Create ArrayList from Existing ArrayList

Question

I've got an ArrayList that looks like this:

ArrayList<String> item;

item [0] --> boom

item [1] --> pow

item [2] --> bang

item [3] --> zing

Now what I'm trying to do is take each item and create an empty ArrayList of strings with that item's name. For instance, the result would be

ArrayList<String> boom; 

ArrayList<String> pow; 

ArrayList<String> bang;

ArrayList<String> zing;

Sorry if this is a simple answer, but I'm still learning.

Please post real code. What type is `item`? Can you post it? — Elliott Frisch, May 25 '15 at 19:28
What would you do with all these variables once you make them??? Wouldn't you have to type their names? — Sergey Kalinichenko, May 25 '15 at 19:29
Would it not be easier just to start with a string array and create arraylists from there? — Mark, May 25 '15 at 19:34
Not sure I follow. Basically, It's for a website scraping project. The code goes to a site, captures the headings in different paragraphs, and then needs to create an Arraylist with each heading name in order to store the paragraph in. I can already get the heading titles, but for organization purposes i need the heading titles in the ArrayList to become new ArrayList titles so I know where to assign each paragraph. @ElliottFrisch - ArrayList item; — REAL O G, May 25 '15 at 19:38
Why not use a e.g. `Map>` for this task? The key could be the heading and the words of the paragraph could be stored inside `List`. Iterating over the keys/ headings is very easy. — Stefan Freitag, May 25 '15 at 19:41

Elliott Frisch · Answer 1 · 2015-05-25T20:09:53.753

If I understand your question, then you might use a Map<String, ArrayList<String>> map, check if the heading is already present (and if it isn't create a new ArrayList<>). Something like,

Map<String, List<String>> headings = new HashMap<>();
// perform processing in a loop, for each heading...
String heading = "Example";
String content = "Body";
// ...
if (!headings.containsKey(heading)) {
    headings.put(heading, new ArrayList<>());
}
List<String> bodies = headings.get(heading);
bodies.add(content);
// .. iterate heading

or you might prepopulate the headings map like

List<String> nameList = Arrays.asList("boom", "pow", "bang");
Map<String, List<String>> headings = new HashMap<>();
for (String name : nameList) {
    headings.put(name, new ArrayList<>());
}
// ...

Razib · Accepted Answer · 2015-05-25T20:04:45.140

In programming language variable names is used to refers a stored value in computer memory. So a variable name can be considered as a key to access the value stored in computer memory. The standard data structure Map have the similar key value structure. So we can user Map here - "boom" as a key and new ArrayList<String>() as an value.

Suppose you have all the names (that is boom, pow, bang) in the nameList -

ArrayList nameList = new ArrayList(){{
    add("boom");
    add("pow");
    add("bang");
}};

Now you want to create 3 ArrayList of Stirng by the name given in nameList. So you put them in a Map<String, List<String> like this -

Map<String, List<String> > vars  = new HashMap<String, List<String>>();

for(int i=0; i<nameList.size(); i++){

   String key = nameList.get(i);
   List<String> value = new ArrayList<String>();
   vars.put(key, value);
}

The complete cod can be -

import java.util.*;

public class ArrayListFromNameList {

    public static void main(String[] args){

        List<String> nameList = new ArrayList<String>(){{
            add("boom");
            add("pow");
            add("bang");
        }};  

        Map<String, List<String> > vars  = new HashMap<String, List<String>>();

        for(int i=0; i<nameList.size(); i++){

           String key = nameList.get(i);
           List<String> value = new ArrayList<String>();
           vars.put(key, value);
        } 

    }

  /* Use the Map vars like this -
  * vars.get("boom") --> will reuturns you an ArrayList<String>();
  * similarly vars.get("pow") --> will returns you an ArrayList<String>();
  */
}

Thank you. This is what I was looking for. Since this is a project on web scraping, and I will need to visit more than one page, I probably won't know the names of the headings without printing them out to the screen. I know that hashmaps don't have an index (or so I thought), but how would I access them without knowing the heading names? — REAL O G, May 25 '15 at 20:16

Create ArrayList from Existing ArrayList

2 Answers2