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.
9 Answers
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);
2 Comments
user288609
What's the difference between list and ArrayList? Why just ArrayList<ArrayList<Foo>> list = new ArrayList<ArrayList<Foo>>(); Thanks
Jigar Joshi
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 alsoArrayList<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
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 );
5 Comments
MarioP
forbidden by java compiler. you cant create a generic array.
x4u
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
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.
MarioP
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?
x4u
It's actually more type safe than the generic collection types as it is guaranteed that there will never be an object in the array that is not a instance of the component type of the array. So you can be 100% sure that the array will always contain ArrayList objects (or null) but never a string for instance. A
ArrayList<ArrayList<T>> can technically hold any object you like and you can manage to put a string in it although you would need to use some nasty casts or reflection to do this. Apart from that the type safety or lack thereof is the same for arrays and collection types.Try:
ArrayList<ArrayList<DataType>> array = new ArrayList<ArrayList<DataType>>();
for (int i = 0; i < n; ++i) {
array.add(new ArrayList<DataType>());
}
8 Comments
trutheality
You can't mix arrays and generics.
trutheality
Correction: you can with
@SuppressWarnings("unchecked"), but you're discouraged to.user288609
But how to add entries for each ArrayList representing a row? Thanks
trutheality
@MarioP: What do you mean? Also, see this answer for a full explanation.
trutheality
@user288609:
array.get(rowNumber).add(element) (like in my answer) |
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.
Comments
if the number of rows is fixed, try something like this:
ArrayList<MyObject>[] = new ArrayList<MyObject>[fixedRows]
4 Comments
MarioP
forbidden by java compiler. you cant create a generic array.
trutheality
Need
@SuppressWarnings("unchecked") for that to always work.
erickzetta
Damn Java generics... Then
ArrayList[] = new ArrayList[fixedRows]. Suck it Java.trutheality
@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.
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);
Comments
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]
Comments
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);
}
3 Comments
MarioP
"ArrayList is a raw type. References to generic type ArrayList<E> should be parameterized" - bad coding style. high risk of
ClassCastException.
CodeRedd
good catch--thanks for the correction! the code above has been fixed.
CodeRedd
Clearly, I need to brush up on my ArrayLists. ^^
Something[][] s = new Something[numRows][]ands[0] = new Something[3]?