Can't adding objects in ArrayList?

Question

Hi I am trying to add an object into an Arraylist, I am using Java. But it does not work as I intended.

Let's say we have a class Sentence, so the code looks like

ArrayList<Sentence> result = new ArrayList<Sentence>();
for (int i =0; i<10;i++)
{
    Sentence s = new Sentence(i.toString(),i);
    //there is a string and an int in this Sentence object need to be set
    result.add(s); 
}

The above one works correctly. But I wish to speed up my code, so I try to only new one obejct, the code become:

ArrayList<Sentence> result = new ArrayList<Sentence>();
Sentence s = new Sentence(" ",0);
for (int i =0; i<10;i++)
{
    s.setString(i.toString());
    s.setInt(i);
    result.add(s); 
}

However, in this case, my result will become empty. I think I do change the content in the object s, but I don't know why it does not work during the result.add(s).

Many thanks to your reply.

Are you sure your result "will become empty"? You should be getting a list of 10 references to the same object. — Cameron Skinner
– Cameron Skinner, Commented Apr 24, 2012 at 5:05

Tim Destan · Accepted Answer · 2012-04-24 05:04:17Z

5

Your s variable is always referring to the same object. It looks like you are adding the same object 10 times, which by the end of the for loop will have its string equal to "9" and its int equal to 9.

answered Apr 24, 2012 at 5:04

Tim Destan

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3 Comments

JLTChiu Over a year ago

Do you mean that my ArrayList will only get one object? and after my sample, it will only get the object with i =9?

Cameron Skinner Over a year ago

The list will contain 10 references to a single Sentence. That Sentence will have i = 9.

Tim Destan Over a year ago

Yes, that's what I mean. You need to call the constructor n times to get n objects. What you're doing now is just making modifications to one object, and storing n references to it.

Community · Accepted Answer · 2017-05-23 12:03:17Z

3

In the second case you adding 10 pointers to single Sentence instance in ArrayList.

You have to make 10 Sentence to insert 10 pointer in ArrayList.

I think you are messing with pass by value and pass by reference in Java, to clarify this, have a look at this post.

This post might also help you.

edited May 23, 2017 at 12:03

CommunityBot

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answered Apr 24, 2012 at 5:04

Adil Soomro

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2 Comments

JLTChiu Over a year ago

I see, so in this case, I can't avoid making these new objects.

Adil Soomro Over a year ago

yea of course, you need to make 10 Sentence objects in the memory.

Donald Herman · Accepted Answer · 2012-04-25 02:18:35Z

0

ArrayList<Sentence> result = new ArrayList<Sentence>();
for (int i =0; i<10;i++)
{
    result.add(new Sentence(i.toString(),i)); 
}

If you want to create less lines of code than you could use this example but it's not necessarily more optimized.

edited Apr 25, 2012 at 2:18

answered Apr 24, 2012 at 5:28

Donald Herman

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Comments

maxaxis · Accepted Answer · 2014-01-11 23:29:17Z

0

In order to prevent duplicate objects. Always instantiate them before using. This way your List would have n number of unique objects,

answered Jan 11, 2014 at 23:29

maxaxis

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