How to create 2 Dimensional ArrayList and set values it in Java?

Question

I have a 2 dimensional array and fill it at start :

public static String [][] BooksList =
    {
        {"Coders at Work",null},
        {"Code Complete",null},
        {"The Mythical Man Month",null},
        {"Don’t Make Me Think, Revisited",null},
        {"The Pragmatic Programmer",null}
    };

I want to add more rows anytime. But array class doesn't allow increase the size of array. So i must use ArrayList like this :

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

But i don't know how to fill it like arrays.

@texasbruce That doesn't allow for increasing the size of the list either. — Joe C
– Joe C, Commented Aug 12, 2017 at 15:05
Have you taken a look at the ArrayList Javadoc? Is anything about it unclear? — Joe C
– Joe C, Commented Aug 12, 2017 at 15:06
The second dimension of your array is always null. Can you explain your need/use case ? It seems a xy problem. — davidxxx
– davidxxx, Commented Aug 12, 2017 at 15:08
@JoeC yes i saw them but i'm confusing .. need a simple example. — Saman
– Saman, Commented Aug 12, 2017 at 15:09

Jonathan · Accepted Answer · 2017-08-12 15:11:28Z

2

You can add items to the list like this:

BooksList.add(Arrays.asList("Cool book", null));

answered Aug 12, 2017 at 15:11

Jonathan

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5 Comments

Saman Over a year ago

Thank you its worked ! and how i access the 2th value of 2th List of ArrayList ?

Jonathan Over a year ago

BooksList.get(1).get(1) Remember lists start at 0

Saman Over a year ago

How to add item to list of specific row ? like: BooksList.get(0).add("YES");

Jonathan Over a year ago

BooksList.get(0).add(int index, E element)

Saman Over a year ago

It doesn't work, i tried this : BooksList.get(0).add(0,"Yes"); also BooksList.get(0).add(1,"YES"); and BooksList.get(0).add(2,"YES");

davidxxx · Accepted Answer · 2017-08-12 17:35:17Z

According to your comment :

It is the simple library system that anytime can add book and every book also have name of person that borrowed it.

You should probably use a Map where the key is a book and the value is a List of borrower (as a book may probably have been booked by several people).
Using a map will ease the retrieval/modification of a specific book.
With array or List, when you want to access/change a specific book, you have to iterate the List until finding it.

You could so create a Map with books :

Map<Book, List<Borrower>> borrowersByBook = new HashMap<>();
borrowersByBook.put(new Book("Coders at Work"), new ArrayList<>());
borrowersByBook.put(new Book("Code Complete"), new ArrayList<>());
...

And add borrowers for books :

 List<Borrower> borrowers = borrowersByBook.get(new Book("Coders at Work"));
 borrowers.add(new Borrower("SaMi GiMiX", LocalDate.of(2017, 8, 12));
 borrowers.add(new Borrower("davidxxx", LocalDate.of(2017, 8, 12));

If you use a Book class instead of a String class to represent a book, you have to override equals()/hashcode() methods.

Hugo sama · Accepted Answer · 2017-08-12 15:15:15Z

1

A two dimensional array can be represented with lists like this :

List<List<String>> BooksList = new ArrayList<>();
//adding elements
BooksList.add( Arrays.asList("ONE","TWO") );
//or like this
List<String> row = new ArrayList<>();
row.add("THREE");
row.add("FOUR");
BooksList.add(row);

answered Aug 12, 2017 at 15:15

Hugo sama

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