
        for (Member member : members)
        {
            // Floor the age: e.g. 37 -> 30
            Integer flooredAge = (member.getAge() / 10) * 10;
            String decoratedMemberId = getDecoratedMemberId(member);
            buf += String.format("memberId=%s, age=%d¥n", decoratedMemberId, flooredAge);
        }
        return buf;
    }

    @Override
    public void close()
            throws IOException
    {
    }

    /**
     * Run a background task that writes a member list to specified files 10 times in background thread
     * so that it doesn't block the caller's thread.
     *
     * Only one thread is allowed to run at once
     *  - When this method is called and another thread is running, the method call should just return w/o starting any thread
     *  - When this method is called and another thread is already finished, the method call should start a new thread
     */
    public void startLoggingMemberList10Times(final String outputFilePrimary, final String outputFileSecondary)
    {
        // Only one thread is allowed to run at once
        if (isRunning) {
            return;
        }
        isRunning = true;

        new Thread(new Runnable() {
            @Override
            public void run()
            {
                int i = 0;
                while (!shouldStop)
                {
                    if (i++ >= 10)
                        break;

                    FileWriter writer0 = null;
                    FileWriter writer1 = null;
                    try {
                        String membersStr = DisappointingGroup.this.getMembersAsStringFlooringAge();

                        writer0 = new FileWriter(new File(outputFilePrimary));
                        writer0.append(membersStr);

                        writer1 = new FileWriter(new File(outputFileSecondary));
                        writer1.append(membersStr);
                    }
                    catch (Exception e) {
                        throw new RuntimeException(
                                "Unexpected error occurred. Please check these file names. outputFilePrimary="
                                        + outputFilePrimary + ", outputFileSecondary=" + outputFileSecondary);
                    }
                    finally {
                        try {
                            if (writer0 != null)
                                writer0.close();

                            if (writer1 != null)
                                writer1.close();
                        }
                        catch (Exception ignored) {
                            // Do nothing since there isn't anything we can do here, right?
                        }
                    }

                    try {
                        Thread.sleep(1000);
                    }
                    catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            }
        }).start();
    }

    /**
     * Stop the background task started by <tt>startLoggingMemberList10Times()</tt>
     */
    public void stopPrintingMemberList()
    {
        shouldStop = true;
    }
}