Finding element in ArrayList<String[]> in Java

Question

What is the most efficient way of finding a ArrayList element according to a "subelement"?

For instance I have an ArrayList named test that is made up of these Arrays:

{a,first}
{b, second}
{c, third}
{d, fourth}

I am trying to create a method so that I can find the second element of each sub element(the array of Strings). My method would be something like:

public static String getElement(String key, ArrayList<String[]> haystack)

so calling the method

getElement("a", test)

would return the String "first". I know I could loop through the whole array and find it that way but I was wondering if there would be a more efficient way. Thanks

Why are you using an arrayList, have you looked into using a hashmap? — Grammin
– Grammin, Commented Jul 13, 2011 at 20:55
Do you call that method more than once? Or is it a one-shot operation? — emboss
– emboss, Commented Jul 13, 2011 at 21:00

Kal · Accepted Answer · 2011-07-13 21:02:20Z

5

I think you might be actually looking for Map implementation.

If not, your option is to bruteforce it, which means you have to scan through the list of elements, then look at the first element in the array and if its equal, return the second element.

for ( String[] a : haystack ) {

       if ( key.equals(a[0])
             return a[1];
}

EDIT --

You could take the arraylist and convert it into a map by iterating over it. This would only be useful if you are given the ArrayList once and you have to repeatedly perform the "get" oepration on it.

Map<String, String> myMap = new HashMap<String, String>();

for ( String[] a : haystack ) 
    myMap.put(a[0] , a[1]);

Once you are done with that, you can just call

myMap.get(key);

edited Jul 13, 2011 at 21:02

answered Jul 13, 2011 at 20:54

Kal

24.9k8 gold badges67 silver badges66 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Ray Toal · Accepted Answer · 2011-07-13 20:55:16Z

0

Please use a Map instead of an ArrayList for this. Or were you given a list by someone else.

If you must use a list, and the list is unsorted, and you only need to do the operation once, you can walk through the outer array.

answered Jul 13, 2011 at 20:55

Ray Toal

88.7k20 gold badges186 silver badges245 bronze badges

1 Comment

Julio Diaz Over a year ago

I was given an ArrayList, is there a way to transform this ArrayList to a Map?

Eng.Fouad · Accepted Answer · 2011-07-13 21:17:07Z

0

You can define Pair:

public class Pair
{
    private String key;
    private String string;
    public Pair(String key, String string)
    {
        this.key = key;
        this.string = string;
    }
    public String getKey(){return key;}
    public String getString(){return string;}
}

and then use it like:

public static String getElement(String key, ArrayList<Pair> list)
for(Pair p : list)
{
    if(p.getKey().equals(key)) return p.getString();
}

edited Jul 13, 2011 at 21:17

answered Jul 13, 2011 at 21:11

Eng.Fouad

118k75 gold badges326 silver badges431 bronze badges

Collectives™ on Stack Overflow

Finding element in ArrayList<String[]> in Java

3 Answers 3

Comments

1 Comment

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

1 Comment

Comments

Your Answer

Sign up or log in

Post as a guest

Related