collection.sort algorithm in java

What is a merge sort method?
The sort () method of the collections framework uses the merge sort algorithm to sort elements of a collection. The merge sort algorithm is based on divide and conquers rule. To learn more about the merge sort, visit Merge Sort Algorithm. Let's take an example of the sort () method.
How does the sort algorithm reorder a list?
The sort algorithm reorders a List so that its elements are in ascending order according to an ordering relationship. Two forms of the operation are provided. The simple form takes a List and sorts it according to its elements' natural ordering. If you're unfamiliar with the concept of natural ordering, read the Object Ordering section.
What is a sort method in Java?
Having said that, the javadocs for different versions say this about the Collections::sort methods: The sorting algorithm is a modified mergesort (in which the merge is omitted if the highest element in the low sublist is less than the lowest element in the high sublist). This algorithm offers guaranteed n log (n) performance.
How to sort a list of objects using collections?
Learn to use Collections.sort () method to sort a list of objects using some examples. By default, the sort () method sorts a given list into ascending order (or natural order ). We can use Collections.reverseOrder () method, which returns a Comparator, for reverse sorting. 1. Sorting in Natural Order and Reverse Order

Sorting with Array

Let’s start by sorting integer arrays first using Arrays.sort() method. We’ll define the following int arrays in a @BeforejUnit method: baeldung.com

Sorting A List

Let’s now use the Collections.sort() API in java.utils.Collections – to sort a Listof Integers: The Listbefore sorting will contain the following elements: And naturally, after sorting: As mentioned in Oracle JavaDoc for Collections.Sort, it uses a modified mergesort and offers guaranteed n log(n)performance. baeldung.com

Sorting A Set

Next, let’s use Collections.sort() to sort a LinkedHashSet. We’re using the LinkedHashSetbecause it maintains insertion order. Notice how, in order to use the sort API in Collections – we’re first wrapping the set in a list: The Comparator.reverseOrder() method reverses the ordering imposed by the natural ordering. baeldung.com

Sorting Map

In this section, we’ll start looking at sorting a Map – both by keys and by values. Let’s first define the map we’ll be sorting: baeldung.com

Sorting Custom Objects

Let’s now work with a custom object: We’ll be using the following EmployeeArray for sorting example in the following sections: We can sort arrays or collections of custom objects either: 1. in the natural order (Using the ComparableInterface) or 2. in the order provided by a Comparator Interface baeldung.com

Sorting with Lambdas

Start with Java 8, we can use Lambdas to implement the ComparatorFunctional Interface. You can have a look at the Lambdas in Java 8writeup to brush up on the syntax. Let’s replace the old comparator: With the equivalent implementation, using Lambda expression: Finally, let’s write the test: As you can see, a much cleaner and more concise logic here

Using Comparator.comparing and Comparator.Thencomparing

Java 8 comes with two new APIs useful for sorting – comparing() and thenComparing() in the Comparatorinterface. These are quite handy for the chaining of multiple conditions of the Comparator. Let’s consider a scenario where we may want to compare Employee by age and then by name: In this example, Employee::getAge is the sorting key for Comparatori

Conclusion

In this article, we saw how we can apply sorting to Array, List, Set, and Map. We also saw a brief introduction about how features of Java 8 could be useful in sorting like usage of Lambdas, comparing() and thenComparing() and parallelSort(). All examples used in the article are available over on GitHub. baeldung.com

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