@Override
public void onCreate() {
    super.onCreate();
    Firebase.setAndroidContext(this);
    // other setup code
}

<TextView
        android:id="@+id/helloText"
        android:text="Hello World!"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"/>

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
  		// ... previous code 
  		
    	final TextView helloText = (TextView) findViewById(R.id.helloText);
		// URL 로 심플하게 스키마에 접근 가능
        Firebase ref = new Firebase("https://jacaopensemina.firebaseIO.com/Test");
        // Get a reference to our posts
        // Attach an listener to read the data at our posts reference
        ref.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot snapshot) {
                helloText.setText(snapshot.getValue().toString());
            }

            @Override
            public void onCancelled(FirebaseError firebaseError) {
                System.out.println("The read failed: " + firebaseError.getMessage());
            }
        });
    }

// 포스트의 댓글
public class Comment {
    String writerId;
    String text;
    String bgUrl;
    long writeTime;

    public String getWriterId() {
        return writerId;
    }

    public void setWriterId(String writerId) {
        this.writerId = writerId;
    }

    public String getText() {
        return text;
    }

    public void setText(String text) {
        this.text = text;
    }

    public String getBgUrl() {
        return bgUrl;
    }

    public void setBgUrl(String bgUrl) {
        this.bgUrl = bgUrl;
    }

    public long getWriteTime() {
        return writeTime;
    }

    public void setWriteTime(long writeTime) {
        this.writeTime = writeTime;
    }
}

// 블로깅 포스트의 도메인 클래스
public class Post {
    String writerId;
    String text;
    String bgUrl;
    long writeTime;
    Map<String, Comment> commentMap = new HashMap<>();


    public String getWriterId() {
        return writerId;
    }

    public void setWriterId(String writerId) {
        this.writerId = writerId;
    }

    public String getText() {
        return text;
    }

    public void setText(String text) {
        this.text = text;
    }

    public String getBgUrl() {
        return bgUrl;
    }

    public void setBgUrl(String bgUrl) {
        this.bgUrl = bgUrl;
    }

    public long getWriteTime() {
        return writeTime;
    }

    public void setWriteTime(long writeTime) {
        this.writeTime = writeTime;
    }

    public Map<String, Comment> getCommentMap() {
        return commentMap;
    }

    public void setCommentMap(Map<String, Comment> commentMap) {
        this.commentMap = commentMap;
    }
}

public class PostActivity extends AppCompatActivity {
    public static final String FIREBASE_POST_URL = "https://jacaseminar0613.firebaseio.com/Posts";
    private RecyclerView mRecyclerView;
    private LinearLayoutManager mLinearLayoutManager;
    private MyAdapter mAdapter;
    private List<Post> mPosts = new ArrayList<>();
    private List<String> mKeys = new ArrayList<>();
    private Query mRef;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        Firebase.setAndroidContext(this);
        JodaTimeAndroid.init(this);
        setContentView(R.layout.activity_post);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        mRecyclerView = (RecyclerView) findViewById(R.id.recyclerView);
        setSupportActionBar(toolbar);

        mLinearLayoutManager = new LinearLayoutManager(this);

        // 데이터의 순서를 역순으로 보여줌
        mLinearLayoutManager.setReverseLayout(true);
        mLinearLayoutManager.setStackFromEnd(true);

        mRecyclerView.setLayoutManager(mLinearLayoutManager);
        mAdapter = new MyAdapter();
        mRecyclerView.setAdapter(mAdapter);

        FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                        .setAction("Action", null).show();
            }
        });

        // Firebase 연동 후 Child Event Listener 등록
        mRef = new Firebase(FIREBASE_POST_URL).orderByChild("writeTime");
        mRef.addChildEventListener(new ChildEventListener() {
            @Override
            public void onChildAdded(DataSnapshot dataSnapshot, String previousChildName) {
                Post value = dataSnapshot.getValue(Post.class);
                String key = dataSnapshot.getKey();
                if (previousChildName == null) {
                    mPosts.add(0, value);
                    mKeys.add(0, key);
                } else {
                    int previousIndex = mKeys.indexOf(previousChildName);
                    int nextIndex = previousIndex + 1;
                    if (nextIndex == mPosts.size()) {
                        mPosts.add(value);
                        mKeys.add(key);
                    } else {
                        mPosts.add(nextIndex, value);
                        mKeys.add(nextIndex, key);
                    }
                }
                mAdapter.notifyDataSetChanged();
            }

            @Override
            public void onChildChanged(DataSnapshot dataSnapshot, String s) {
                String key = dataSnapshot.getKey();
                Post value = dataSnapshot.getValue(Post.class);
                int index = mKeys.indexOf(key);
                mPosts.set(index, value);

                mAdapter.notifyDataSetChanged();
            }

            @Override
            public void onChildRemoved(DataSnapshot dataSnapshot) {
                // A model was removed from the list. Remove it from our list and the name mapping
                String key = dataSnapshot.getKey();
                int index = mKeys.indexOf(key);

                mKeys.remove(index);
                mPosts.remove(index);

                mAdapter.notifyDataSetChanged();
            }

            @Override
            public void onChildMoved(DataSnapshot dataSnapshot, String previousChildName) {
                // A model changed position in the list. Update our list accordingly
                String key = dataSnapshot.getKey();
                Post newModel = dataSnapshot.getValue(Post.class);
                int index = mKeys.indexOf(key);
                mPosts.remove(index);
                mKeys.remove(index);
                if (previousChildName == null) {
                    mPosts.add(0, newModel);
                    mKeys.add(0, key);
                } else {
                    int previousIndex = mKeys.indexOf(previousChildName);
                    int nextIndex = previousIndex + 1;
                    if (nextIndex == mPosts.size()) {
                        mPosts.add(newModel);
                        mKeys.add(key);
                    } else {
                        mPosts.add(nextIndex, newModel);
                        mKeys.add(nextIndex, key);
                    }
                }
                mAdapter.notifyDataSetChanged();
            }

            @Override
            public void onCancelled(FirebaseError firebaseError) {
                firebaseError.toException().printStackTrace();
            }
        });
    }

    class MyViewHolder extends RecyclerView.ViewHolder {
        TextView text;
        ImageView background;
        TextView timeText;
        TextView commentCount;
        CardView cardView;

        public MyViewHolder(View itemView) {
            super(itemView);
            text = (TextView) itemView.findViewById(R.id.text);
            background = (ImageView) itemView.findViewById(R.id.background);
            timeText = (TextView) itemView.findViewById(R.id.timeText);
            commentCount = (TextView) itemView.findViewById(R.id.commentCount);
            cardView = (CardView) itemView.findViewById(R.id.card_view);
        }
    }

    class MyAdapter extends RecyclerView.Adapter<MyViewHolder> {

        @Override
        public MyViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
            View itemView = getLayoutInflater().inflate(R.layout.card_post, null);
            MyViewHolder myViewHolder = new MyViewHolder(itemView);
            return myViewHolder;
        }

        @Override
        public void onBindViewHolder(MyViewHolder holder, int position) {
            Post post = mPosts.get(position);
            holder.text.setText(post.getText());
            holder.commentCount.setText("" + post.getCommentMap().size());
            holder.timeText.setText(getDiffTimeText(post.getWriteTime()));
            Glide.with(PostActivity.this).load(post.getBgUrl()).centerCrop().into(holder.background);
        }

        @Override
        public int getItemCount() {
            return mPosts.size();
        }
    }

    String getDiffTimeText(long targetTime) {
        DateTime curDateTime = new DateTime();
        DateTime targetDateTime = new DateTime().withMillis(targetTime);

        int diffDay = Days.daysBetween(curDateTime, targetDateTime).getDays();
        int diffHours = Hours.hoursBetween(targetDateTime, curDateTime).getHours();
        int diffMinutes = Minutes.minutesBetween(targetDateTime, curDateTime).getMinutes();
        if (diffDay == 0) {
            if(diffHours == 0 && diffMinutes == 0){
                return "방금전";
            }
            if(diffHours > 0){
                return "" + diffHours + "시간 전";
            }
            return "" + diffMinutes + "분 전";

        } else {
            SimpleDateFormat format = new SimpleDateFormat("yyyy-MM-dd HH:mm");
            return format.format(new Date(targetTime));
        }
    }
}

public class PostActivity extends AppCompatActivity {
	...
    private static final int REQUEST_WRITE = 0;
    ...
    @Override
    protected void onCreate(Bundle savedInstanceState) {
    ...
    	FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Intent intent = new Intent(PostActivity.this, WriteActivity.class);
                startActivityForResult(intent, REQUEST_WRITE);
            }
        });

public class WriteActivity extends AppCompatActivity {
    private EditText mInput;
    private LinearLayoutManager mLinearLayoutManager;
    private RecyclerView mRecyclerView;
    private List<String> bgUrls = new ArrayList<>();
    private final String DEFAULT_BG_URL = "http://postfiles1.naver.net/20151124_256/akj61300_1448357467095Vrg0b_JPEG/bg1.jpg?type=w2";
    private ImageView mWriteBackground;
    private int mCardBgPosition = 0;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_write);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        // assets 에 json 에서 배경이미지 목록을 불러옴
        try {
            InputStream stream = getAssets().open("bg_urls.json");
            String[] bgArray = new Gson().fromJson(new InputStreamReader(stream), String[].class);
            if (bgArray != null && bgArray.length > 0) {
                bgUrls.addAll(Arrays.asList(bgArray));
            }
            if (bgUrls.isEmpty()) {
                bgUrls.add(DEFAULT_BG_URL);
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
        mInput = (EditText) findViewById(R.id.input);
        mWriteBackground = (ImageView) findViewById(R.id.writeBackground);
        mRecyclerView = (RecyclerView) findViewById(R.id.recyclerView);
        mLinearLayoutManager = new LinearLayoutManager(this);
        mLinearLayoutManager.setOrientation(LinearLayoutManager.HORIZONTAL);
        mRecyclerView.setLayoutManager(mLinearLayoutManager);
        mRecyclerView.setAdapter(new MyImageAdapter());

        FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                String text = mInput.getText().toString();
                if(TextUtils.isEmpty(text)){
                    return;
                }
                Post post = new Post();
                post.setText(text);
                post.setBgUrl(bgUrls.get(mCardBgPosition));
                post.setWriterId(getMyId());
                post.setWriteTime(System.currentTimeMillis());

                new Firebase(PostActivity.FIREBASE_POST_URL).push().setValue(post);
                setResult(RESULT_OK);
                finish();
            }
        });
    }

    class MyImageAdapter extends RecyclerView.Adapter<MyImageViewHolder> {

        @Override
        public MyImageViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
            View itemView = getLayoutInflater().inflate(R.layout.card_image, null);
            MyImageViewHolder myImageViewHolder = new MyImageViewHolder(itemView);
            return myImageViewHolder;
        }

        @Override
        public void onBindViewHolder(MyImageViewHolder holder, final int position) {
            final String imgSrc = bgUrls.get(position);
            Glide.with(WriteActivity.this).load(imgSrc).centerCrop().into(holder.background);
            holder.background.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View v) {
                    Glide.with(WriteActivity.this).load(imgSrc).centerCrop().into(mWriteBackground);
                    mCardBgPosition = position;
                }
            });
        }

        @Override
        public int getItemCount() {
            return bgUrls.size();
        }
    }

    class MyImageViewHolder extends RecyclerView.ViewHolder {
        ImageView background;

        public MyImageViewHolder(View itemView) {
            super(itemView);
            background = (ImageView) itemView.findViewById(R.id.background);
        }
    }

    private String getMyId(){
        return Settings.Secure.getString(this.getContentResolver(),
                Settings.Secure.ANDROID_ID);
    }
}

public class Post implements Serializable{
...
}

public class Comment implements Serializable{
...
}

@Override
public void onBindViewHolder(MyViewHolder holder, final int position) {
    final Post post = mPosts.get(position);
    holder.text.setText(post.getText());
    holder.commentCount.setText("" + post.getCommentMap().size());
    holder.timeText.setText(getDiffTimeText(post.getWriteTime()));
    Glide.with(PostActivity.this).load(post.getBgUrl()).centerCrop().into(holder.background);
    holder.cardView.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Intent intent = new Intent(PostActivity.this, DetailActivity.class);
            intent.putExtra("postId", mKeys.get(position));
            intent.putExtra("post", post);
            startActivity(intent);
        }
    });
}

public class DetailActivity extends AppCompatActivity {
    private RecyclerView mRecyclerView;
    private LinearLayoutManager mLinearLayoutManager;
    private TextView mText;
    private ImageView mCardImage;
    private String mPostId;
    private Post mPost;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_detail);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        mRecyclerView = (RecyclerView) findViewById(R.id.recyclerView);
        mText = (TextView) findViewById(R.id.text);
        mCardImage = (ImageView) findViewById(R.id.cardImage);

        mPost = (Post) getIntent().getSerializableExtra("post");
        mPostId = getIntent().getStringExtra("postId");

        Glide.with(this).load(mPost.getBgUrl()).centerCrop().into(mCardImage);
        mText.setText(mPost.getText());

        setSupportActionBar(toolbar);

        FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                        .setAction("Action", null).show();
            }
        });

        mLinearLayoutManager = new LinearLayoutManager(this);
        mLinearLayoutManager.setOrientation(LinearLayoutManager.HORIZONTAL);
        mRecyclerView.setLayoutManager(mLinearLayoutManager);
        mRecyclerView.setAdapter(new MyCommentAdapter(mPost.getCommentMap()));
    }

    private class MyCommentAdapter extends RecyclerView.Adapter<MyCommentViewHolder> {
        List<Comment> comments = new ArrayList<>();
        String[] keys;

        MyCommentAdapter(Map<String, Comment> commentMap) {
            List<Comment> list = new ArrayList<>(commentMap.values());
            Collections.sort(list, new Comparator<Comment>() {
                public int compare(Comment o1, Comment o2) {
                    Collator collator = Collator.getInstance();
                    String strLhs = String.format("%030d", o1.getWriteTime());
                    String strRhs = String.format("%030d", o2.getWriteTime());
                    return collator.compare(strRhs, strLhs);
                }
            });
            comments.addAll(list);
        }

        @Override
        public MyCommentViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
            View itemView = getLayoutInflater().inflate(R.layout.card_comment, null);
            MyCommentViewHolder viewHolder = new MyCommentViewHolder(itemView);
            return viewHolder;
        }

        @Override
        public void onBindViewHolder(MyCommentViewHolder holder, int position) {
            Comment comment = comments.get(position);
            Glide.with(DetailActivity.this).load(comment.getBgUrl()).centerCrop().into(holder.background);
            holder.commentText.setText(comment.getText());
        }

        @Override
        public int getItemCount() {
            return comments.size();
        }
    }

    class MyCommentViewHolder extends RecyclerView.ViewHolder {
        ImageView background;
        TextView commentText;

        public MyCommentViewHolder(View itemView) {
            super(itemView);
            background = (ImageView) itemView.findViewById(R.id.background);
            commentText = (TextView) itemView.findViewById(R.id.commentText);
        }
    }
}

public class DetailActivity extends AppCompatActivity {
...
	@Override
    protected void onCreate(Bundle savedInstanceState) {
    ...
    FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Intent intent = new Intent(DetailActivity.this, WriteActivity.class);
                intent.putExtra("postId", mPostId);
                startActivity(intent);
                finish();
            }
        });

public class WriteActivity extends AppCompatActivity {
...
	private String mPostId;
	
	@Override
	protected void onCreate(Bundle savedInstanceState) {
		 super.onCreate(savedInstanceState);
		 setContentView(R.layout.activity_write);
		 Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
		 setSupportActionBar(toolbar);
		 
		 mPostId = getIntent().getStringExtra("postId");
		 if(!TextUtils.isEmpty(mPostId)){
		 	getSupportActionBar().setTitle("Comment Write Activity");
		 }
		 ...
		 FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
		 fab.setOnClickListener(new View.OnClickListener() {
		 	@Override
		 	public void onClick(View view) {
		 		String text = mInput.getText().toString();
		 		if(TextUtils.isEmpty(text)){
                    return;
                }
                if(TextUtils.isEmpty(mPostId)){
                    Post post = new Post();
                    post.setText(text);
                    post.setBgUrl(bgUrls.get(mCardBgPosition));
                    post.setWriterId(getMyId());
                    post.setWriteTime(System.currentTimeMillis());

                    new Firebase(PostActivity.FIREBASE_POST_URL).push().setValue(post);
                    setResult(RESULT_OK);
                }else{
                    Comment comment = new Comment();
                    comment.setText(text);
                    comment.setBgUrl(bgUrls.get(mCardBgPosition));
                    comment.setWriterId(getMyId());
                    comment.setWriteTime(System.currentTimeMillis());
                    new Firebase(PostActivity.FIREBASE_POST_URL).child(mPostId)
                            .child("commentMap").push().setValue(comment);
                    setResult(RESULT_OK);
                }
                finish();
		 	}
		 });
    ...