How to work with ArrayList of ArrayLists of Integers?

Question

I'm trying to use an ArrayList of ArrayLists of Integers in Java, code:

Attributes and Constructor

int V;
    ArrayList<ArrayList<Integer>>  gpond;
    ArrayList<Integer> vpond;
    
    GrPond(int v){ //Number of total nodes (arraylists in the main arraylist)
        V=v;
        gpond = new ArrayList<>();
        for(int i=0;i<V;i++){
            gpond.add(new ArrayList<Integer>(V));
        }
    }

Im using this method to add the ArrayLists of Integers to the main ArrayList (gpond)

 void VPond(int v, int w,int val){ // v=origin, w=destination, val=value.
        ArrayList<Integer> temp = new ArrayList<Integer>(V);
        for(int i=0;i<V;i++){
            temp.add(null);
        }
        temp.set(w, val); //I want to set in the given index a certain value
        gpond.add(v, temp); //Then, add that ArrayList with the value (val) 
                            //at the given index (w), to the main ArrayList.
    }

And trying to print it using:

void printGraph(){
        for(int i=0;i<V;i++){
            System.out.println("node "+i);
            if(gpond.get(i)!=null){  //Just want to print those who are occupied
                for(int j=0;j<V;j++){ //j<V, because each ArrayList<Integer> has maximum V values
                    if(gpond.get(i).get(j)!=null) //Just print the indexes with a given value
                        System.out.println("["+i+"]-> ["+j+"] | val: "+gpond.get(i).get(j));
                }
            }
        }
        
        for(ArrayList<Integer> e: gpond){ //Just want to test each ArrayList of the main ArrayList
            System.out.println("e: "+ e);
        }
    }

What I expect it to do if the input is, lets say:

v,w,val:
0 1 2
0 2 1
1 0 1 

e: [null, 2, null], [null, null, 1] //or something like this. e=origin, [index->value]
e: [1, null, null]

is to print

node 0
[0]->[1] | val: 2
[0]->[2] | val: 1
[1]->[0] | val: 1

Instead its printing:

node 0
[0]->[2] | val: 1
node 1
[1]->[0] | val: 1
node 2
[2]->[1] | val: 2

e: [null, null, 1]
e: [1, null, null]
e: [null, 2, null]

I've tried changing set for add in temp and/or gpond in VPond
I think it's a pretty little mistake but I just can't figure it out

Answer 1

Your problem is that you always create a complete new List in your method VPond. So you loose your old values. You should create the new List in the constructor of GrPond and just set the new value in VPond .

GrPond(int v){ //Number of total nodes (arraylists in the main arraylist)
    V = v;
    gpond = new ArrayList<>();
    for(int i=0;i<V;i++){
        ArrayList<Integer> temp = new ArrayList<Integer>(V);
        for(int j=0;j<V;j++){
            temp.add(null);
        }
        gpond.add(temp);
    }
}

void VPond(int v, int w,int val){ // v=origin, w=destination, val=value.
    List<Integer> temp=gpond.get(v);
    temp.set(w, val); //I want to set in the given index a certain value
}

And please, think about the names of your variables. You use v, w, val, V, VPond, GrPond, i and j. If you touch this code a week after you wrote it you will be completely confused (just as I was). You even write comments to translate your variable names, why don't you use these meanings directly?

int sizeOfLists;
List<List<Integer>> gpond;
List<Integer> vpond;

GrPond(int listSize) {
    sizeOfLists = listSize;
    gpond = new ArrayList<>();
    for (int listIndex = 0; listIndex < sizeOfLists; listIndex++) {
        List<Integer> temp = new ArrayList<>(sizeOfLists);
        for (int indexInList = 0; indexInList < sizeOfLists; indexInList++) {
            temp.add(null);
        }
        gpond.add(temp);
    }
}

void VPond(int listIndex, int indexInList, int value) { listIndex=listIndex, indexInList=indexInList
    List<Integer> list = gpond.get(listIndex);
    list.set(indexInList, value);
}