Comparator#compareTo returns an int; while getTime is obviously long.
It would be nicer written like this:
.sort(Comparator.comparingLong(Message::getTime))
Baeldung
baeldung.com › home › java › java – powerful comparison with lambdas
Java – Powerful Comparison with Lambdas | Baeldung
January 8, 2024 - In this tutorial, we’re going to take a first look at the Lambda support in Java 8, specifically how to leverage it to write the Comparator and sort a Collection.
Top answer 1 of 7
91
Comparator#compareTo returns an int; while getTime is obviously long.
It would be nicer written like this:
.sort(Comparator.comparingLong(Message::getTime))
2 of 7
38
Lambda
The lambda can be seen as the shorthand of somewhat cumbersome anonymous class:
Java8 version:
Collections.sort(list, (o1, o2) -> o1.getTime() - o2.getTime());
Pre-Java8 version:
Collections.sort(list, new Comparator<Message>() {
@Override
public int compare(Message o1, Message o2) {
return o1.getTime() - o2.getTime();
}
});
So, every time you are confused how to write a right lambda, you may try to write a pre-lambda version, and see how it is wrong.
Application
In your specific problem, you can see the compare returns int, where your getTime returns long, which is the source of error.
You may use either method as other answer method, like:
Long.compare(o1.getTime(),o2.getTime())
Notice
- You should avoid using
-inComparator, which may causes overflow, in some cases, and crash your program.
Videos
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BeginnersBook
beginnersbook.com › 2017 › 10 › java-8-lambda-comparator-example-for-sorting-list-of-custom-objects
Java 8 Lambda Comparator example for Sorting List of Custom Objects
Comparator sortingByName = new Comparator() { @Override public int compare(Student s1, Student s2) { return s1.getName().compareTo(s2.getName()); } }; Using Lambda expression: The Java 8 equivalent code using Lambda expression would look like this:
CodeJava
codejava.net › java-core › the-java-language › java-8-lambda-collections-comparator-example
How to use Java Lambda expression for sorting a List using comparator
Comparator<Book> titleComparator = new Comparator<Book>() { public int compare(Book book1, Book book2) { return book1.getTitle().compareTo(book2.getTitle()); } };And sort the above list like this: Collections.sort(listBooks, titleComparator);Print the list: System.out.println("\nAfter sorting by title:"); System.out.println(listBooks);Output: After sorting by title: [Code Complete-42.5, Effective Java-50.0, Head First Java-38.9, Thinking in Java-30.0] Since Java 8 with Lambda expressions support, we can write a comparator in a more concise way as follows:
Mkyong
mkyong.com › home › java8 › java 8 lambda : comparator example
Java 8 Lambda : Comparator example - Mkyong.com
August 5, 2015 - //List.sort() since Java 8 listDevs.sort(new Comparator<Developer>() { @Override public int compare(Developer o1, Developer o2) { return o2.getAge() - o1.getAge(); } }); ... package com.mkyong.java8; import java.math.BigDecimal; import java.util.ArrayList; import java.util.List; public class TestSorting { public static void main(String[] args) { List<Developer> listDevs = getDevelopers(); System.out.println("Before Sort"); for (Developer developer : listDevs) { System.out.println(developer); } System.out.println("After Sort"); //lambda here!
Spring Framework Guru
springframework.guru › home › comparison and sorting with lambda
Comparison and Sorting with Lambda - Spring Framework Guru
January 5, 2021 - You can compare and sort Product objects based on various properties both with and without lambda expressions. However, when you use lambda expressions it makes your code concise and easier to read.
Java8
java8.org › sort-a-collection-using-lambda-comparators
Sort a collection with lambda comparator - Java Resources
class Person { public String name; public int age; } // TODO we should add some persons to this list List<Person> persons = new ArrayList<>(); // Using a lambda expression to implement the Comparator Collections.sort(persons, (p1, p2) -> p1.name.compareTo(p2.name)); ... Collections.sort(persons, ...
GeeksforGeeks
geeksforgeeks.org › java › java-lambda-expression-with-collections
Java Lambda Expression with Collections - GeeksforGeeks
July 11, 2025 - Expression: - Using lambda expression in place of comparator object for defining our own sorting in collections. ... import java.util.*; public class Demo { public static void main(String[] args) { ArrayList<Integer> al = new ArrayList<Integer>(); al.add(205); al.add(102); al.add(98); al.add(275); al.add(203); System.out.println("Elements of the ArrayList " + "before sorting : " + al); // using lambda expression in place of comparator object Collections.sort(al, (o1, o2) -> (o1 > o2) ?
HowToDoInJava
howtodoinjava.com › home › java 8 › java comparator with lambda
Java Comparator with Lambda (with Examples) - HowToDoInJava
February 6, 2023 - ... The Comparator interface is used to sort a collection of objects that can be compared. The object comparison can be made using Comparable interface as well, but it restricts us by comparing objects in a specific single way only.
amitph
amitph.com › home › java › comparator with java lambda expression examples
Comparator with Java Lambda Expression Examples - amitph
November 22, 2024 - Learn how to use Java Lambda Expression-based Comparator to sort collections in forward and reverse directions easily.
GitHub
gist.github.com › Mountain-Biker › 444bd628563ff07dac22de5fe2e04238
Comparable, Comparator and lambda expression #Java #Comparable #Comparator · GitHub
Before Sorting : [John, Roger, Steven] After Sorting by age : [Roger, John, Steven] Java 8 provides new ways of defining Comparators by using lambda expressions, and the comparing() static factory method.
Top answer 1 of 4
89
The cleanest way would be:
CopyArrays.sort(months, Comparator.comparingInt(String::length));
or, with a static import:
CopyArrays.sort(months, comparingInt(String::length));
However, this would work too but is more verbose:
CopyArrays.sort(months,
(String a, String b) -> a.length() - b.length());
Or shorter:
CopyArrays.sort(months, (a, b) -> a.length() - b.length());
Finally your last one:
CopyArrays.sort(months,
(String a, String b) -> { return Integer.signum(a.length() - b.length()) };
);
has the ; misplaced - it should be:
CopyArrays.sort(months,
(String a, String b) -> { return Integer.signum(a.length() - b.length()); }
);
2 of 4
3
You're looking for this:
CopyArrays.sort(months, (a, b) -> Integer.signum(a.length() - b.length()));
Reddit
reddit.com › r/javahelp › lambda expression as a comparator?
r/javahelp on Reddit: lambda expression as a comparator?
September 23, 2023 -
public static int longestStrChain(String[] words) {
Arrays.sort(words, (a, b) -> a.length() - b.length());
HashMap<String, Integer> dp = new HashMap<>();
int max_chain = 0;
for (String word : words) {
dp.put(word, 1);
for (int i = 0; i < word.length(); i++) {
String prev_word = word.substring(0, i) + word.substring(i + 1);
if (dp.containsKey(prev_word)) {
dp.put(word, Math.max(dp.get(word), dp.get(prev_word) + 1));
}
}
max_chain = Math.max(max_chain, dp.get(word));
}
return max_chain;
}Can someone help me understand how doesArrays.sort(words, (a, b) -> a.length() - b.length()); actually work? they said that it's for sorting String array, descending order. I don't even know how this
does work i use the debugging tool and seems like it's looping through the array, i was fine with the for-each-loop sorting until this came in.
for(int i = 0; i<words.length;i++) {
for (int j = i; j < words.length; j++) {
if (words[j].length()<words[i].length()){
swap(j,i,words);
}could someone please explain this to me? the Array.sort is looping too so maybe it's the same?
Top answer 1 of 2
2
the Array.sort is looping too so maybe it's the same? Yes, the Arrays.sort(...) method is basically doing the same thing as the for loop you describe; except that instead of for loops, it will use a more efficient sorting algorithm, such as merge sort . The lambda is a shorthand way of writing a Comparator class. The comparator has a single compare method, which takes two parameters, and basically says how those two object should be ordered. The sorting algorithm uses the comparator to sort the array -- basically the same as the code in your if statement's condition.
2 of 2
1
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TutorialsPoint
tutorialspoint.com › how-to-write-the-comparator-as-a-lambda-expression-in-java
How to write the comparator as a lambda expression in Java?
In the below example, we can sort the employee list by name using the Comparator interface. import java.util.ArrayList; import java.util.Collections; import java.util.List; class Employee { int id; String name; double salary; public Employee(int id, String name, double salary) { super(); this.id = id; this.name = name; this.salary = salary; } } public class LambdaComparatorTest { public static void main(String[] args) { List<Employee> list = new ArrayList<Employee>(); // Adding employees list.add(new Employee(115, "Adithya", 25000.00)); list.add(new Employee(125, "Jai", 30000.00)); list.add(ne
GeeksforGeeks
geeksforgeeks.org › java › java-comparator-interface
Java Comparator Interface - GeeksforGeeks
Final list shows sorted order by both name and age fields. Java 8 introduced a more simple way to write comparators using lambda expressions.
Published April 20, 2016
Oracle
oracle.com › java › technical details
Java 8: Lambdas, Part 2
As a matter of fact, this “I ... out of the box. On the Comparator class, a comparing method takes a function (a lambda) that extracts a comparison key out of the object and returns a Comparator that sorts based on that....
Java Guides
javaguides.net › 2020 › 04 › java-8-lambda-sort-list-in-ascending-and-descending-order.html
Java 8 Lambda - Sort List in Ascending and Descending Order | Comparator Example
September 14, 2020 - class MySort implements Comparator < Employee > { @Override public int compare(Employee o1, Employee o2) { return (int)(o2.getSalary() - o1.getSalary()); } } Note that we have done a single line of change to sort Employee by their salary in descending order. return (int)(o2.getSalary() - o1.getSalary()); In this example, we will see how to sort a list of employees by name in ascending and descending order using Lambda Expressions: package com.java.tutorials.sorting; import java.util.ArrayList; import java.util.Collections; import java.util.Comparator; import java.util.List; public class SortLi
Coderanch
coderanch.com › t › 781949 › java › Comparator-lambda
Comparator with lambda (Features new in Java 8 forum at Coderanch)
The range of arguments they are being tested on is too small to detect the error. You can read about object ordering in the Java™ Tutorials, which will tell you the correct thing to write. You can simply write Comparator<Integer> comp = Comparator.naturalOrder(); beausee Integer has a natural ordering.
Oracle
docs.oracle.com › javase › 8 › docs › api › java › util › Comparator.html
Comparator (Java Platform SE 8 )
October 20, 2025 - @FunctionalInterface public interface Comparator<T> A comparison function, which imposes a total ordering on some collection of objects. Comparators can be passed to a sort method (such as Collections.sort or Arrays.sort) to allow precise control over the sort order.
Java67
java67.com › 2014 › 11 › java-8-comparator-example-using-lambda-expression.html
How to implement Comparator and Comparable in Java with Lambda Expression & method reference? Example | Java67
To complete these tasks we need to create two custom Comparator implementations, one to sort TrainingCourse by title and the other to sort it by price. To show the stark difference in the number of lines of code you need to do this prior to Java 8 and in JDK 1.8, I have implemented that two Comparator first using Anonymous class and later using the lambda expression.