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.

Answer 1

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.