How do I implement nested ArrayList?

Question

I want to implement a data structure which looks something like this.

{{RowID, N1, N2, N3},
 {RowID, N4, N5, N6},
 {RowID, N7, N8, N9}}

And goes on. It basically is a table in Java with 3 columns and RowID. What data structure should I use and how do I implement it as in code?

What have you tried? Have you read up on the Java collection APIs (hint: Map, HashMap, List, ArrayList). — Jim Garrison, Nov 08 '10 at 17:28

score 15 · Answer 1 · answered Nov 08 '10 at 17:30

15

Make an ArrayList of ArrayLists. E.g.:

ArrayList<ArrayList> arrayListOfLists = new ArrayList<ArrayList>();
arrayListOfLists.add(anotherArrayList);
//etc...

answered Nov 08 '10 at 17:30

Rafe Kettler

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This is the only exact answer for the question. +1 – Sep 30 '11 at 02:40

score 6 · Answer 2 · answered Nov 08 '10 at 17:27

There are several options. One way is to declare a class that represents a row.

 public class MyRow{
     private long rowId;
     private int col1;
     private int col2;
     private int col3;
     //etc
 }

Obviously you choose appropriate data types and variable names.

Then you can create an ArrayList of this type:

   List<MyRow> rows = new ArrayList<MyRow>();

This is especially useful if the number of columns will not vary.

score 3 · Accepted Answer · answered Nov 08 '10 at 19:01

Assuming that RowID is a long and the column data are Doubles, I would implement this construct as:

import java.util.HashMap;
import java.util.Map;
...
Map<Long, Double[]> table = new HashMap<Long, Double[]>();

To store a row:

Long rowID = 1234L;
table.put(rowID, new Double {0.1, 0.2, 0.3});

To access a row:

Double[] row = table.get(rowID);

Replace Double[] with whatever data type you desire Int[], String[], Object[] ...

You may loop through this data with an iterator:

import java.util.Iterator;
import java.util.Map.Entry;
...
Iterator<Entry<Long, Double[]>> iter = table.entrySet().iterator();
while (iter.hasNext()) {
    Entry entry = iter.next();
    rowID = entry.getKey();
    row = entry.getValue();
};

To iterate the data in the order data was inserted, use LinkedHashMap in place of HashMap.

score 2 · Answer 4 · answered Nov 08 '10 at 17:28

2

You could use a Map<Integer, ArrayList<MyObject>> where the key to the map would be your RowID.

answered Nov 08 '10 at 17:28

mR_fr0g

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score 1 · Answer 5 · answered Nov 08 '10 at 17:27

1

I would create a bean object that contains the data for each row. That has an advantage over a "nested ArrayList" because the data members are strongly-typed.

Next, I would insert these beans into a List, probably a LinkedList unless you know the number of them ahead of time. If so, I would switch to an ArrayList.

If order is not important, you could use a HashSet or HashMap instead, depending on if you are only iterating them (Set) or need to do key lookups by RowID (Map). If you use one of these data structures, you will need to override equals() and hashCode() for your bean.

answered Nov 08 '10 at 17:27

@Jeremy: While `ArrayList` generally is the go-to `List` implementation when you have no specific idea of how the list will be used, I wouldn't say `LinkedList` is almost always a bad idea. If you aren't going to be doing any random access on the list and will be adding to or removing elements from the the list, `LinkedList` may well be faster, for example. Just need to know the relative costs of different operations on each type and what you'll be doing. – ColinD Nov 08 '10 at 17:57
@Jeremy: Got a reference for that? I'm curious. – ColinD Nov 08 '10 at 18:08
@Jeremy: Check out this [blog post](http://blog.publicobject.com/2010/07/caliper-confirms-reality-linkedlist-vs.html) and [corresponding microbenchmark](http://microbenchmarks.appspot.com/run/limpbizkit@gmail.com/com.publicobject.blog.ListAsQueueBenchmark) by Jesse Wilson at Google which shows how easily `LinkedList` beats `ArrayList` for a use case like I described. – ColinD Nov 08 '10 at 18:14
I think the core issue here is we don't know how the question asker is using the data structure. There are several implementations: aside from the linked/array list, a hash set/map may also be useful. Without more information, we simply do not know. – Nov 09 '10 at 02:01

score 0 · Answer 6 · answered Oct 07 '14 at 11:21

0

Java provides list casting, so for example you can do this in following way:

ArrayList<List<someObject>> ArrayListOfLists = new ArrayList<List<someObject>>();

answered Oct 07 '14 at 11:21

kiedysktos

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How do I implement nested ArrayList?

6 Answers6

Linked