How do I add arrays to an Arraylist of Arrays (Arraylist<Integer[]>)

Question

I want to save the steps pf my player-character in a Sokoban-game. So at first i want to fill an Array of int x and y, called "pos" with the actual position of the character. And than i want to add this array to an ArrayList of Arrays, called "moves".

Array for one player-position:

int[] pos = new int [2];

ArrayList for all steps, the player made in the level:

Arraylist<Integer[]> moves = new ArrayList<>();

"int[]" makes an error, if placed inside the pointy brackets at the ArrayList.

How do i add the Array pos to the ArrayList moves?

Please share the exact text of the error and the code that causes it — Mureinik
– Mureinik, Commented May 11, 2019 at 22:13
List<Integer[]> moves = new ArrayList<Integer[]>(); works for me — billjamesdev
– billjamesdev, Commented May 11, 2019 at 22:16
@Drachenbauer how are you compiling your java code? ArrayList<int[]> moves = new ArrayList<>(); is valid java code — Ferrybig
– Ferrybig, Commented May 11, 2019 at 22:25

Sumit Bhardwaj · Accepted Answer · 2019-05-11 22:28:21Z

2

This works completely fine with int[]. However int[] != Integer[], Use same in both places.

int[] pos = new int[2];
ArrayList<int[]> arrayList = new ArrayList<>();
pos[0] = 1;
pos[1] = 2;
arrayList.add(pos);
System.out.println(arrayList.get(0)[0]+ " "+ arrayList.get(0)[1]);

answered May 11, 2019 at 22:28

Sumit Bhardwaj

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3 Comments

Drachenbauer Over a year ago

i use this: public void savePos() { pos[0] = x; pos[1] = y; moves.add(pos); } It´s very similar to yours. The Array and the ArrayList are class-variables. Every game-move, x or y changes. But it doesn´t only add the new vaules to the end of the ArrayList, it also replaces all existing list-elements with the new x and y vaules. Does the ArrayList hold instancas of the Array, wich change their vaules, if the vaules in the original array change? if yes, how do i add the vaules of each game-move as a new Array, that does not more interact with the original Array "pos"?

Drachenbauer Over a year ago

Now i use this: public void savePos() { moves.add(new int[] {x, y}); } and use the Array pos only in the method for the actual undo move-function and not more to save the positions.

Sumit Bhardwaj Over a year ago

ArrayList moves stores just the reference of the int[] pos as a list element, not the whole array. Since you had the same array pos, updating values of this on each move and adding it to the list, it will store a new element indeed i.e. same reference of pos, hence you will get only the latest value. The correct way will be to initialize a new array pos for every move, like you've done already

Michael Borgwardt · Accepted Answer · 2019-05-11 22:32:06Z

int and Integer are two different things (I struggled with this as well when I first learned Java a long time ago.

int is a so-called primitive datatype, which means it is not an object. Integer is a wrapper class, basically a class whose only purpose is to contain an int so that it can be handled as an object. Since Java 5, there is "autoboxing", which means you can assign int values to Integer variables and vice versa, and the compiler will handle it silently. This tends to muddle the difference, but it still is there and important - such as in this case.

So if you want to store instances of int[] in an ArrayList, you need an ArrayList<int[]>, not an ArrayList<Integer[]>.

However, it is impossible to have an ArrayList<int> because ArrayList can only store Objects (the generic types exist only for the compiler), not primitives.

Oh yes, and to add an element to the list, use

Arraylist<int[]> moves = new ArrayList<>();
arrayList.add(pos);

You could have found this in the API doc for ArrayList - you should really learn to use the API doc, you will need it all the time to answer questions such as this.

Ahmed Elsaghier · Accepted Answer · 2019-05-11 23:00:22Z

0

You can also make class (for example Pos) and make x and y as attributes and make constructor. The you can make the type of arraylist is Pos ArrayList arrayList = new ArrayList<>(); arrayList.add(new Pos(3,2));

answered May 11, 2019 at 23:00

Ahmed Elsaghier

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