CODEWITHSUNDEEP
javaandroidgenericssqlite
Oroboros102
Oroboros102

Reputation: 2254

How to hold collection of different type?

I'm writing method wrapping db SELECT statement, that receives SQL query and runs it. Result is collected in Object[][]. Every cell in database may be Integer or String.

I have problem with return array. It's Object[][], so I loose type.

public Object[][] select(String query) {
    Object[][] result = new Object[cursor.getCount()][cursor.getColumnCount()];
    Cursor cursor = db.rawQuery(query, null);

    //Iterate over cursor (rows)
    do {
        //Iterate over columns (cells with data of different type)
        for(int columnIndex = 0; columnIndex < cursor.getColumnCount(); columnIndex++) {
            int type = cursor.getType(columnIndex);

            //assume, that there are only two types - String or Integer (actualy there are 3 more types)
            if(type == Cursor.FIELD_TYPE_INTEGER) {
                result[cursor.getPosition()][columnIndex] = cursor.getInt(columnIndex);
            }
            else if(type == Cursor.FIELD_TYPE_STRING) {
                result[cursor.getPosition()][columnIndex] = cursor.getString(columnIndex);
            }
        }
    } while (cursor.moveToNext());

    return result;
}

I do not want to loose type. E.g. want to be able do like this (pseudocode):

//pseudocode:
result = select("SELECT age, name FROM people")
print( "Hello, " + result[0][0] + ". In ten years you'll be " )
print( result[0][0] + 10 )
//Prints: Hello, John. In ten years you'll be 56

Every book about Java says, that I can't have array of different types. It's clear.

But how should I hold all this Integers and Strings from DB? I tried to use Collections and Generics, but no luck - new to Java.

Also tried to use some kind of Result value holder:

private class Result {
    private Class type;
    private Object value;

        ...

    public ??? get() { //??? - int or string - here I have some problems :)
        return this.type.cast(this.value);
    }

    //do NOT want to have these:
    public int getInt() { return (int) value; }
    public int getString() { return (String) value; }
}

Method select(...) returns Result[][] in this case.

Answered myself: Understood, that I was trying to code in Java like I did in php. With strong typing I can not make straightforward method, returning all types without explicit casting.

After all I decided to create common data objects and fill them in on selects.

Upvotes: 0

Views: 899

Answers (5)

obataku
obataku

Reputation: 29646

I would probably do something like...

public final class Row {

  private final Map<String, ?> columns;

  protected Row(final Map<String, ?> columns) {
    this.columns = columns;
  }

  public <T> T get(final String name) {
    return (T) columns.get(name);
  }  

  public <T> T get(final String name, final Class<T> type) {
    return (T) columns.get(name);
  }
}

public final Iterable<Row> select(final String query) {
  final Cursor cursor = db.rawQuery(query, null);
  final int numRow = cursor.getCount();
  final int numCol = cursor.getColumnCount();
  final Row[] rows = new Row[numRow];
  for (int row = 0; row < numRow; ++row) {
    final Map<String, Object> cols = new HashMap<String, Object>(numCol);
    for (int col = 0; col < numCol; ++col) {
      final String name = cursor.getColumnName(col);
      final Object val = null;
      final int type = cursor.getType(col);
      switch (type) {
        case Cursor.FIELD_TYPE_INTEGER:
          val = cursor.getInt(col);
          break;
        case Cursor.FIELD_TYPE_STRING:
          val = cursor.getString(col);
          break;
        ...
      }
      map.put(name, val);
    }
    rows[row] = new Row(cols);
    cursor.moveToNext();
  }
  return Arrays.asList(rows); 
}

private final Row selectFirst(final String query) {
  return select(query).iterator().next();
}

Followed by...

final Row result = selectFirst("SELECT age, name FROM people");
print("Hello, " + result.get("name") + ". In ten years you'll be "
          + (result.<Integer>get("age") + 10)).

... or, equivalently:

final Row result = selectFirst("SELECT age, name FROM people");
final String name = result.get("name");
final int age = result.get("age");
print("Hello, " + name + ". In ten years you'll be " + (age + 10));

Using variables allows javac to infer the type parameters for get (String and Integer, respectively).

Since select returns Iterable<Row>, you can use it in a for loop:

for (final Row row : select(...)) {
  /* operate on the row */
}

Note that I'm not sure if this will even compile without a tweak or two. Haven't tested it, sorry -- but the general idea should.

Upvotes: 2

davidmontoyago
davidmontoyago

Reputation: 1833

Return a List of Maps with the columns names or indexes as keys like this:

public List<Map<Integer,Result>> select(String query)

When extracting your data do this:

List<Map<Integer,Result>> data = new ArrayList<Map<Integer,Result>>();
...
Map<String,Result> elem = new...
elem.put(columnIndex, new Result(..));

data.add(elem);

Collections are surely more powerful than Arrays.

EDIT Used the Result holder class you provided.

Upvotes: 0

Byter
Byter

Reputation: 1132

use a Map<String,List<Result>>

The Result class can store the actual value and its type

class Result {

   Object value;
   DataType type;

}

enum DataType {

  String,
  Integer

}

Upvotes: 0

Serdar Dogruyol
Serdar Dogruyol

Reputation: 5157

Why dont you a create a Result class which wraps whatever you want and instantiate it?

Upvotes: 3

kosa
kosa

Reputation: 66657

Best way may be create a java bean with getter/setter with the properties you will get from database. For example if you are getting employee

class Employee
{
int empID;
String empName;

public void setEmpID(String eID)
{
empID= eID;
}

public int getEmpID()
{
return eID;
}
.......
}

Upvotes: 1

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