From Arraylist to Array

I want to know if it is safe/advisable to convert from ArrayList to Array? I have a text file with each line a string:

1236
1233
4566
4568
....

I want to read them into array list and then i convert it to Array. Is it advisable/legal to do that?

thanks

Solution 1:

Yes it is safe to convert an ArrayList to an Array. Whether it is a good idea depends on your intended use. Do you need the operations that ArrayList provides? If so, keep it an ArrayList. Else convert away!

ArrayList<Integer> foo = new ArrayList<Integer>();
foo.add(1);
foo.add(1);
foo.add(2);
foo.add(3);
foo.add(5);
Integer[] bar = foo.toArray(new Integer[foo.size()]);
System.out.println("bar.length = " + bar.length);

outputs

bar.length = 5

Solution 2:

This is the best way (IMHO).

List<String> myArrayList = new ArrayList<String>();
//.....
String[] myArray = myArrayList.toArray(new String[myArrayList.size()]);

This code works also:

String[] myArray = myArrayList.toArray(new String[0]);

But it less effective: the string array is created twice: first time zero-length array is created, then the real-size array is created, filled and returned. So, if since you know the needed size (from list.size()) you should create array that is big enough to put all elements. In this case it is not re-allocated.