java插入HashMap会弄乱排序

Question

基于这个类

public class Record {
    public static final String TABLE_NAME = "records";

    public static final String COLUMN_ID = "id";
    public static final String COLUMN_LONGITUDE = "longitude";
    public static final String COLUMN_LATITUDE = "latitude";
    public static final String COLUMN_SPEED = "speed";
    public static final String COLUMN_TIMESTAMP = "timestamp";

    private int id;
    private String longitude;
    private String latitude;
    private String speed;
    private String timestamp;

    public static final String CREATE_TABLE =
            "CREATE TABLE " + TABLE_NAME + "("
                + COLUMN_ID + " INTEGER PRIMARY KEY AUTOINCREMENT,"
                + COLUMN_LONGITUDE + " TEXT,"
                + COLUMN_LATITUDE + " TEXT,"
                + COLUMN_SPEED + " TEXT,"
                + COLUMN_TIMESTAMP + " TIMESTAMP DEFAULT (DATETIME('now','localtime'))"
            + ")";

    public Record(){
    }

    public Record(int id, String longitude, String latitude, String speed, String timestamp){
        this.id = id;
        this.longitude = longitude;
        this.latitude = latitude;
        this.speed = speed;
        this.timestamp = timestamp;
    }

    public int getId(){
        return id;
    }

    public String getLongitude(){
        return longitude;
    }

    public String getLatitude(){
        return latitude;
    }

    public String getSpeed(){
        return speed;
    }

    public String getTimestamp(){
        return timestamp;
    }

    public void setId(int id){
        this.id = id;
    }

    public void setLongitude(String longitude){
        this.longitude = longitude;
    }

    public void setLatitude(String latitude){
        this.latitude = latitude;
    }

    public void setSpeed(String speed){
        this.speed = speed;
    }

    public void setTimestamp(String timestamp){
        this.timestamp = timestamp;
    }
}

我已经创建了我的SQLite模型

public class DatabaseHelper extends SQLiteOpenHelper {

    private static final int DATABASE_VERSION = 1;

    // Database Name
    private static final String DATABASE_NAME = "speeds";

    public DatabaseHelper(Context context) {
        super(context, DATABASE_NAME, null, DATABASE_VERSION);
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        db.execSQL(Record.CREATE_TABLE);
    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        db.execSQL("DROP TABLE IF EXISTS " + Record.TABLE_NAME);

        onCreate(db);
    }

    public void insertRecord(String longitude, String latitude, String speed){
        SQLiteDatabase db = this.getWritableDatabase();
        ContentValues values = new ContentValues();

        values.put(Record.COLUMN_LONGITUDE, longitude);
        values.put(Record.COLUMN_LATITUDE, latitude);
        values.put(Record.COLUMN_SPEED, speed);

        System.out.println("VALUEEEEEEEES" + values);

        db.insert(Record.TABLE_NAME, null, values);
        db.close();
    }

    public List<Record> getAllRecords(){
        List<Record> records = new ArrayList<>();

        String selectQuery = "SELECT * FROM " + Record.TABLE_NAME + " ORDER BY " + Record.COLUMN_TIMESTAMP + " DESC";

        SQLiteDatabase db = this.getWritableDatabase();
        Cursor cursor = db.rawQuery(selectQuery, null);

        if(cursor.moveToFirst()){
            do{
                Record record = new Record();
                record.setId(cursor.getInt(cursor.getColumnIndex(Record.COLUMN_ID)));
                record.setLongitude(cursor.getString(cursor.getColumnIndex(Record.COLUMN_LONGITUDE)));
                record.setLatitude(cursor.getString(cursor.getColumnIndex(Record.COLUMN_LATITUDE)));
                record.setSpeed(cursor.getString(cursor.getColumnIndex(Record.COLUMN_SPEED)));
                record.setTimestamp(cursor.getString(cursor.getColumnIndex(Record.COLUMN_TIMESTAMP)));

                records.add(record);
            } while (cursor.moveToNext());
        }
        db.close();

        return records;
    }

    public List<Record> getLastTenRecords(){
        List<Record> records = new ArrayList<>();

        String selectQuery = "SELECT * FROM " + Record.TABLE_NAME + " ORDER BY " + Record.COLUMN_TIMESTAMP + " DESC LIMIT 10";

        SQLiteDatabase db = this.getWritableDatabase();
        Cursor cursor = db.rawQuery(selectQuery, null);

        if(cursor.moveToFirst()){
            do{
                Record record = new Record();
                record.setId(cursor.getInt(cursor.getColumnIndex(Record.COLUMN_ID)));
                record.setLongitude(cursor.getString(cursor.getColumnIndex(Record.COLUMN_LONGITUDE)));
                record.setLatitude(cursor.getString(cursor.getColumnIndex(Record.COLUMN_LATITUDE)));
                record.setSpeed(cursor.getString(cursor.getColumnIndex(Record.COLUMN_SPEED)));
                record.setTimestamp(cursor.getString(cursor.getColumnIndex(Record.COLUMN_TIMESTAMP)));

                records.add(record);
            } while (cursor.moveToNext());
        }
        db.close();

        return records;
    }
}

现在，我试图显示ExpandableListView中的一些数据。在我的场景中，我按COLUMN_TIMESTAMP对sql查询的响应进行排序，并尝试以HashMap<String , List<String>类型对其进行解析，以便ExpandableListView能够识别它

问题是，即使对结果进行了排序（我在逐个循环和打印时可以看到），在我使用此方法解析并放入HashMap后，排序也消失了

public static HashMap<String, List<String>> getLastTenData(Context context) {
        HashMap<String, List<String>> expandableListDetail = new HashMap<>();

        DatabaseHelper databaseHelper = new DatabaseHelper(context);

        List<Record> recordList = databaseHelper.getLastTenRecords();
        for(int i=0; i<recordList.size(); i++){
            List<String> myList = new ArrayList<>();
            myList.add("Y: "+recordList.get(i).getLatitude());
            myList.add("X: " + recordList.get(i).getLongitude());
            myList.add("Km/h: "+recordList.get(i).getSpeed());
            expandableListDetail.put(recordList.get(i).getTimestamp(), myList);
            System.out.println("check "+String.valueOf(i)+"    "+recordList.get(i).getTimestamp());
        }

        System.out.println(expandableListDetail);

        return expandableListDetail;
    }

我的意思是expandableListDetail在应该按键排序时没有按键排序。 有什么帮助吗

Answer 1

如果你已经回答了一个类似的问题（关于HashSet）here

类似地，HashMap也是无序的，因此向它添加值，然后在映射上迭代，将不会按照最初添加的顺序生成项目

为什么LinkedHashMap解决了这个问题？（从@Deadpool了解到这个想法）

Hash table and linked list implementation of the Map interface, with predictable iteration order. This implementation differs from HashMap in that it maintains a doubly-linked list running through all of its entries. This linked list defines the iteration ordering, which is normally the order in which keys were inserted into the map (insertion-order).

为什么TreeMap解决了这个问题？（从@tomgeraghty3中学习这个想法）

The map is sorted according to the natural ordering of its keys