Add Integer in ArrayList<ArrayList<Integer>>

Question

please help meto add integer in the ArrayList inside an ArrayList.. Here is the code..

ArrayList<ArrayList<Integer>> player = new ArrayList<ArrayList<Integer>>(10);
ArrayList<Integer> array = new ArrayList<Integer>(10);
array.add(1);
array.add(2);
array.add(3);
array.add(4);
array.add(5);

player.add(array);
player.add(array);

If i check what's inside array and player using debug..

array[1,2,3,4,5]
player[[1,2,3,4,5],[1,2,3,4,5]]

Now, i want to add Integer on player's ArrayList slot 0 using this:

player.get(0).add(6);

but instead of getting of this:

player[[1,2,3,4,5,6],[1,2,3,4,5]]

i got this:

player[[1,2,3,4,5,6],[1,2,3,4,5,6]]

In short, player's ArrayLost slot 0 and slot 1 received the integer that i'ved add..

Please help.. Thanks in advance.. :)

You are adding the same List of String twice. As the List data type is by reference all changes done to array affects player.get(0) and player.get(1). To solve this, you need to create two distinct Lists for player. — Binkan Salaryman
– Binkan Salaryman, Commented Nov 11, 2015 at 8:19

Tim Biegeleisen · Accepted Answer · 2015-11-11 08:25:10Z

3

You added the same ArrayList to both entries of the player ArrayList. Both entries of the player ArrayList are exactly the same object.

You should make a second ArrayList:

ArrayList<Integer> array2 = new ArrayList<Integer>();

Then add it as the second item of the player ArrayList.

edited Nov 11, 2015 at 8:25

Tim Biegeleisen

526k32 gold badges323 silver badges399 bronze badges

answered Nov 11, 2015 at 8:19

Matthew Gunn

4,5291 gold badge15 silver badges31 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Binkan Salaryman Over a year ago

Hint: To make a copy of an ArrayList, you can use the copy constructor ArrayList#<init>(Collection)

Michael De Chavez Over a year ago

Thank you guys.. i will check your anwers.. :)

ETartaren · Accepted Answer · 2015-11-11 08:33:07Z

0

ArrayList<ArrayList<Integer>> player = new ArrayList<ArrayList<Integer>>(10);
ArrayList<Integer> array = new ArrayList<Integer>(10);
array.add(1);
array.add(2);
array.add(3);
array.add(4);
array.add(5);
ArrayList<Integer> array1 = new ArrayList<Integer>(array);
player.add(array);
player.add(array1);

Already answered...

answered Nov 11, 2015 at 8:33

ETartaren

1704 silver badges16 bronze badges

Comments

Denis Lukenich · Accepted Answer · 2015-11-11 08:26:16Z

0

As others mentioned you actually add the same element to both lists.

If you do not want to fill both ArrayLists with the initial values. You can keep your code and only change the following lines:

player.add(array);
player.add(array);

to:

player.add(array);
player.add(new ArrayList(array)); // alternatively player.add(array.clone());

This makes a new ArrayList to the second entry in players.

answered Nov 11, 2015 at 8:26

Denis Lukenich

3,1941 gold badge24 silver badges39 bronze badges

Comments

Community · Accepted Answer · 2017-05-23 11:44:30Z

0

The problem with your code is that - when you add the array to player object you pass the same reference of array object.

When you do this player.get(0) you are getting the same object which is ArrayList<Integer> array = new ArrayList<Integer>(10); and then when you do the adding player.get(0).add(6); you are pointing to the reference of array object and java passes objects by reference. see this SO thread

edited May 23, 2017 at 11:44

CommunityBot

11 silver badge

answered Nov 11, 2015 at 8:27

Nikolay Rusev

4,2803 gold badges21 silver badges29 bronze badges

Comments

Staffan Runnsjö · Accepted Answer · 2015-11-11 08:46:10Z

0

That is because adding array twice to player does not add the values of array to player twice, it only adds two copies of the reference to array to player

player[[1,2,3,4,5],[1,2,3,4,5]]

is really

player[array,array]

You can test this with for example jUnits assertTrue:

assertTrue(player.get(0) == player.get(1))

edited Nov 11, 2015 at 8:46

answered Nov 11, 2015 at 8:22

Staffan Runnsjö

867 bronze badges

3 Comments

Matthew Gunn Over a year ago

That's not correct. Java IS pass by value! When you pass in objects to a function, object references are passed by value.

Staffan Runnsjö Over a year ago

In a way, yes, and in another way, no. It passes the value of the reference to the object, which is what you write, so yes. But it passes the reference to the object, which is what you are (usually) interested in with the code. That is why pass-by-reference is a convenient way of thinking, although it is technically false.

Matthew Gunn Over a year ago

I would avoid the whole pass by reference vs. pass by value distinction when talking with a Java beginner. It's an important distinction for C, Fortran, etc.. but it's likely just to blow completely over a beginner's head. I'd emphasize that Object variables in JAVA don't hold the literal objects themselves but pointers to the object. That's a clear way of thinking about the mistake here. It also has the benefit of being technically correct rather than technically false.

Collectives™ on Stack Overflow

Add Integer in ArrayList<ArrayList<Integer>>

5 Answers 5

2 Comments

Comments

Comments

Comments

3 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

5 Answers 5

2 Comments

Comments

Comments

Comments

3 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related