ArrayList of ArrayLists, how to use inner lists?

Question

I have an ArrayList object to which I want to dynamically add ArrayLists.

However, since the compiler doesn't know what objects are going to be inside the outer ArrayList before the loop to add them runs, I can't use the inner ArrayLists as I normally would.

My code looks somewhat like this:

ArrayList list = new ArrayList();

 for(int i=8;i>0;i--)
 {
     list.add(i, new ArrayList());
 }

 for(int i=8;i>0;i--)
 {         
    tileRows.get(i).add( //add other stuff );
 }

How can I tell the compiler that the items in list are going to be of ArrayList type?

Bear in mind that I'm completely new to Java. If this is a stupid question I apologise.

score 2 · Accepted Answer · edited May 23 '17 at 10:29

You can declare a List<Whatever>, and Whatever can be another interface/class that supports generics, like List<String>, so putting all this together you can declare a List<List<String>> and the compiler will be happy. Based on your code above:

List<List<String>> tileRows= new ArrayList<List<String>>();
for(int i=8;i>0;i--) {
    list.add(i, new ArrayList<String>());
}

for(int i=8;i>0;i--) {
    //here the compiler knows tileRows.get(i) is a List<String>
    tileRows.get(i).add("foo");
    //uncomment the below line and you will get a compiler error
    //tileRows.get(i).add(new Object());
}

Remember to always program to an interface, not to a direct class implementation.

Thank you, this is exactly what I was after. I suspected it would be something simple like this. — Obversity, Feb 25 '14 at 00:23

score 2 · Answer 2 · answered Feb 24 '14 at 15:33

You do it the same way you do with any other type of Object. You cast it:

((ArrayList)tileRows.get(i)).add( //add other stuff );

Or better yet, you use generics so you don't have to do that cast:

//this tells the compiler that your ArrayList will contain ArrayLists
ArrayList<ArrayList<Whatever>> list = new ArrayList()<>;
//now the compiler knows the get() function returns an ArrayList, so you don't have to cast it
tileRows.get(i).add( //add other stuff );

score 2 · Answer 3 · answered Feb 24 '14 at 15:34

This is Java Generic, and the correct use is some like this:

 ArrayList<ArrayList> list = new ArrayList<ArrayList>();

 for(int i=8;i>0;i--)
 {
     list.add(i, new ArrayList());
 }

 for(int i=8;i>0;i--)
 {         
    tileRows.get(i).add( //add other stuff );
 }

score 1 · Answer 4 · answered Feb 24 '14 at 15:33

1

List<List<String>> list = new ArrayList<>();

answered Feb 24 '14 at 15:33

Kakarot

4,252
2
16
18

score 1 · Answer 5 · answered Feb 24 '14 at 15:33

1

List<ArrayList<Object>> list = new ArrayList<>();

in Java 7

answered Feb 24 '14 at 15:33

Philipp Sander

10,139
6
45
78

ArrayList of ArrayLists, how to use inner lists?

5 Answers5