Correct Way to Use "Hashtable"?

Question

I am new to Hashtables, and am trying to understand how they fully work. I need to determine if a string occurs in a large file that contains around 100,000 strings, each on their own line. I have been told that a HashTable would be much more efficient than a LinkedList or ArrayList to runtime being O(n) for both.

I have looked into the HashTable Class in Java, but do not understand how exactly I would enter in every string from the file due to the "put" method needing a key, as well as the object.

I imagine I could use a Scanner to run through every string in my file, but how would I enter them into a Hashtable, and how would the contains() method be utilized in a HashTable?

Do not use `Hashtable`, instead use `Map` backed by `HashMap`. — Luiggi Mendoza, Feb 10 '14 at 22:22
Use a `HashSet` for your case, it uses hashing again, but it only need the value, not the key. Although `HashMap` performs much better than legacy `Hashtable` which is there just for backward-compatibility. — Amir Pashazadeh, Feb 10 '14 at 22:22
And here's [the documentation](http://docs.oracle.com/javase/7/docs/api/java/util/HashSet.html) for `HashSet`. — afsantos, Feb 10 '14 at 22:24
@AmirPashazadeh in fact, you need the key, not the value. The *value* entered will be used in a backing `Map`. — Luiggi Mendoza, Feb 10 '14 at 22:24
@AmirPashazadeh refer to [`Collections#newSetFromMap(Map)`](http://docs.oracle.com/javase/7/docs/api/java/util/Collections.html#newSetFromMap(java.util.Map)) — Luiggi Mendoza, Feb 10 '14 at 22:27

score 3 · Answer 1 · answered Feb 10 '14 at 22:23

3

To me, this sounds like a Usecase of HashSet. Check that out!

You put only values in, which are stored in a distinct way (no doubled values). You can then just call the contains method, to check wether your String is inside the Set.

answered Feb 10 '14 at 22:23

markusthoemmes

3,080
14
23

Mike B · Answer 2 · 2014-02-10T22:34:18.230

1

You could just put the strings in to a HashSet.

Set yourStrings = new HashSet<String>();

for (String line : yourFile) {
    yourStrings.add(line);
}

Then to check if a particular string is present:

if (yourStrings.contains("Hi!")) {
    // It's present
}
else {
    // It's not present
}

edited Feb 10 '14 at 22:34

answered Feb 10 '14 at 22:25

Mike B

5,390
2
23
45

score 1 · Answer 3 · answered Feb 10 '14 at 22:27

Hashtable is pretty old, and has been superseded by HashMap. There's also HashSet. Both use a hash table, but they have different purposes. A HashMap is used when you want to associate some sort of value with each key; for example, you could use it to look up somebody's name and get their phone number. A HashSet, however, just stores the keys without any values. You'd use that just to add names to the set, and then later check to see if the name is in the set.

As Luiggi mentioned in the comments, HashMap and HashSet are just specific implementations of Map and Set; those implementations use hash tables, but other implementations of those classes are available. You'll need to use HashMap and HashSet when constructing the table, but you should usually declare your variables simply as Map and Set, because that way you could replace the HashMap or HashSet with some other class that implements the same methods. That way you're not tied to a specific implementation.

score 1 · Answer 4 · answered Feb 10 '14 at 23:19

You need a HashSet to store each line of your file.

*You would probably need a HashMap only if you were interested in the number of occurrences of each String in the file.

import java.io.File;
import java.io.FileNotFoundException;
import java.util.HashSet;
import java.util.Scanner;
import java.util.Set;


public class MyFileUtils {

//this can be omitted, just added to increase speed 
//when requiring multiple searches in the same file, to avoid recalculations.
//Use it only if you need to search in one file ONLY
private static Set<String> stringLines = null;

/*
 * Get a HashSet of all the (distinct) lines in a file
 */
public static Set<String> getStringLinesFromFile (String filePath) {
    //this can be omitted, just added to support fast multiple calls of this function 
    if (stringLines != null) return stringLines;

    Set<String> stringLines = new HashSet<String>(); 

    Scanner scanner = null;
    try {
        scanner = new Scanner(new File(filePath));
        while (scanner.hasNextLine())
            stringLines.add(scanner.nextLine());
    } catch (FileNotFoundException e) {
        System.out.println("File does not exist");
    } finally {
        if(scanner != null)
            scanner.close();
    }
    //as the first line, this can be omitted, just added to support fast multiple calls of this function 
    MyFileUtils.stringLines = stringLines;

    return stringLines;
}

/*
 * Call this method to search for a stringLine in a file
 */
public static boolean checkIfStringExistsInFile(String filePath, String aStringLine) {
    return getStringLinesFromFile(filePath).contains(aStringLine);
}

//Test
public static void main (String args[]) {
    System.out.println(checkIfStringExistsInFile("test.txt", "Hello World"));
}

}

Correct Way to Use "Hashtable"?

4 Answers4