Arraylists and arrays

Question

I have 2 arraylists situated inside an array as follows:

public Object[] loopthrougharrays() {
 Object[] tables = new Object[2];
 tables[0] = list;
 tables[1] = listB15;
 return tables;
}

My 2 arraylists are called list and listB15.

I can then call my arraylists from another method like

loopthrougharrays()[1] = new ArrayList();

which is listB15.

However if I try to add an item to the ArrayList like

loopthrougharrays()[1].add(s)

where s is a variable

Java doesn't recognize the loopthrougharrays()[1] as an ArrayList.

How can I add the variable via this method?

I appear to get the following error:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException

My entire code works fine if I do the following:

listB15 = new ArrayList();
listB15.add(s)

as I would expect.

• Tom: You have an array of Object, how should the compiler know what instances it contains? And btw: you get the NPE because loopthrougharrays() creates a new array and that doesn't care about your loopthrougharrays()[1] = new ArrayList(); statement.
• Turing85: tl;dr: make tables an ArrayList[] (or even better a List[]).

Answer 1

The problem is you are doing assignments on method calls. You are calling the method two times. so the second line creates another array list

loopthrougharrays()[1] = new ArrayList();
loopthrougharrays()[1].add(s) //This one will call the method again
// and get new array list and the previous value is lost

Simple fix

 Object[] getTwoArrays = loopthrougharrays()
 ArrayList L0 = <ArrayList> getTwoArrays[0];
 ArrayList L1 = <ArrayList> getTwoArrays[1];
 L1 = new ArrayList();
 L1.add(s);

Here the loopthrougharrays() is called once and the return value is stored into a local reference and then it works

While I try to provide a quick fix provided, i would not return null arraylits in a object array and then initialize outside the method and do assignments. it's clumsy.

A lesser evil way is

 class SomeX {
 private List Ll = new ArrayList();
 private List L2 = new ArrayList();
 public addToL1(Object s) {
 L1.add(s);
 }
 public addToL2(Object s) {
 L2.add(s)
 }
 }
 //main method
 SomeX x = new SomeX();
 x.addToL1(s);
 x.addToL2(s);

Answer 2

I don't know what do you want to do with that, but it is not a good practice. However it could work, if you cast your Object array element to an ArrayList like:

((ArrayList)t.loopthrougharrays()[1]).add(s);

And delete loopthrougharrays()[1] = new ArrayList(); (like @Tom comment says, thanks) or replace it with

ArrayList myNewArrayList = (ArrayList)loopthrougharrays()[1];
myNewArrayList.add(s);

Answer 3

Cast it before you add anything to your list.

((ArrayList<YourVariableType>)(loopthrougharrays()[1])).add(s);

But remember, this is not a good practice. I'm pretty sure, you can achieve what you want to achieve with a better design and with best practices.

Answer 4

Your tables is an array of objects ,

tables[1] = listB15; // tables still remains as array of object and listB15 is considered as an object since Object is the Super class .

To achieve wt u want you can try these methods :

1. ((List<String>) tables[1]).add("hello world"); // cast it with List<String> (for example)

2. create a list

   List<List<String>> tables = new ArrayList<List<String>>();
    tables1.add(list) ; 
    tables1.add(listB15) ; 
    tables1.get(1).add("hello world");
    System.out.println(tables);