Initialising a 2D array in Java

Question

In array, we can initialise something like this:

int[] arr = {1, 2, 3, 4, 5};

But for two dimensional array, why the below is not correct?

public class Main {
    public static void main(String[] args) {
       int[][] arr = new int[3][];
       arr[0] = new int[]{1,2,3}; //This is the correct way

       arr[0] = {1,2,3};  // ->Why this is not right? 
    }
}

are you looking for an answer other than "because of the java language specification" ? — Patrick Parker, Jul 01 '18 at 11:24
check this answer https://stackoverflow.com/questions/12231453/syntax-for-creating-a-two-dimensional-array — bembas, Jul 01 '18 at 11:28

score 1 · Accepted Answer · answered Jul 01 '18 at 11:31

1

Yes, you are correct the second way of creating a 2D array will not work because when the compiler will compile the code because it couldn't decide its type.

answered Jul 01 '18 at 11:31

Asheesh Sahu

342
2
12

thank you for your answer Asheesh, but when the first line of code execute: `int[][] arr = new int[3][]` does it indicate that all elements within would be all `int` type? – Reacher Jul 01 '18 at 11:36
1

@Ruizhi arrays in java are reference types, so when you declare 2d array like `new int[3][]` this means that it will contain an array with 3 null references for another int arrays `[null, null, null]` – Arashigor Jul 01 '18 at 11:43
yes because when you write int[ ][ ] arr this means that you are going to create an Integer array. and new int[3][ ] would allocate memory at runtime . – Asheesh Sahu Jul 01 '18 at 11:44
@Asheesh Sahu I know Arrays are Object in java and memory is allocated to Objects only by using new keyword. but why for 1D array, it is allowed to use short hand like: `int[] arr = {1 , 2 , 3 }`, but not allowed in 2D array ? – Reacher Jul 01 '18 at 11:47
@Arashigor I think you solved my confusion, thank you – Reacher Jul 01 '18 at 11:48
@Arashigor thank you, I still want to ask: null references to another `int` type, we can use `new` keyword to allocate memory at runtime, but why for 1D array, it is allowed to use short hand like: int[] arr = {1 , 2 , 3 }, and not allowed in 2D array ? – Reacher Jul 01 '18 at 12:01
@Ruizhi The int[] arr is just the reference to the array of integer. If you create an array with 10 integers, it is the same – an array is allocated and a reference is returned. While, Actually, we can only have the one-dimensional array in Java. 2D arrays are basically the just one-dimensional array of one-dimensional array. So to address that other next array we have to tell the compiler about that. – Asheesh Sahu Jul 01 '18 at 12:03

Arashigor · Answer 2 · 2018-07-01T11:34:00.543

0

You can only use this syntactic sugar during variable initialization time. Meaning that Java Language Specification allows only arr[0] = new int[]{1, 2, 3}; or int[][] arr = {{1,2,3},{1,2,3},{1,2,3}};. Difference is that the first variant is a variable assignment and the second one is variable initialization.

edited Jul 01 '18 at 11:34

answered Jul 01 '18 at 11:24

Arashigor

171
3
12

thanks, but why I can use array int[] arr ={1,2,3}; I can't use it in 2D array initialisation ? – Reacher Jul 01 '18 at 11:28
You can initialize inline 2D array this way `int[][] arr = {{1,2,3},{1,2,3},{1,2,3}};` – Arashigor Jul 01 '18 at 11:30
@Ruizhi check updated answer, hope it will bring clarity – Arashigor Jul 01 '18 at 11:36
Thank you Arashigor – Reacher Jul 01 '18 at 11:39

score 0 · Answer 3 · answered Jul 01 '18 at 11:27

0

The space for the second dimension would be allocate only with the code new int [ ] { 1, 2, 3}. Before that the compiler doesn't know what the type of the array items and what the dimension is.

answered Jul 01 '18 at 11:27

sven0379

11
4

thank you for your answer, but when the `int[][] arr = new int[3][];` execute, does it tell the compiler that all the elements would be `int` type, and it would be 2 dimension – Reacher Jul 01 '18 at 11:34

Initialising a 2D array in Java

3 Answers3