Java Sort map by key explanation

Java Sort map by key explanation

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java sort hashmap by value
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I recently found a great way to sort a map that contains GregorianCalendar as keys.

Map<GregorianCalendar, String> map = new HashMap<>();

Map<GregorianCalendar, String> sortedMap = map.entrySet().stream().sorted(Map.Entry.comparingByKey()).collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue,
            (oldValue, newValue) -> oldValue, LinkedHashMap::new));

Can someone help me to understand each procedure called in this command especially going on from the stream function ?


map.entrySet().stream() produces a Stream of the entries of the input Map (i.e. a Stream<Map.Entry<GregorianCalendar,String>>.

.sorted(Map.Entry.comparingByKey()) sorts the elements of this Stream by the keys of the entries (it relies on the fact that the key type - GregorianCalendar - implements Comparable).

.collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, (oldValue, newValue) -> oldValue, LinkedHashMap::new))

produces a LinkedHashMap that contains the same entries as the input Map. Since you are using LinkedHashMap, the insertion order is maintained, and since you sorted the entries of the Stream by the keys, you get a Map whose insertion order (and iteration order) is according to the order of the keys.

Of course you can get a sorted Map more easily by putting all the entries of the input Map in a TreeMap:

Map<GregorianCalendar, String> sortedMap = new TreeMap<>(map);

This has the advantage of maintaining the order even if you add new entries to the Map.

Sorting a HashMap according to keys in Java, A Computer Science portal for geeks. It contains well written, well thought and well explained computer science and programming articles, quizzes and  Sort Map by Key using Stream APIs In java 8, Map.Entry class has static method comparingByKey () to help you in sorting by keys. This method returns a Comparator that compares Map.Entry in natural order on key. Alternatively, you can pass a custom Comparator to use in sorting.

First you sort entries by Key via sorted(Map.Entry.comparingByKey())

then these are are collected to a Map (the implementation underneath uses HashMap), where the Key is of type GregorianCalendar and value is of type String. The code also supplies a merge function:

(oldValue, newValue) -> oldValue

where they keep the last value, when there is a collision, like a "last-wins" policy. So when these is a collision ( two same GregorianCalendar values ), you will always keep the second one, whatever second means here. Since your source for streaming is a HashMap that does not have a defined order, this "second" can change if you have add or remove values from your initial map.

Once those are sorted and you collect them to a HashMap (via collect), you order can break, so the collecting happens in a LinkedHashMap basically preserving that sorted order

How to sort HashMap in Java by Keys and Values, In this example we are sorting the HashMap based on the keys using the TreeMap collection class. package; import java.util.HashMap   Sorting a HashMap according to keys in Java We are given the details of marks scored by students in form of a HashMap, where name of the student is the Key and marks scored is the Value. Our task is to sort the map according to the key values i.e the names of the students in the alphabetical (lexicographical) order.

I just want to leave a generic method for such sorting because it was a bit tricky for me

private <K extends Comparable<? super K>,V> Map<K, V> sortMapByKey(Map<K, V> map) {

        return  map.entrySet().stream()
                .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, (oldValue, newValue) -> oldValue,

Sort a HashMap in Java, As we know, keys in TreeMap are sorted using their natural order. This is a good solution when we want to sort the key-value pairs by their key. 1. Sort by Key. 1.1 Uses java.util.TreeMap, it will sort the Map by keys automatically.

How to Sort HashMap in Java based on Keys and Values, There are two ways to sort HashMap by keys, first by using TreeMap and second by using LinkedHashMap. If you want to sort using TreeMap then it's simple, just   Stack Overflow Public questions and answers; Teams Private questions and answers for your team; Enterprise Private self-hosted questions and answers for your enterprise; Jobs Programming and related technical career opportunities

How to sort HashMap by values in Java 8 using Lambdas and , In the past, I have shown you how to sort a HashMap by values in Java, but that and comparingByValue() which can be used to sort a Map by key and values. on Pluralsight, which provide an in-depth explanation of new Java 8 features. Previous Next HashMap does not preserve order of element but what if you want to sort it by keys or values. In this post, we will see how to sort HashMap by keys or values. Java HashMap tutorial: HashMap in java HashMap internal working hash and indexfor method in HashMap hashcode and equals in java sort HashMap by keys and values Difference between HashMap and HashSet Difference between

How to sort a Map by keys in Java 8 - Example Tutorial, Theoretically, you cannot sort a Map because it doesn't provide any ordering guarantee. For example, when you iterate over a HashMap, you don  As we know, keys in TreeMap are sorted using their natural order. This is a good solution when we want to sort the key-value pairs by their key. So the idea is to push all the data from our HashMap into the TreeMap. For starters, let's define a HashMap and initialize it with some data: