Ugh... Hashmaps, ArrayLists, Java and me. Any help? - Overclock.net

kdbolt70 · 09-11-07

So I'm working on this program where I'm hashing a word and have an ArrayList of Integers (not ints, as ArrayList only works with objects) as the value. I'm trying to reference the ArrayList and add a new entry to it if the word is in the hashmap, otherwise add a new ArrayList to the value of the hashmap. It is not letting me do a .add to the returned ArrayList object if I do find a match in the hashmap. Here's code:

Code:

HashMap<String, ArrayList<Integer>> words = new HashMap<String, ArrayList<Integer>>();

int count = 0;
		String line = "";
		String tempWord = "";
		File srcFile = new File(file); 
		
		BufferedReader in = new BufferedReader(new FileReader(srcFile)); 
		do {
			line = in.readLine(); 
			StringTokenizer tokenizer = new StringTokenizer(line);
			tempWord = tokenizer.nextToken();
			if(words.containsKey(tempWord)){
				words.get(tempWord).add(new Integer(count));
			}
			else{
				ArrayList<Integer> list = new ArrayList<Integer>();
				list.add(new Integer(count));
				words.put(tempWord, list);
			}
			  
			count++;
		}while (in.ready());

I realize I'm not doing error checking or exception handling, I'm just trying to understand the best way to access the ArrayList returned from the .get function. the underlined portion is where it is erroring.

**bigvaL** · 09-11-07

words.get(tempWord).add(new Integer(count));

That's incorrect syntax I believe.. Look it up on javadocs and it'll give you the correct syntax.

kdbolt70 · 09-11-07

Quote:

Originally Posted by bigvaL

words.get(tempWord).add(new Integer(count));

That's incorrect syntax I believe.. Look it up on javadocs and it'll give you the correct syntax.

I realize the syntax is wrong, I just don't know how to make it right.

Code:

words.get(tempWord)

Returns an object, which I want to be of type ArrayList. Did I do the declaration correctly when I declared the HashMap? If so, doesn't it know that the type is an ArrayList (which is very similar to a vector) and therefore know I can do .add with it? Otherwise, how would I assign the returned object to a variable to be able to add to the vector? when I try:

Code:

				
ArrayList<Integer> a = (words.get(tempWord);
a.add(new Integer(count));

It says "cannot convert from object to ArrayList<Integer>"

Sorry for being nubbish at this, this is my first java coding, I'm more of a C person.Array

kdbolt70 · 09-11-07

Ok, I've gotten it down to this, and I don't know why its not letting me do a.add(i);

Code:

ArrayList<Integer> a = (ArrayList<Integer>) words.get(tempWord);
Integer i = new Integer(count);
a.add(i);
words.put(tempWord, a);

a is obviously declared as an arraylist<Integer>, and i is obviously an Integer, so why can I not add to a?

mikester · 09-11-07

Quote:

Originally Posted by kdbolt70

Ok, I've gotten it down to this, and I don't know why its not letting me do a.add(i);

Code:

ArrayList<Integer> a = (ArrayList<Integer>) words.get(tempWord);
Integer i = new Integer(count);
a.add(i);
words.put(tempWord, a);

a is obviously declared as an arraylist<Integer>, and i is obviously an Integer, so why can I not add to a?

What do you mean by "it's not letting me add"? What is it doing - compile time error, runtime exception, etc.?

Just looking at this code, I can see one glaring error - you're not handling the possibility of "tempWord" not being in the hashmap to start with. In that case, it will return "null". You should handle this case by creating a new empty array list and then adding "i", like this:

Code:

ArrayList<Integer> a = (ArrayList<Integer>) words.get(tempWord);
if (a == null)
    a = new ArrayList<Integer>();
Integer i = new Integer(count);
a.add(i);
words.put(tempWord, a);

Pasha · 09-11-07

I don't get your code.

Your calling for the object in the arraylist with get method. Then you're adding with the add method to the object that was just got. words.get(tempWord).add(new Integer(count));

Shouldn't you just add to the ArrayList like so, words.add(new Integer(count));

also here

Code:

				
ArrayList<Integer> a = (words.get(tempWord);
a.add(new Integer(count));

It says "cannot convert from object to ArrayList<Integer>"

you missed a ). it should be
ArrayList<Integer> a = (words.get(tempWord));

kdbolt70 · 09-11-07

Quote:

Originally Posted by mikester

What do you mean by "it's not letting me add"? What is it doing - compile time error, runtime exception, etc.?

Just looking at this code, I can see one glaring error - you're not handling the possibility of "tempWord" not being in the hashmap to start with. In that case, it will return "null". You should handle this case by creating a new empty array list and then adding "i", like this:

Code:

ArrayList<Integer> a = (ArrayList<Integer>) words.get(tempWord);
if (a == null)
    a = new ArrayList<Integer>();
Integer i = new Integer(count);
a.add(i);
words.put(tempWord, a);

Well, to start off, this initially was a giant error with the fact that I've never used the eclipse editor before. It gives you all these cute little warnings, but they don't go away till you recompile them, even though they appear automatically. This caused me to believe my a.add(i); was still erroring, however that is in fact correct. This is me learning my lessons the hard way.

Next, I'm sorry I didn't post the entirety of the code, I actually do a

Quote:

if(words.containsKey(tempWord)){

before I even do this add. This checks if the word is already in the hashmap. I have an else block if it is a new word added that does what you suggested, add a new arraylist with one element.

Quote:

Originally Posted by Pasha

I don't get your code.

Your calling for the object in the arraylist with get method. Then you're adding with the add method to the object that was just got. words.get(tempWord).add(new Integer(count));

Shouldn't you just add to the ArrayList like so, words.add(new Integer(count));

also here

Code:

				
ArrayList<Integer> a = (words.get(tempWord);
a.add(new Integer(count));

It says "cannot convert from object to ArrayList<Integer>"

you missed a ). it should be
ArrayList<Integer> a = (words.get(tempWord));

I think you're misunderstanding my code. Its kind of complicated, but we're going for efficiency, which is why I'm using a hashmap that maps each key to a dynamic ArrayList. The line

Code:

words.add(new Integer(count));

Would be correct if I was mapping the key to a single Integer, but I actually need it mapped to a vector (in this case, ArrayList) of Integers.

Edit: I think I see what you think is happening. the object words is not an ArrayList, it is actually the name of the hashmap. so a word.get() returns an object of type ArrayList.

mjoc13 · 09-12-07

Edit: NM i think you got it

A side note you can turn the auto "compilation on save" on, so it recompiles every time you save a class.

Go "Project->Build Automatically"

decompiled · 09-13-07

I did a project like this in my Data Structures class a while ago. Hashing is so much fun.

kdbolt70 · 09-14-07

Quote:

Originally Posted by decompiled

I did a project like this in my Data Structures class a while ago. Hashing is so much fun.

Yeah, the concept of hashing is pretty cool. I've done this type of thing a couple times in C++, but never in Java, which is what is causing me troubles. Nevertheless, I got the project done later in the evening. I'm on vacation now

		Overclock.net - Overclocking.net > Software, Programming and Coding > Coding and Programming
Ugh... Hashmaps, ArrayLists, Java and me. Any help?

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)