priority queue java comparator lambda

Java Lambdas: Comparator with PriorityQueue

stackoverflow.com › questions › 68404736 › java-lambdas-comparator-with-priorityqueue

Your two ways to define the Comparator are equivalent. Your lambda version actually expands to the other version after compiling, because Comparator is a functional interface.

Here's a third shorter way to define your Comparator, using a static factory method on the Comparator interface:

PriorityQueue<Integer> heap3 = new PriorityQueue<>(Comparator.reverseOrder());

Answer from marstran on Stack Overflow

Stack Overflow

stackoverflow.com › questions › 68404736 › java-lambdas-comparator-with-priorityqueue

Java Lambdas: Comparator with PriorityQueue - Stack Overflow

Top answer

1 of 1

Your two ways to define the Comparator are equivalent. Your lambda version actually expands to the other version after compiling, because Comparator is a functional interface.

Here's a third shorter way to define your Comparator, using a static factory method on the Comparator interface:

PriorityQueue<Integer> heap3 = new PriorityQueue<>(Comparator.reverseOrder());

GeeksforGeeks

geeksforgeeks.org › java › priority-queue-of-pair-in-java-with-examples

Priority Queue of Pair in Java with Examples - GeeksforGeeks

July 23, 2025 - PriorityQueue<Pair<K, V>> = new PriorityQueue<>(Comparator.comparing(Pair :: getKey)) ... Since Pair<K, V> class was the part of JavaFX and JavaFX was removed from JDK since JDK 11. So Pairs can be used till JDK 10. The below source code might not run on most of the online IDEs, better to try it on offline software. ... import java.util.*; import java.io.*; import javafx.util.Pair; class GFG { public static void main(String[] args) { // Priority Queue implementing min heap of Pairs // Creating instance of PriorityQueue by passing // Lambda expression as a constructor parameter.

Discussions

java 8 - Comparator as lambda expression in PriorityQueue - Stack Overflow

PriorityQueue constructor: PriorityQueue(int initialCapacity, Comparator comparator) 2021-07-20T03:59:32.23Z+00:00 ... Find the answer to your question by asking. Ask question ... See similar questions with these tags. ... New site design and philosophy for Stack Overflow: Starting February 24, 2026... 511 What is the point of the diamond operator (<>) in Java... More on stackoverflow.com

stackoverflow.com

lambda function doesn't work in priority_queue in c++ (when using [&]) - Stack Overflow

explicit priority_queue(const Compare& x = Compare(), Container&& y = Container()); As you can see, this involves default-constructing an object of type Compare. Prior to C++20, lambdas are not default constructible. More on stackoverflow.com

stackoverflow.com

PriorityQueue with lambda expression not working as expected?

Can you first explain what are you trying to do and why? What's the expected output? EDIT: sorry, I didn't realize there is a link. Just a moment. EDIT2: So you are supposed to kind of "shuffle" the string. On lines 12-14, does this code work as you'd expect? Edit3: I ran your code with the input you gave and I printed the queue. Looks exactly what it should. Why not? More on reddit.com

r/learnjava

December 27, 2021

How do you make Java's PriorityQueue custom sort?

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r/javahelp

November 10, 2021

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reddit.com › r/learnjava › priorityqueue with lambda expression not working as expected?

r/learnjava on Reddit: PriorityQueue with lambda expression not working as expected?

December 27, 2021 -

I am working on this LeetCode problem and I'm using a PriorityQueue to maintain a heap based on the respective value in a HashMap like this

    Map<Character, Integer> countMap = new HashMap<>();
    PriorityQueue<Character> heap = new PriorityQueue<>((c1, c2) -> countMap.get(c2)-countMap.get(c1));

However I noticed in my code that when I modify values in countMap, the heap is not reordering itself using the Lambda expression (for example the input "bfrbs") and I can't understand why? Is there something I am misunderstanding? Here is what my full code looks like:

public String reorganizeString(String s) {
    StringBuilder sb = new StringBuilder();
    Map<Character, Integer> countMap = new HashMap<>();
    PriorityQueue<Character> heap = new PriorityQueue<>((c1, c2) -> countMap.get(c2)-countMap.get(c1));
    
    // O(N)
    for(char c : s.toCharArray()){
        countMap.put(c, countMap.getOrDefault(c, 0) + 1);
    }
    
    // O(N*Log(N))
    for(char c : countMap.keySet()){
        heap.add(c);
    }
    
    // O(N*Log(N)) ?
    while(countMap.get(heap.peek()) > 0){           
        char c1 = heap.remove();

        if(countMap.get(heap.peek()) == 0){
            if(countMap.get(c1) > 1) return "";
            sb.append(c1);
            break;
        }
        
        char c2 = heap.peek();
        
        if(sb.length() > 0 && sb.charAt(sb.length()-1) == c1){
            sb.append(c2);
            sb.append(c1);
        } else {
            sb.append(c1);
            sb.append(c2);                
        }
        
        countMap.put(c1, countMap.get(c1)-1);
        countMap.put(c2, countMap.get(c2)-1);
        heap.add(c1);
    }
    
    // O(N)
    return sb.toString();
}

Top answer

1 of 3

2 of 3

The priority queue will not just update by itself when you modify an external object, and by doing it, you can break the heap rules.

Oracle

docs.oracle.com › javase › 8 › docs › api › java › util › PriorityQueue.html

PriorityQueue (Java Platform SE 8 )

3 weeks ago - Creates a PriorityQueue containing the elements in the specified sorted set. This priority queue will be ordered according to the same ordering as the given sorted set. ... ClassCastException - if elements of the specified sorted set cannot be compared to one another according to the sorted ...

CalliCoder

callicoder.com › java-priority-queue

Java Priority Queue Tutorial with Examples | CalliCoder

February 18, 2022 - Comparator<String> stringLengt... created using lambda expression like this => Comparator<String> stringLengthComparator = (s1, s2) -> { return s1.length() - s2.length(); }; Which can be shortened even further like this => ...

HackMD

hackmd.io › @csci0200 › Hk-2l0knd

Lab 6: Priority Queues and Comparators - HackMD

There is a built-in `compare` method on `Integers` that returns a negative number when the first number is smaller than the second, a positive number when the first is larger, and 0 if they are equal.

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.

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Scaler

scaler.com › home › topics › java priorityqueue comparator

Java PriorityQueue Comparator - Priority queue

May 4, 2023 - It stores the data with the maximum priority at the top so you can extract your data according to its relevant importance. It uses a function that is used for comparison to decide the ordering of the elements and this function is called the ...

GeeksforGeeks

geeksforgeeks.org › java › priority-queue-in-reverse-order-in-java

Priority Queue in Reverse Order in Java - GeeksforGeeks

July 23, 2025 - Lambda expression since Java 8 has come to use, lambda function names its input parameters a and b and returns (b-a), which is basically what the int comparator class does except it returns a-b. ... // Java program to demonstrate the // working ...

Stack Overflow

stackoverflow.com › questions › 68449329 › comparator-as-lambda-expression-in-priorityqueue

java 8 - Comparator as lambda expression in PriorityQueue - Stack Overflow

error: incompatible types: incompatible parameter types in lambda expression · PriorityQueue<int[]> pq = new PriorityQueue(10, ( (int[] a, int[] b) -> a[0] - b[0] )); java-8 · comparator · priority-queue · Share · Improve this question · Follow · edited Jul 20, 2021 at 3:58 ·

ConcretePage

concretepage.com › java › java-priorityqueue-comparator

Java PriorityQueue with Comparator

December 21, 2022 - public PriorityQueue(int initialCapacity, Comparator<? super E> comparator) Creates a PriorityQueue using specified initial capacity and the comparator to order the elements. If comparator is null, elements will be ordered in natural order.

YouTube

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Understanding Java Lambdas: Comparator with PriorityQueue - YouTube

02:13

Dive into the world of `Java Lambdas` and explore how to implement comparators in `PriorityQueue`. Learn the similarities and differences of using lambda exp...

Published October 11, 2025

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HappyCoders.eu

happycoders.eu › algorithms › priorityqueue-java

Java PriorityQueue (+ Code Examples)

November 27, 2024 - In the last part of this tutorial series, I will show you how to implement a priority queue using a heap yourself. With the java.util.PriorityQueue class, the dequeue order results either from the elements' natural order¹ or according to a comparator¹ passed to the constructor.

OpenJDK

cr.openjdk.org › ~briangoetz › lambda › lambda-state-final.html

State of the Lambda

As an example of how the second rule comes into play, say the Collection and List interfaces provided different defaults for removeAll, and Queue inherits the default method from Collection; in the following implements clause, the List declaration would have priority over the Collection declaration inherited by Queue:

Stack Overflow

stackoverflow.com › questions › 50635728 › lambda-function-doesnt-work-in-priority-queue-in-c-when-using › 50635995

lambda function doesn't work in priority_queue in c++ (when using [&]) - Stack Overflow