Question

I just want to merge two ArrayLists and have the contents in one ArrayList. Both lists contain the object which is an instance of the same class.
The object reference themselves are different though. However I am getting this unexpected size for the combined arraylist. I use JAVA 1.4

ArrayList a1 = new ArrayList();
ArrayList b1 = new ArrayList();


ClassA cls1A = new ClassA();

ClassA cls1B = new ClassA();


a1.add(cls1A);

b1.add(cls1B);


a1.size()  = 100;

b1.size() = 50;


//merge the two arraylist contents into one

//1st method and its result

 a1.addAll(b1);


 //Expected Result
 a1.size = 150  

 //but 

 //Obtained result
 a1.size = 6789  


//2nd method and its result

 Collections.copy(a1, b1)

 //Expected result
  a1.size() = 150

 //but

 //Obtained result
 a1.size = 6789  

How can I have an ArrayList which displays the combined size??

Answer 1

I came up with the following solution. First get the size of both the arraylists, then increase the capacity(by using ensureCapacity method of ArrayList class) of the arraylist to which the two needs to be merged. Then add the objects of the 2nd arraylist from the last index of the first arraylist.

     a1.add(cls1A);
     b1.add(cls1B);
        int p = a1.size();

        int w = b1.size();

        int j = (p + w);

        a1.ensureCapacity(j);   



        for(int r = 0; r <w; r++)
        {

        ClassA cls1B = new ClassA();
        Object obj  = b1.get(r);                
        cls1B = (ClassA)obj;
        a1.add(p, cls1B);
        p++;
        }