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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.
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.
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For example, let's take a look at how we can sort String keys by length in a HashMap , using the length of the Strings, and a custom comparator: Map<String, Integer> sortedMap = new TreeMap<>(new Comparator<String>() { @Override public int compare(String o1, String o2) { int lengthDifference = o1. length() - o2.
Example: Sort a map by values Inside the method, we first created a list named capitalList from the map capitals . We then use the sort() method of Collections to sort elements of the list. The sort() method takes two parameters: list to be sorted and a comparator. In our case, the comparator is a lambda expression.
No two mapped values can have equal key values. By default, a Map in C++ is sorted in increasing order based on its key.
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.
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.
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Assuming TreeMap is not good for you (and assuming you can't use generics):
List sortedKeys=new ArrayList(yourMap.keySet()); Collections.sort(sortedKeys); // Do what you need with sortedKeys. If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
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