如何使用数组和泛型在Java实现队列？

队列是遵循先进先出规则(先进先出)的线性数据结构。我们不仅可以为整数实现 Queue ，还可以为字符串、浮点数或字符实现 Queue 。队列中有 5 个主要操作：

1. 入队() 将元素 x 添加到队列的前面
2. 出队() 移除队列的最后一个元素
3. 正面() 返回前面的元素
4. 后部() 返回后部元素
5. 空的() 返回是否 队列是否为空

Note: Time complexity is of order 1 for all operations

执行：

例子

Java

// Java Program to Implement Queue using Array and Generics
 
// Importing input output classes
import java.io.*;
// Importing all utility classes
import java.util.*;
 
// Class 1
// Helper Class(user defined - generic queue class)
class queue {
    // front and rear variables are initially initiated to
    // -1 pointing to no element that control queue
    int front = -1, rear = -1;
 
    // Creating an object of ArratList class of T type
    ArrayList A = new ArrayList<>();
 
    // Method 1
    // Returns value of element at front
    T front()
    {
        // If it is not pointing to any element in queue
        if (front == -1)
 
            return null;
 
        // else return the front element
        return A.get(front);
    }
    // Method 2
    // Returns value of element at rear
    T rear()
    {
        // If it is not pointing to any element in queue
        if (rear == -1)
            return null;
        return A.get(rear);
    }
    // Method 3
    // Inserts element at the front of queue
    void enqueue(T X)
    {
        // If queue is empty
        if (this.empty()) {
            front = 0;
            rear = 0;
            A.add(X);
        }
 
        // If queue is not empty
        else {
            front++;
            if (A.size() > front) {
 
                // Mov front pointer to next
                A.set(front, X);
            }
            else
 
                // Add element to the queue
                A.add(X);
        }
    }
    // Method 4
    // Deletes elements from the rear from queue
    void dequeue()
    {
        // if queue doesn't have any elements
        if (this.empty())
 
            // Display message when queue is already empty
            System.out.println("Queue is already empty");
 
        // If queue has only one element
        else if (front == rear) {
 
            // Both are pointing to same element
            front = rear = -1;
        }
 
        // If queue has more than one element
        else {
 
            // Incrememnt the rear
            rear++;
        }
    }
 
    // Method 5
    // Checks whether the queue is empty
    boolean empty()
    {
        // Both are initialized to same value
        // as assigned at declaration means no queue made
        if (front == -1 && rear == -1)
            return true;
        return false;
    }
    // Method 6
    // Print the queue
 
    // @Override
    public String toString()
    {
        if (this.empty())
            return "";
 
        String Ans = "";
 
        for (int i = rear; i < front; i++) {
            Ans += String.valueOf(A.get(i)) + "->";
        }
 
        Ans += String.valueOf(A.get(front));
 
        return Ans;
    }
}
 
// Class 2
// Main class
class GFG {
 
    // Main driver method
    public static void main(String args[])
    {
        // Case 1 : Integer Queue
 
        // Creating object of queue Class (user defined)
        // Declaring object of integer type
        queue q1 = new queue<>();
 
        // Pushing elements to the integer object created
        // Custom input integer entries
        q1.enqueue(5);
        q1.enqueue(10);
        q1.enqueue(20);
 
        // Print the queue after inserting integer elements
        System.out.println(
            "q1 after enqueue of 3 elements:\n" + q1);
        q1.dequeue();
        System.out.println("q1 after dequeue :\n" + q1);
 
        // Case 2 : String Queue
 
        // Creating object of queue Class (user defined)
        // Declaring object of string type
        queue q2 = new queue<>();
 
        // Pushing elements to the String object created
        // Custom input string entries
        q2.enqueue("hello");
        q2.enqueue("world");
        q2.enqueue("GFG");
 
        // Print the queue after inserting string elements
        System.out.println(
            "\nq2 after enqueue of 3 elements:\n" + q2);
 
        // Printing front and rear of the above queue
        System.out.println("q2 front = " + q2.front()
                           + ", q2 rear = " + q2.rear());
 
        // Case 3 : Float Queue
 
        // Creating object of queue Class (user defined)
        // Declaring object of float type
        queue q3 = new queue<>();
 
        // Display message only
        System.out.println(
            "\nCreated new Float type queue q3...");
 
        // Display whether queue is empty or not
        // using the empty() method
        System.out.println(
            "Checking if queue is empty or not :\n"
            + q3.empty());
    }
}

输出

q1 after enqueue of 3 elements:
5->10->20
q1 after dequeue :
10->20

q2 after enqueue of 3 elements:
hello->world->GFG
q2 front = GFG, q2 rear = hello

Created new Float type queue q3...
Checking if queue is empty or not :
true

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