In this tutorial, I will introduces Kotlin Priority Queue and examples that shows you how to work with it.
Related Posts:
– Kotlin Queue tutorial with examples
– Kotlin Comparator & Sort List of objects example
– Kotlin – Compare Objects & Sort List with Comparable Example
Contents
Overview
A Priority Queue is a special type of Queue in that all the elements are ordered by natural ordering or by a custom Comparator provided at the time of creation.
The front of the Priority Queue contains the least element according to the ordering, and the rear contains the greatest element.
- Remove an element from the Priority Queue => Dequeue the least element.
- Peek an element from the Priority Queue => Get the least element.
The PriorityQueue class is part of Collections framework and implements the Queue interface. This is class hierarchy:
Create Priority Queue in Kotlin
We’re gonna create several Priority Queues and add some elements to them.
Then we remove one by one item from the Queues and see how the least element is removed first followed by the next least element and so on.
– Priority Queue of Int items:
package com.bezkoder.kotlin.priorityqueue
import java.util.PriorityQueue
fun main(args: Array<String>) {
val nums: PriorityQueue<Int> = PriorityQueue<Int>()
// Add items (enqueue)
nums.add(800)
nums.add(50)
nums.add(200)
nums.add(550)
println("peek: " + nums.peek())
// Remove items (dequeue)
while (!nums.isEmpty()) {
println(nums.remove())
}
}
Output:
peek: 50
50
200
550
800
– Priority Queue of String items:
package com.bezkoder.kotlin.priorityqueue
import java.util.PriorityQueue
fun main(args: Array<String>) {
val names: PriorityQueue<String> = PriorityQueue<String>()
// Add items (enqueue)
names.add("Jack")
names.add("Alex")
names.add("Katherin")
names.add("Watson")
names.add("Jason")
names.add("Hector")
println("peek: " + names.peek())
// Remove items (dequeue)
while (!names.isEmpty()) {
println(names.remove())
}
}
Output:
peek: Alex
Alex
Hector
Jack
Jason
Katherin
Watson
Kotlin Priority Queue with Comparator
How about creating a Priority Queue of String items in which the String with the smallest length is processed first.
We need a Comparator that compares two Strings by their length.
To create Comparator, we use compareBy function.
package com.bezkoder.kotlin.priorityqueue
import java.util.PriorityQueue
fun main(args: Array<String>) {
val compareByLength: Comparator<String> = compareBy { it.length }
val namesQueueByLength = PriorityQueue<String>(compareByLength)
namesQueueByLength.add("Jack")
namesQueueByLength.add("Alex")
namesQueueByLength.add("Katherin")
namesQueueByLength.add("Watsons")
namesQueueByLength.add("Jason")
namesQueueByLength.add("Hector")
while (!namesQueueByLength.isEmpty()) {
println(namesQueueByLength.remove())
}
}
Output:
Jack
Alex
Jason
Hector
Watsons
Katherin
Kotlin Priority Queue of custom objects
Assume that we have a Customer class:
Customer.kt
package com.bezkoder.kotlin.priorityqueue
data class Customer(val name: String, val age: Int) {
}
Now look at the case that we need to create a Priority Queue of Customer objects and order them by age, then by name. How to do this?
Using Comparator
We’re gonna define a custom Comparator. One way is to create CustomerComparator class with companion object like following code:
CustomerComparator.kt
package com.bezkoder.kotlin.priorityqueue
class CustomerComparator {
companion object : Comparator<Customer> {
override fun compare(a: Customer, b: Customer): Int = when {
a.age != b.age -> a.age - b.age
else -> a.name.compareTo(b.name)
}
}
}
Now we only need to declare PriorityQueue<Customer> with CustomerComparator.
app.kt
package com.bezkoder.kotlin.priorityqueue
import java.util.PriorityQueue
fun main(args: Array<String>) {
val customers = PriorityQueue<Customer>(CustomerComparator)
customers.add(Customer("Jack", 26))
customers.add(Customer("Alex", 35))
customers.add(Customer("Katherin", 21))
customers.add(Customer("Watsons", 18))
customers.add(Customer("Jason", 32))
customers.add(Customer("Hector", 18))
while (!customers.isEmpty()) {
println(customers.remove())
}
}
Output:
Customer(name=Hector, age=18)
Customer(name=Watsons, age=18)
Customer(name=Katherin, age=21)
Customer(name=Jack, age=26)
Customer(name=Jason, age=32)
Customer(name=Alex, age=35)
Using Comparable
Another way is to implements Comparable interface and override compareTo() method.
So we rewrite our Customer class like this.
Customer.kt
package com.bezkoder.kotlin.priorityqueue
data class Customer(val name: String, val age: Int) : Comparable<Customer> {
override fun compareTo(other: Customer) = when {
age != other.age -> age - other.age
else -> name.compareTo(other.name)
}
}
Now we don’t need any Comparator when creating Priority Queue.
app.kt
package com.bezkoder.kotlin.priorityqueue
import java.util.PriorityQueue
fun main(args: Array<String>) {
val customers = PriorityQueue<Customer>()
customers.add(Customer("Jack", 26))
customers.add(Customer("Alex", 35))
customers.add(Customer("Katherin", 21))
customers.add(Customer("Watsons", 18))
customers.add(Customer("Jason", 32))
customers.add(Customer("Hector", 18))
while (!customers.isEmpty()) {
println(customers.remove())
}
}
Output:
Customer(name=Hector, age=18)
Customer(name=Watsons, age=18)
Customer(name=Katherin, age=21)
Customer(name=Jack, age=26)
Customer(name=Jason, age=32)
Customer(name=Alex, age=35)
Conclusion
Today we’ve known what is a Priority Queue, how to create and use a Kotlin Priority Queue with a Comparator, and way to create Priority Queue with custom objects.
You can also learn how to use Comparator or Comparable to sort List of objects in the articles:
– Kotlin Comparator & Sort List of objects example
– Kotlin – Compare Objects & Sort List with Comparable Example
Happy Learning! See you again.
