Sort Map by keys in Java

In this post, we will discuss various methods to sort Map in Java according to the natural ordering of its keys.

 

1. Using TreeMap

TreeMap is a Red-Black tree based implementation of Map which is sorted according to the natural ordering of its keys. We can pass an unsorted map to TreeMap constructor which will then construct a new tree map containing the same mappings as the given map but ordered according to the natural ordering of its keys.

Java

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Output:

Sorted Map by Keys :
{China=Beijing, India=New Delhi, Japan=Tokyo, USA=Washington D.C.}

 

Using Guava’s TreeMap –

Google’s Guava library also provides TreeMap implementation which we can use to create a mutable, empty TreeMap instance which is sorted according to a Comparator provided at map creation time. We can pass Guava’s Ordering.natural() for natural ordering of keys.

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2. Using LinkedHashMap

LinkedHashMap is a hash table and linked list implementation of the Map interface, with predictable iteration order which is the order in which keys were inserted into the map. We can use this property to produce a copy of a map that is sorted according to the natural ordering of its keys.

The idea is to create a List of keys present in the map and sort it. Then for every key in the sorted list, we insert key-value pair in an empty LinkedHashMap. The resultant LinkedHashMap will be sorted by keys.

Java

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Output:

Sorted Map by Keys :
{China=Beijing, India=New Delhi, Japan=Tokyo, USA=Washington D.C.}

 

3. Java 8

We can also use Java 8 Stream API to sort Map by keys. Below are the steps:

  1. Obtain stream from Set view of the mappings contained in the map.
     
  2. Sort the stream in natural order of keys using Stream.sorted() method by passing comparator returned by Map.Entry.comparingByKey().
     
  3. Collect all sorted elements in a LinkedHashMap using Stream.collect() with Collectors.toMap().

 
Please note that a Stream is a sequence of items, not a sequence of key/value pairs. So we can’t construct a Map out of a stream without specifying how to extract keys and values from it. Java 8 provides Collectors.toMap() method for this purpose. We need to use overloaded version of toMap() that returns LinkedHashMap instead of HashMap to retain the sorted order.

Java

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Output:

Sorted Map by Keys :
{China=Beijing, India=New Delhi, Japan=Tokyo, USA=Washington D.C.}

Below is another approach that doesn’t involve using collector. It uses Stream.forEachOrdered() method on the sorted stream that inserts each element of the stream into a new LinkedHashMap.

Java

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Output:

Sorted Map by Keys :
{China=Beijing, India=New Delhi, Japan=Tokyo, USA=Washington D.C.}

 
Related Post: Sort Map by Values in Java

 
References:

1. TreeMap Javadoc – Java SE 8
2. LinkedHashMap Javadoc – Java SE 8 Javadoc

 
Thanks for reading.




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