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How can I group values in ArrayList

Let's say I have List<Example> list = new ArrayList<Example>();

and Example is

public class Example{

private Date date;
private Integer no;
private Integer salary;
}

and I have initial data

01.01.2009 , 1 , 10000
02.01.2009, 1, 20000
01.01.2009, 2, 30000

and I want to transform this to two new lists so I can have in the first
arrayList grouped values by distinct date, and third row summed;

01.01.2009, 40000
02.01.2009, 20000

and second list distinct date+no and third row summed;

01.01.2009 , 1, 10000
01.01.2009, 2, 30000
02.01.2009, 1 , 20000

How can i do this transformation ?

Thanks in advance.




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In article <gv36ca$abr$>,
"news.t-com.hr" <> wrote:

> How can I group values in ArrayList
>
> Let's say I have List<Example> list = new ArrayList<Example>();
>
> and Example is
>
> public class Example{
>
> private Date date;
> private Integer no;
> private Integer salary;
> }
>
> and I have initial data
>
> 01.01.2009 , 1 , 10000
> 02.01.2009, 1, 20000
> 01.01.2009, 2, 30000
>
> and I want to transform this to two new lists so I can have in the first
> arrayList grouped values by distinct date, and third row summed;
>
> 01.01.2009, 40000
> 02.01.2009, 20000
>
> and second list distinct date+no and third row summed;
>
> 01.01.2009 , 1, 10000
> 01.01.2009, 2, 30000
> 02.01.2009, 1 , 20000
>
> How can i do this transformation ?

Well, you can loop through the list and create whatever cross-tabulation
you want: for (Example e : list) { ... }. In the first case, a HashMap
can be used to group records by date; in the second, Collections.sort()
with a suitable Comparator can handle the sorting. As Date implements
Comparable, the required Comparator is particularly straightforward.

Personally, I'd set up and query a relational database:

<http://java.sun.com/docs/books/tutorial/jdbc/basics/index.html>

--
John B. Matthews
trashgod at gmail dot com
<http://sites.google.com/site/drjohnbmatthews>

thanks, i'll try that

"Piotr Kobzda" <> wrote in message
news:gv3mgu$bhk$...
> news.t-com.hr wrote:
>
> [...]
>> How can i do this transformation ?
>
> There are many solutions to your problem possible.
>
> One, a kind of general solution, may be using the following simple
> utility:
>
>
> import java.util.HashMap;
> import java.util.Map;
> import java.util.Map.Entry;
>
> /**
> * @param <E> the type of elements being grouped
> * @param <K> the type of grouping key
> * @param <V> the type of value associated with each group
> */
> public abstract class Grouper<E, K, V>
> {
> private static class ValueHolder<V>
> {
> V value;
>
> ValueHolder(V value)
> {
> this.value = value;
> }
> }
>
> private Map<K, ValueHolder<V>> map = new HashMap<K, ValueHolder<V>>();
>
> protected abstract K keyOf(E e);
> protected abstract V createValue(E e, K key);
> protected abstract V updateValue(E e, K key, V value);
>
> public void add(E e)
> {
> K key = keyOf(e);
> ValueHolder<V> valueHolder = map.get(key);
> if (valueHolder == null)
> {
> valueHolder = new ValueHolder<V>(createValue(e, key));
> map.put(key, valueHolder);
> }
> else
> {
> valueHolder.value = updateValue(e, key, valueHolder.value);
> }
> }
>
> public Map<K, V> toMap()
> {
> Map<K, V> res = new HashMap<K, V>();
> for (Entry<K, ValueHolder<V>> e : map.entrySet())
> {
> res.put(e.getKey(), e.getValue().value);
> }
> return res;
> }
>
> }
>
>
> The implementation provided may be redesigned and/or improved depending on
> needs (especially implementing Iterable<Entry<K, V>> to allow direct
> iteration of the inner map might be useful). But the idea presented seems
> to be flexible enough in many typical grouping scenarios.
>
>
> Below is an example based on your initial posting demonstrating intended
> use of my simple utility:
>
>
> import java.text.DateFormat;
> import java.text.ParseException;
> import java.text.SimpleDateFormat;
> import java.util.ArrayList;
> import java.util.Arrays;
> import java.util.Date;
> import java.util.List;
> import java.util.Map.Entry;
>
> public class GrouperDemo
> {
>
> static class Example
> {
> Date date;
> Integer no;
> Integer salary;
>
> static final DateFormat dateFmt = new SimpleDateFormat("d.M.y");
>
> Example(String date, Integer no, Integer salary)
> throws ParseException
> {
> this(dateFmt.parse(date), no, salary);
> }
>
> Example(Date date, Integer no, Integer salary)
> {
> this.date = date;
> this.no = no;
> this.salary = salary;
> }
>
> @Override
> public String toString()
> {
> return String.format("[%tF, %s, %s]", date, no, salary);
> }
> }
>
>
> static class DateNo
> {
> Date date;
> Integer no;
>
> DateNo(Date date, Integer no)
> {
> this.date = date;
> this.no = no;
> }
>
> @Override
> public boolean equals(Object obj)
> {
> return obj instanceof DateNo && equals((DateNo) obj);
> }
>
> public boolean equals(DateNo other)
> {
> return date.equals(other.date) && no.equals(other.no);
> }
>
> @Override
> public int hashCode()
> {
> return date.hashCode() ^ no.hashCode();
> }
>
> @Override
> public String toString()
> {
> return String.format("[%tF, %s]", date, no);
> }
> }
>
>
> public static void main(String[] args) throws Exception
> {
> List<Example> list
> = Arrays.asList(
> new Example("01.01.2009", 1, 10000),
> new Example("02.01.2009", 1, 20000),
> new Example("01.01.2009", 2, 30000));
>
> System.out.println(list);
>
>
> Grouper<Example, Date, Integer> gruper1
> = new Grouper<Example, Date, Integer>()
> {
>
> @Override
> protected Date keyOf(Example e)
> {
> return e.date;
> }
>
> @Override
> protected Integer createValue(Example e, Date key)
> {
> return e.salary;
> }
>
> @Override
> protected Integer updateValue(Example e, Date key, Integer value)
> {
> return value + e.salary;
> }
>
> };
>
>
> Grouper<Example, DateNo, Integer> gruper2
> = new Grouper<Example, DateNo, Integer>()
> {
>
> @Override
> protected DateNo keyOf(Example e)
> {
> return new DateNo(e.date, e.no);
> }
>
> @Override
> protected Integer createValue(Example e, DateNo key)
> {
> return e.salary;
> }
>
> @Override
> protected Integer updateValue(Example e, DateNo key,
> Integer value)
> {
> return value + e.salary;
> }
>
> };
>
> for (Example e : list)
> {
> gruper1.add(e);
> gruper2.add(e);
> }
>
> List<DateNo> res1 = new ArrayList<DateNo>();
> for (Entry<Date, Integer> e : gruper1.toMap().entrySet())
> {
> res1.add(new DateNo(e.getKey(), e.getValue()));
> }
>
> List<Example> res2 = new ArrayList<Example>();
> for (Entry<DateNo, Integer> e : gruper2.toMap().entrySet())
> {
> res2.add(
> new Example(e.getKey().date, e.getKey().no, e.getValue()));
> }
>
>
> System.out.println(res1);
> System.out.println(res2);
> }
> }
>
>
> piotr
>
> __________ Information from ESET NOD32 Antivirus, version of virus
> signature database 4095 (20090521) __________
>
> The message was checked by ESET NOD32 Antivirus.
>
> http://www.eset.com
>
>
>



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