adding to an 2d arraylist code example

Example 1: 2d arraylist in java

int vertexCount = 3;
ArrayList<ArrayList<Integer>> graph = new ArrayList<>(vertexCount);
//Next, we'll initialize each element of ArrayList with another ArrayList:

for(int i=0; i < vertexCount; i++) {
    graph.add(new ArrayList());
}

Example 2: how to create a 2d arraylist java

ArrayList<ArrayList<Object>> a = new ArrayList<ArrayList<Object>>();