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1

I need to implement a 2D dynamic array. The number of rows is fixed, say n. But the number of columns for each row is not fixed and equivalent. For instance, the first row has 3 elements and the second row has 5 elements. How to do this in Java using Arraylist. Thanks.

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Does - in your example - the first row always have 3 elements? If so, why don't you simply create a (e.g.) Something[][] s = new Something[numRows][] and s[0] = new Something[3]? –  Tedil Jun 3 '11 at 20:30

9 Answers 9

up vote 2 down vote accepted

How about List<List<Foo>> ?

For Example: 

List<List<Foo>> list = new ArrayList<List<Foo>>();

List<Foo> row1 = new ArrayList<Foo>();
row1.add(new Foo());
row1.add(new Foo());
row1.add(new Foo());
list.add(row1);

List<Foo> row2 = new ArrayList<Foo>();
row2.add(new Foo());
row2.add(new Foo());

list.add(row2);
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What's the difference between list and ArrayList? Why just ArrayList<ArrayList<Foo>> list = new ArrayList<ArrayList<Foo>>(); Thanks –  user288609 Jun 3 '11 at 20:48
2  
List is an interface , ArrayList is its specific implementation. well if you have List<Foo> then you can refer to an LinkedList or some other implementation also –  Jigar Joshi Jun 3 '11 at 20:56
up vote 0 down vote

As you say, you can make an array of arraylists and use the ArrayList(int initial capacity) constructor to set the capacity of each column:

ArrayList<YourObject>[] rows=new ArrayList<YourObjects>[n];
for(i=0;i<n;i++){
rows[i]=ArrayList<YourObjects>(initialsize);
}
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1  
"ArrayList is a raw type. References to generic type ArrayList<E> should be parameterized" - bad coding style. high risk of ClassCastException. –  MarioP Jun 3 '11 at 20:36
    
good catch--thanks for the correction! the code above has been fixed. –  CodeRedd Jun 4 '11 at 2:50
    
now it won't compile ^^ –  MarioP Jun 4 '11 at 10:44
    
Clearly, I need to brush up on my ArrayLists. ^^ –  CodeRedd Jun 7 '11 at 18:56
up vote 2 down vote

You can either use a array for the rows since this dimenstion is fixed:

@SuppressWarnings("unchecked")
ArrayList<T>[] arr = new ArrayList[ fixedsize];

or use nested ArrayLists:

List<List<T>> list = new ArrayList<List<T>>( fixedsize );
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forbidden by java compiler. you cant create a generic array. –  MarioP Jun 3 '11 at 20:42
    
Nope it's not forbidden! You can very well create arrays of parameterized types and you can test my code. It compiles and works as expected. –  x4u Jun 3 '11 at 20:46
    
well, i couldnt do it. what the %@§$ am i doing wrong? o.O –  MarioP Jun 3 '11 at 20:57
    
Oops, sorry, you are right I had a error in my code and the cast can be omitted. I now fixed it and it will compile now. –  x4u Jun 3 '11 at 21:08
    
aaah yeah, there we go. didn't see that before. can i assume that this is typesafe, considerung the actual variable isn't a raw type? –  MarioP Jun 3 '11 at 21:12
up vote 0 down vote

I would create an array of ArrayList (ArrayList[3] rows = new ArrayList[3] if the rows were 3) Then for each row create column classes and insert them into an ArrayList. and then place the ArrayList into the Array. the row array's index can be used to keep track of the row number. Remember arrays start there indexes at 0 so the row number would be rows[index+1]

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up vote 0 down vote 
List<ArrayList<SomeObject>> twoDList = new ArrayList<List<SomeObject>>(n);
for( int i=0; i<n; i++ )
    twoDList.add( new ArrayList<SomeObject>() );

Use as:

twoDList.get(rownumber).add(newElementInColumn);
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up vote 1 down vote 
ArrayList<ArrayList<SomeObject>> twodlist = new ArrayList<ArrayList<SomeObject>>();
ArrayList<SomeObject> row = new ArrayList<SomeObject>();
row.add(new SomeObject(/* whatever */));
// etc
twodlist.add(row);
row = new ArrayList<SomeObject>();
// etc
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2  
Almost, except 2dlist isn't a valid Java identifier. :) –  Ted Hopp Jun 3 '11 at 20:28
    
@Ted Hopp, where do see that identifier? ;-) thanks for correction :-) –  MarioP Jun 3 '11 at 20:33
up vote 0 down vote

if the number of rows is fixed, try something like this:

ArrayList<MyObject>[] = new ArrayList<MyObject>[fixedRows]
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forbidden by java compiler. you cant create a generic array. –  MarioP Jun 3 '11 at 20:39
    
Need @SuppressWarnings("unchecked") for that to always work. –  trutheality Jun 3 '11 at 20:40
    
Damn Java generics... Then ArrayList[] = new ArrayList[fixedRows]. Suck it Java. –  erickzetta Jun 3 '11 at 20:45
    
now it asks for a ClassCastException ^^ –  MarioP Jun 3 '11 at 20:48
    
@erikzetta: it is indeed annoying, I've given up and switched to all-generics when I have cases like that -- the overhead is pretty negligible anyway. –  trutheality Jun 3 '11 at 20:55
up vote 0 down vote

You could create an array of ArrayList elements because your row count is fixed.

ArrayList[] dynamicArray = new ArrayList[n]();

Note: You'll need to allocate an ArrayList object in each entry in the array. So...

for (int loop = 0; loop < n; loop++)
dynamicArray[loop] = new ArrayList();

OR if you'd like both rows and columns to be dynamic you could create an ArrayList of ArrayLists....

ArrayList<ArrayList<T>> dynamicArray = new ArrayList<ArrayList<T>>();

Once again, you'll need to create an array list in each new entry to dynamicArray.

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up vote 1 down vote

Try:

ArrayList<ArrayList<DataType>> array = new ArrayList<ArrayList<DataType>>();
for (int i = 0; i < n; ++i) {
    array.add(new ArrayList<DataType>());
}
share|improve this answer
1  
You can't mix arrays and generics. –  trutheality Jun 3 '11 at 20:29
    
Ack. Amended answer. –  Ted Hopp Jun 3 '11 at 20:30
    
Correction: you can with @SuppressWarnings("unchecked"), but you're discouraged to. –  trutheality Jun 3 '11 at 20:37
    
@trutheality: code snipped? couldn't make that working... –  MarioP Jun 3 '11 at 20:44
    
But how to add entries for each ArrayList representing a row? Thanks –  user288609 Jun 3 '11 at 20:47

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