How to sort Map values by key in Java?

Question

I have a Map that has strings for both keys and values.

Data is like following:

"question1", "1"
"question9", "1"
"question2", "4"
"question5", "2"

I want to sort the map based on its keys. So, in the end, I will have question1, question2, question3....and so on.

Eventually, I am trying to get two strings out of this Map.

• First String: Questions ( in order 1 ..10)
• Second String: Answers (in the same order as the question)

Right now I have the following:

Iterator it = paramMap.entrySet().iterator();
while (it.hasNext()) {
    Map.Entry pairs = (Map.Entry) it.next();
    questionAnswers += pairs.getKey() + ",";
}

This gets me the questions in a string but they are not in order.

Answer 1

Short answer

Use a TreeMap. This is precisely what it's for.

If this map is passed to you and you cannot determine the type, then you can do the following:

SortedSet<String> keys = new TreeSet<>(map.keySet());
for (String key : keys) { 
   String value = map.get(key);
   // do something
}

This will iterate across the map in natural order of the keys.

---

Longer answer

Technically, you can use anything that implements SortedMap, but except for rare cases this amounts to TreeMap, just as using a Map implementation typically amounts to HashMap.

For cases where your keys are a complex type that doesn't implement Comparable or you don't want to use the natural order then TreeMap and TreeSet have additional constructors that let you pass in a Comparator:

// placed inline for the demonstration, but doesn't have to be a lambda expression
Comparator<Foo> comparator = (Foo o1, Foo o2) -> {
        ...
    }

SortedSet<Foo> keys = new TreeSet<>(comparator);
keys.addAll(map.keySet());

Remember when using a TreeMap or TreeSet that it will have different performance characteristics than HashMap or HashSet. Roughly speaking operations that find or insert an element will go from O(1) to O(Log(N)).

In a HashMap, moving from 1000 items to 10,000 doesn't really affect your time to lookup an element, but for a TreeMap the lookup time will be about 3 times slower (assuming Log₂). Moving from 1000 to 100,000 will be about 6 times slower for every element lookup.