ArrayList<Integer> a =new ArrayList<Integer>();
ArrayList<ArrayList<Integer>> j =new ArrayList<ArrayList<Integer>>();
a.add(1);
a.add(2);
a.add(3);
for(int c=0; c<10; c++){
j.add(a);
}
j.get(3).add(1);
System.out.println(j);
Does anyone know why this code adds 1 to every element of j as opposed to only the third element, and what can I do to fix this?
This is what happens when you add array list a to array list j 10 times.
This is what happens when you add 1 to array list a.
So basically all 10 indexes of ArrayList j points to a single ArrayList a. Hence, printing of value from any index of j will always gives you the same result.
To let each index point to a different array list:
j.get(9).add(9, <my Int>);You use the very same ArrayList a instance in every element of j.
You have to create a new instance of ArrayList for every element of j if you want them to be different.
for(int c=0; c<10; c++)
{
j.add(new ArrayList<>(a));
}
In your code a is the pointer to the memory location where the ArrayList resides.
Actually j.get(3) references the arrayList a as every j.get(c), that's why every cells of j pointing on a are changed. the next image shows how it works.
