How can I initialize an Array of ArrayList<String>?
I tried this syntax but it didn't work:
ArrayList<String>[] subsection = new ArrayList<String>[4];
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1ArrayList array[] = new ArrayList[4];Samir Mangroliya– Samir Mangroliya2012-04-19 11:40:00 +00:00Commented Apr 19, 2012 at 11:40
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@SamirMangroliya it wouldn't work with genericsDejell– Dejell2013-11-24 14:53:22 +00:00Commented Nov 24, 2013 at 14:53
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4 Answers
you can define like this :
ArrayList<String>[] lists = (ArrayList<String>[])new ArrayList[10];
lists[0] = new ArrayList<String>();
lists[0].add("Hello");
lists[0].add("World");
String str1 = lists[0].get(0);
String str2 = lists[0].get(1);
System.out.println(str1 + " " + str2);
1 Comment
SVK PETO
Unchecked cast: 'java.util.ArrayList[]' to 'java.util.ArrayList<java.lang.String>[]'
That syntax works fine for the non-generic ArrayList. (ideone)
But it won't work for the generic ArrayList<E>: (ideone)
This code:
ArrayList<String>[] subsection = new ArrayList<String>[4];
Gives a compiler error:
Main.java:8: generic array creation
ArrayList<String>[] subsection = new ArrayList<String>[4];
For the generic version use an ArrayList<ArrayList<E>>:
ArrayList<ArrayList<String>> subsection = new ArrayList<ArrayList<String>>();
3 Comments
Jakub Zaverka
But then the OP will end up with an ArrayList of ArrayLists, not with an array of ArrayLists. It is almost the same, but the are differences.
Gruber
If the generic version is preferred, better use the Interface
List<List<String>> subsection = new ArrayList<ArrayList<String>>(); for the variables type.
Louis Wasserman
The point is that generics and arrays just don't play nicely together.
Okay after comment, I thought well... your right why not.
Figured it out.
ArrayList[] test = new ArrayList[4];
test[3] = new ArrayList<String>();
test[3].add("HI");
System.out.println(test[3].get(0));
Though I will be honest, I am not really sure WHY this works.
Once you assign the first item of test as a new Collection, it will only allow all other items in the array to be that type. So you couldn't do
test[3] = new ArrayList<String>();
test[2] = new HashSet<String>();
answered Apr 19, 2012 at 13:09
Dan Ciborowski - MSFT
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2 Comments
Dave Newton
Are you sure about that? Why couldn't you have an array of collections?
Dan Ciborowski - MSFT
After Dave's comment, I figured out the OP's answer.
Look into generics as type clarification process, you can assign typed value to a variable of raw type AND vice versa. In core generics are a shortcut for the programmers to avoid making type casting too much, which also helps to catch some logical errors at compile time. At the very basics ArrayList will always implicitly have items of type Object.
So
test[i] = new ArrayList<String>(); because test[i] has type of ArrayList.
The bit
test[3] = new ArrayList<String>();
test[2] = new HashSet<String>();
did not work - as was expected, because HashSet simply is not a subclass of ArrayList. Generics has nothing to do here. Strip away the generics and you'll see the obvious reason.
However,
test[2] = new ArrayList<String>();
test[3] = new ArrayList<HashSet>();
will work nicely, because both items are ArrayLists.
Hope this made sense...
