NoName NoName - 1 year ago 105
Java Question

LinkedList object add() function not updating values - java

I am creating an inverted index dictionary, which takes a million or so tweets from a file, stores the words from these tweets as the Keys in the dictionary (HashMap) and a pointer to a postings list (LinkedList) which contains the document ID (tweet username, date etc.) as the Value of the key.

My function stores the words as the key for the HashMap with no problem and should store an object pointer to the postings list for each occurrence of the word as the value for the key. But for some reason when I try to update the list it doesn't work. Once the entire file has been read through, the HashMap contains the keys with null Objects as their values.

The code here:

String line = scanner.nextLine();
String[] lineArr = line.split(" ");
DocID id = new DocID(lineArr[0], lineArr[1],lineArr[2]);
for(int i=3; i<lineArr.length; i++){
ListPointer list = new ListPointer();
if(dict.containsKey(lineArr[i].toLowerCase())) list = dict.get(lineArr[i]);
dict.put(lineArr[i].toLowerCase(), list);

should store an Object with a list attribute as the value, effectively acting as a pointer to a list. If a similar key exists in the table, the value is obtained and the list attribute of that value should be updated and set again as the value for that key.

I know using a LinkedList as the value of the HashMap rather than using an object containing an inherent list would be better, but we were instructed that the postings list should be stored separately and shouldn't be an attribute of the dictionary class, and the dictionary should just contain a pointer to its relevant postings list.

So far these are the objects with their members:

public static HashMap<String, ListPointer> dict;

public static class DocID{
public String userID;
public String date;
public String time;

public DocID(String dte, String tme, String id){
this.userID = id; = dte;
this.time = tme;

public static class ListPointer{
public static LinkedList<DocID> postings;
public ListPointer(){
postings = new LinkedList<DocID>();

I could understand if it was an overwriting error, but no, the value of each key in the HashMap upon complete read through of the file is null and I have no idea why this could be?

Answer Source

Your postings member shouldn't be static. You have a single instance shared across all ListPointer instances, and you overwrite it with an empty LinkedList<DocID> each time the ListPointer constructor is invoked.


public static LinkedList<DocID> postings;


public LinkedList<DocID> postings;


You have another problem in the retrieval from the Map :


if(dict.containsKey(lineArr[i].toLowerCase())) list = dict.get(lineArr[i]);


if(dict.containsKey(lineArr[i].toLowerCase())) list = dict.get(lineArr[i].toLowerCase());

If you are passing a lower case String to containsKey, you must pass the same lower case String to get. Otherwise get will return null if the original key wasn't lower case.

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