How to Maintain order of insertion [duplicate]

Question

I want to add a key, value pair into a hashtable (or any other collection) but have to maintain insertion order. How can I do this?

Like I'll add 1 as key "one" as value, 2 as key and "two" as value.

The output should be:

1:one
2:two

Pompano

Answer 1

Here are the characteristic differences of some important Map implementations:

• LinkedHashMap: "with predictable iteration order [...] which is normally the order in which keys were inserted into the map (insertion-order)."
• HashMap: "makes no guarantees as to the order of the map"
• TreeMap: "is sorted according to the natural ordering of its keys, or by a Comparator"
  • i.e. it's a SortedMap

So it looks like LinkedHashMap is what you need in this case.

Here's a snippet to illustrate the differences; it also shows a common way to iterate over all entries of a Map, and how using an interface to refer to objects allow great flexibility of choice of implementation.

import java.util.*;
public class MapExample {
    public static void main(String[] args) {
        populateThenDump(new HashMap<String,Integer>());
        populateThenDump(new TreeMap<String,Integer>());
        populateThenDump(new LinkedHashMap<String,Integer>());
    }
    static void populateThenDump(Map<String,Integer> map) {
        System.out.println(map.getClass().getName());

        map.put("Zero",  0);
        map.put("One",   1);
        map.put("Two",   2);
        map.put("Three", 3);
        map.put("Four",  4);

        for (Map.Entry<String,Integer> entry : map.entrySet()) {
            System.out.println(entry.getKey() + " => " + entry.getValue());
        }
    }
}

The output of the above snippet is (as seen on ideone.com):

java.util.HashMap          // unordered, results may vary
Three => 3
Zero => 0
One => 1
Four => 4
Two => 2
java.util.TreeMap          // ordered by String keys lexicographically
Four => 4
One => 1
Three => 3
Two => 2
Zero => 0
java.util.LinkedHashMap    // insertion order
Zero => 0
One => 1
Two => 2
Three => 3
Four => 4

Related questions

• Iterate Over Map
• iterating over and removing from a map
  • If you want to modify the map while iterating, you'd need to use its Iterator.

Similar questions

• How to keep the order of elements in hashtable
• Does entrySet() in a LinkedHashMap also guarantee order?
• Java Class that implements Map and keeps insertion order?
• Ordered List Map implementation in Java

Capapie

Anybody else nervous about the doc's use of the world 'normally'? The excerpt "normally the order in which keys were inserted into the map" seems a little uncertain. Has anyone had any problems? – Adviser

Answer 2

For hash table, use LinkedHashMap class.

Parlance

Thanks for adding link. I wanted to find javadocs for 1.6 on sun/oracle site, but didn't find it. – Favors

Answer 3

You are searching for the LinkedHashMap class.

Microwave