Read file into HashMap Java

Question

I'm trying to read lines from file into Arraylist. Here is my writer :

private Map<Integer, ArrayList<Integer>> motPage = 
                                             new HashMap<Integer, ArrayList<Integer>>();

private void writer() throws UnsupportedEncodingException, FileNotFoundException, IOException{

  try (Writer writer = new BufferedWriter(
                          new OutputStreamWriter(
                             new FileOutputStream("/home/kdiri/workJuno/motorRecherche/src/kemal.txt"), "utf-8"))) {
    for(Map.Entry<Integer, ArrayList<Integer>> entry : motPage.entrySet()){
      writer.write(entry.getKey() + " : " + entry.getValue() + "\n");
    }
  }
}

And this is an exemple result in the file kemal.txt :

0 : [38, 38, 38, 38, 199, 199, 199, 199, 3004, 3004, 3004, 3004, 23, 23]

My question is how can I read it these lines efficiently into Hashmap again ? Because size of file is about 500MB. Thank you in advance.

Answer 1

As JonSkeet said, you should start with something working. Find below one possible way. The snippet is kept quite verbose to show the principle.

String line = "0 : [38, 38, 38, 38, 199, 199, 199, 199, 3004, 3004, 3004, 3004, 
    23, 23]";

int firstSpace = line.indexOf(" ");
int leftSquareBracket = line.indexOf("[");
int rightSquareBracket = line.indexOf("]");

String keyString = line.substring(0, firstSpace);        
String[] valuesString = line.substring(leftSquareBracket + 1, rightSquareBracket)
    .split(", ");

int key = new Integer(keyString);
List<Integer> values = new ArrayList<>(valuesString.length);
for (String value : valuesString) {
    values.add(new Integer(value));
}

Map<Integer, List<Integer>> motPage = new HashMap<>();
motPage.put(key, values);

Btw. read ... these lines efficiently into Hashmap depends on your requirements. Efficiency could be for example:

• read speed of the huge file
• convertion speed String to Integer
• small size of the bytecode
• less object generation
• ... there could be other as well

When the snippet does not fulfil your efficiency criteria. Start to tune the part which impacts your criteria.