ArrayList grouping?

N

news.t-com.hr

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.




__________ Information from ESET NOD32 Antivirus, version of virus signature database 4092 (20090520) __________

The message was checked by ESET NOD32 Antivirus.

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J

John B. Matthews

"news.t-com.hr said:
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>
 
N

news.t-com.hr

thanks, i'll try that

Piotr Kobzda said:
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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The message was checked by ESET NOD32 Antivirus.

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