elements of HashMap are in the wrong order [duplicate]

Question

I need to read two columns (both String) from a file and keep the values of the first column in a HashMap where the Integer is the counter.

For example if the file I am reading is

Apple Fruit
PC    Device
Pen   Tool
...

and the code is

    String line="";
    int counter=1;
BufferedReader reader = new BufferedReader(new InputStreamReader(new FileInputStream("test.txt"),"Unicode"));
    while ((line = reader.readLine()) != null)
    {
        String[] words;
        words= st.split(" ");
            tokens.put(counter, words[0]);
        counter+=1;
    }

The problem is when I print The HashMap values, I found the values are in different order of that in the origianl file

        for (Map.Entry<Integer, String> token:tokens.entrySet())
    {
        System.out.println(token.getKey() + token.getValue());
    }

I got the following

1   Apple
3   Pen
4   whatever
2   ..etc

I do not know what is the problem?! can you please help me with that

Answer 1

As the documentation clearly states, HashMaps are unordered.
The enumeration order is determined by the hascodes of the keys.

If you want to preserve insertion order when enumerating the map, use LinkedHashMap.
If you want enumeration order to follow the natural ordering of the keys, use TreeMap.

Answer 2

HashMap is not ordered. You don't have any control over what order the items appear in. If you want an ordered map, you can use, for example, TreeMap.

EDIT: thanks to those that brought this up: TreeMap will keep items in the natural sorting order (i.e. in your case in alphabetical order). LinkedHashMap will preserve the order of insertion.

Answer 3

Use LinkedHashMap , if you want insertion order back. By default HashMap's iterator doesn't guarantee insertion order.

Answer 4

HashMap is not an ordered Collection. (It is not even a Collection) It is more like a dictionary.

If you want to perserve the order of the items you can either use a TreeMap or a LinkedHashMap.

The difference between these is that TreeMap keeps them ordered by their natural sort order while LinkedHashMap keeps the insertion order.

In most cases where you would like to have something like with using a List you can use the LinkedHashMap.

TreeMap uses a red-black tree for its internal implementation while LinkedHashMap uses doubly linked list.

My suggestion is that you should refer to the official documentation. There you will find thorough explanations.

Additionally the idiomatic way to iterate over Maps is this:

for(Integer key : map.keySet()) {
    String myString = map.get(key);
    // ...
}