Adding to a linkedList in a HashMap<String, LinkedList>

Question

I want to add to a linkedlist within a hashmap. ex john:--> jack-->black-->crack susan:--> sally,sammy,silly ect

Im not quite sure how to do this. Do i need a new linkedList for each name and if so how do i dynamically create one. Here is some sample code i made to try.

import java.util.*;
import java.io.*;
public class test {
    public static void main(String args[]) throws FileNotFoundException{
        HashMap<String, LinkedList<String>> testMap =  new HashMap<String, LinkedList<String>>();
        File testFile = new File("testFile.txt");
        Scanner enterFile = new Scanner(testFile);
        String nextline = "";
        LinkedList<String> numberList = new LinkedList<String>();
        int x = 0;
        while(enterFile.hasNextLine()){
            nextline = enterFile.nextLine();
            testMap.put(nextline.substring(0,1),numberList);
            for(int i = 1; i < nextline.length() - 1; i++){
                System.out.println(nextline);
                testMap.put(nextline.substring(0,1),testMap.add(nextline.substring(i,i+1)));
            }
            x++;
        }
        LinkedList<String> printHashList = new LinkedList<String>();
        printHashList = testMap.get(1);
        if(printHashList.peek() != "p"){
            System.out.println(printHashList.peek());
        }
    }
}

Srry if this is not a good post this is my first one

This is a much better first post than many I have seen, keep up the good work — Neilos
– Neilos, Commented Oct 21, 2014 at 4:00

Alex Suo · Accepted Answer · 2014-10-21 04:17:50Z

2

public void putToMap(String name) {
    String firstLetter = name.substring(0, 1);

    List<String> names = testMap.get(firstLetter);
    if (names == null) {
        names = new LinkedList<String> ();
        testMap.put(firstLetter, names);
    }

    names.add(name);
}

answered Oct 21, 2014 at 4:17

Alex Suo

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3 Comments

Chris Over a year ago

Right algorithm, but method should be static, and should pass the testMap as a parameter (if accessing from the main method).

Alex Suo Over a year ago

Yeah. I just don't care ... :)

etherous Over a year ago

Guava is a very fat library to include for only this one feature. I use this method

Chris · Accepted Answer · 2017-02-14 21:06:40Z

1

Alex's answer is the common (and most lightweight) solution to your problem, and the one you should choose as your answer(if he modifies it as per my comment), but just wanted to make you aware that another solution is to use a LinkedListMultiMap (which is a class in the Guava library).

LinkedListMultiMap is a simple way to deal with maps of lists (but carries an additional library overhead of the Guava library). You can add multiple values individually per key, which I believe is your desired behaviour.

edited Feb 14, 2017 at 21:06

answered Oct 21, 2014 at 4:28

Chris

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2 Comments

Alex Suo Over a year ago

I remember Google Multimap used to be quite popular. Not sure if still being used now as I have a strong tendency to choose simple solutions unless the cost to write it up doesn't justify...

Chris Over a year ago

I was simply offering another solution for reference purposes. Personally, I'd choose your solution.

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