How do I create an array list of array lists in java? [duplicate]

Question

I currently have an array of arrays and that would work fantastic, except I just realized that I don't know the length of all of the arrays that I need. So I have to switch to ArrayList. I'm used to having an array of arrays and I know how to iterate through them. How do you do the same thing with ArrayLists?

The following works... until I need to change line size through my iterations.

Person[][] nameList = new Person[yearsBack][lines];
for (int i=0; i<yearsBack; i++) {
    for (int j=0; j<lines; j++) {
        nameList[i][j] = new Person(name, gender, rank);

Have you seen the Java tutorials on using Lists and generics? — dimo414
– dimo414, Commented Mar 13, 2015 at 5:42
You might be interested in this post stackoverflow.com/questions/4401850/… — sreeAravinth
– sreeAravinth, Commented Mar 13, 2015 at 5:45

Brandon · Accepted Answer · 2015-03-13 05:36:19Z

1

This should do it:

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

To loop through:

for(List<Object> list : listOfLists){
    for(Object obj : list){
        //do stuff
    }
}

answered Mar 13, 2015 at 5:36

Brandon

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2 Comments

Gil Over a year ago

How do you access specific points in the ArrayList like you can with Arrays? And by that I mean, the way you can access them in nested for loops with list[i][j]?

Brandon Over a year ago

Use list.get(i) where i is an index

kondu · Accepted Answer · 2015-03-13 05:54:06Z

0

ArrayList < Objects > can hold the Arraylist Objects and thus you can have the array of arraylists.

edited Mar 13, 2015 at 5:54

answered Mar 13, 2015 at 5:39

kondu

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Comments

raghavanm · Accepted Answer · 2015-03-13 05:54:18Z

import java.util.ArrayList;

public class Test {

    public static void main(String[] args) {
        ArrayList<Object> mainAl = new ArrayList<Object>();
        ArrayList<Integer> al1 = new ArrayList<Integer>();
        ArrayList<String> al2 = new ArrayList<String>();
        ArrayList<Double> al3 = new ArrayList<Double>();
        ArrayList<ArrayList<Object>> al4 = new ArrayList<ArrayList<Object>>();
        al1.add(1);
        al1.add(2);
        al1.add(3);
        al1.add(4);
        al2.add("Hello");
        al2.add("how");
        al2.add("are");
        al2.add("you");
        al3.add(100.12);
        al3.add(200.34);
        al3.add(300.43);
        al3.add(400.56);
        mainAl.add(al1);
        mainAl.add(al2);
        mainAl.add(al3);
        for (Object obj : mainAl) {
            System.out.println(obj);
        }
        al4.add(mainAl);
        for (Object obj : al4) {
            System.out.println(obj);
        }
    }
}

I hope this example is helpful for you.

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