Java PriorityQueue的用法

一、创建一个PriorityQueue实例

import java.util.PriorityQueue;

public class CreatePriorityQueueExample {
    public static void main(String[] args) {
        // Create a Priority Queue
        PriorityQueue numbers = new PriorityQueue<>();

        // Add items to a Priority Queue (ENQUEUE)
        numbers.add(750);
        numbers.add(500);
        numbers.add(900);
        numbers.add(100);

        // Remove items from the Priority Queue (DEQUEUE)
        while (!numbers.isEmpty()) {
            System.out.println(numbers.remove());
        }

    }
}

# Output
100
500
750
900

二、创建一个String类型的PriorityQueue实例

import java.util.PriorityQueue;

public class CreatePriorityQueueStringExample {
    public static void main(String[] args) {
        // Create a Priority Queue
        PriorityQueue namePriorityQueue = new PriorityQueue<>();

        // Add items to a Priority Queue (ENQUEUE)
        namePriorityQueue.add("Lisa");
        namePriorityQueue.add("Robert");
        namePriorityQueue.add("John");
        namePriorityQueue.add("Chris");
        namePriorityQueue.add("Angelina");
        namePriorityQueue.add("Joe");

        // Remove items from the Priority Queue (DEQUEUE)
        while (!namePriorityQueue.isEmpty()) {
            System.out.println(namePriorityQueue.remove());
        }

    }
}

# Output
Angelina
Chris
Joe
John
Lisa
Robert

三、使用自定义的Comparator

import java.util.Comparator;
import java.util.PriorityQueue;

public class PriorityQueueCustomComparatorExample {
    public static void main(String[] args) {
        // A custom comparator that compares two Strings by their length.
        Comparator stringLengthComparator = new Comparator() {
            @Override
            public int compare(String s1, String s2) {
                return s1.length() - s2.length();
            }
        };

        /*
        The above Comparator can also be created using lambda expression like this =>
        Comparator stringLengthComparator = (s1, s2) -> {
            return s1.length() - s2.length();
        };

        Which can be shortened even further like this =>
        Comparator stringLengthComparator = Comparator.comparingInt(String::length);
        */

        // Create a Priority Queue with a custom Comparator
        PriorityQueue namePriorityQueue = new PriorityQueue<>(stringLengthComparator);

        // Add items to a Priority Queue (ENQUEUE)
        namePriorityQueue.add("Lisa");
        namePriorityQueue.add("Robert");
        namePriorityQueue.add("John");
        namePriorityQueue.add("Chris");
        namePriorityQueue.add("Angelina");
        namePriorityQueue.add("Joe");

        // Remove items from the Priority Queue (DEQUEUE)
        while (!namePriorityQueue.isEmpty()) {
            System.out.println(namePriorityQueue.remove());
        }
    }
}

# Output
Joe
John
Lisa
Chris
Robert
Angelina

四、创建一个自定义类型的PriorityQueue实例

import java.util.Objects;
import java.util.PriorityQueue;

class Employee implements Comparable {
    private String name;
    private double salary;

    public Employee(String name, double salary) {
        this.name = name;
        this.salary = salary;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public double getSalary() {
        return salary;
    }

    public void setSalary(double salary) {
        this.salary = salary;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Employee employee = (Employee) o;
        return Double.compare(employee.salary, salary) == 0 &&
                Objects.equals(name, employee.name);
    }

    @Override
    public int hashCode() {
        return Objects.hash(name, salary);
    }

    @Override
    public String toString() {
        return "Employee{" +
                "name='" + name + '\'' +
                ", salary=" + salary +
                '}';
    }

    // Compare two employee objects by their salary
    @Override
    public int compareTo(Employee employee) {
        if(this.getSalary() > employee.getSalary()) {
            return 1;
        } else if (this.getSalary() < employee.getSalary()) {
            return -1;
        } else {
            return 0;
        }
    }
}


public class PriorityQueueUserDefinedObjectExample {
    public static void main(String[] args) {
        /*
           The requirement for a PriorityQueue of user defined objects is that

           1. Either the class should implement the Comparable interface and provide
              the implementation for the compareTo() function.
           2. Or you should provide a custom Comparator while creating the PriorityQueue.
        */

        // Create a PriorityQueue
        PriorityQueue employeePriorityQueue = new PriorityQueue<>();

        // Add items to the Priority Queue
        employeePriorityQueue.add(new Employee("Rajeev", 100000.00));
        employeePriorityQueue.add(new Employee("Chris", 145000.00));
        employeePriorityQueue.add(new Employee("Andrea", 115000.00));
        employeePriorityQueue.add(new Employee("Jack", 167000.00));

        /*
            The compareTo() method implemented in the Employee class is used to determine
            in what order the objects should be dequeued.
        */
        while (!employeePriorityQueue.isEmpty()) {
            System.out.println(employeePriorityQueue.remove());
        }
    }
}

# Output
Employee{name='Rajeev', salary=100000.0}
Employee{name='Andrea', salary=115000.0}
Employee{name='Chris', salary=145000.0}
Employee{name='Jack', salary=167000.0}