In c++ we can have this kind of vectors:
vector < int > v[20];
I have to create the equivalent in java of a program in c++. I can declare
static ArrayList < ArrayList<Integer > > v;
but it's not quite what I want. I want it to be more intuitive. I am trying to find a way to easily add elements in my structure. For example if I have a graph and I am reading a road from a to b, in C++ a simply use v[a].push_back(b).
First of all vector<int> v[20] is not vectors of vector.
Secondly ArrayList could be the equivalent of std::vectors in Java.
The general equivalent of vector.push_back could be like this ie, using generic list:-
List<Integer> x = new ArrayList<Integer>();
x.add(1);
List<Integer>[] v = (List<Integer>[]) Array.newInstance(Integer.class, 20);
{
for (int i = 0; i < v.length; ++i) {
v[i] = new ArrayList<>();
}
}
Sadly, Java is not so concise or well designed here. Array.newInstance serves for all dimesions: int[][][] would be Array.newInstance(int.class, 10, 10, 10). Hence it returns an Object as least denominator, and needs to be cast.
The initial array contains only nulls, and needs an initialisation.
Correction:
Should of course be List.class not Integer.class.
Integer.class, specifically? I mean, is there a technical reason, is it common convention, or just your personal preference?
List<int> is not possible, a List contains objects only. There are non-standard libraries for an int list. However with boxing you may put an int into the list etcetera.List.class.import java.util.Vector;
public class Main {
public static void main(String[] args) {
// TODO Auto-generated method stub
Vector<Integer>[] anArray = (Vector<Integer>[]) new Vector[100];
for(int i = 0; i < anArray.length; i++)
anArray[i] = new Vector<Integer>();
anArray[2].add(2);//....
}
}
vector<int> v[20]is not a vector of vectors.