Java实现队列——顺序队列、链式队列

Java实现队列——顺序队列、链式队列

概念

先进者先出,这就是典型的“队列”。(First In, First Out,FIFO)。

我们知道,栈只支持两个基本操作:入栈push()和出栈pop()。队列跟栈非常相似,支持的操作也很有限,最基本的操作也是两个:入队和出队。入队 enqueue(),让一个数据到队列尾部;出队 dequeue(),从队列头部取一个元素。

栈和队列

所以,队列跟栈一样,也是一种操作受限的线性表数据结构。

跟栈类似,用数组实现的队列叫做顺序队列,用链表实现的队列叫做链式队列。下面我们看下如何用Java代码如何实现。

顺序队列

代码如下:

public class QueueBasedArray implements QueueInterface {
    private String[] values;// 数组
    private int capacity = 0;// 数组容量
    private int head = 0;// 头部下标
    private int tail = 0;// 尾部下标

    public QueueBasedArray(int capacity) {
        values = new String[capacity];
        this.capacity = capacity;
    }

    @Override
    public Boolean enqueue(String value) {
        // tail == capacity 表示队列末尾没有空间了
        if (tail == capacity) {
            // tail == capacity && head == 0 表示整个队列都占满了。
            if (head == 0) {
                return false;
            }
            // 数据搬移
            for (int i = head; i < tail; i++) {
                values[i - head] = values[i];
            }
            // 搬移完成后更新 head 和 tail
            tail -= head;
            head = 0;
        }
        values[tail] = value;
        tail++;
        return true;
    }

    @Override
    public String dequeue() {
        // 如果 head == tail 表示队列为空
        if (0 == tail) {
            return null;
        }
        String result = values[head];
        head++;
        return result;
    }

    @Override
    public String toString() {
        return "QueueBasedArray{" +
                "values=" + Arrays.toString(values) +
                ", capacity=" + capacity +
                ", head=" + head +
                ", tail=" + tail +
                '}';
    }
}

测试代码:

    QueueBasedArray qba = new QueueBasedArray(10);
    System.out.println(qba);

    for (int i = 0; i < 10; i++) {
        qba.enqueue("" + i + i + i);
    }
    System.out.println(qba);

    for (int i = 0; i < 10; i++) {
        qba.dequeue();
        System.out.println(qba);
    }

    for (int i = 11; i < 20; i++) {
        qba.enqueue("" + i + i + i);
    }
    System.out.println(qba);

输出结果:符合预期

QueueBasedArray{values=[null, null, null, null, null, null, null, null, null, null], capacity=10, head=0, tail=0}
QueueBasedArray{values=[000, 111, 222, 333, 444, 555, 666, 777, 888, 999], capacity=10, head=0, tail=10}
QueueBasedArray{values=[000, 111, 222, 333, 444, 555, 666, 777, 888, 999], capacity=10, head=1, tail=10}
QueueBasedArray{values=[000, 111, 222, 333, 444, 555, 666, 777, 888, 999], capacity=10, head=2, tail=10}
QueueBasedArray{values=[000, 111, 222, 333, 444, 555, 666, 777, 888, 999], capacity=10, head=3, tail=10}
QueueBasedArray{values=[000, 111, 222, 333, 444, 555, 666, 777, 888, 999], capacity=10, head=4, tail=10}
QueueBasedArray{values=[000, 111, 222, 333, 444, 555, 666, 777, 888, 999], capacity=10, head=5, tail=10}
QueueBasedArray{values=[000, 111, 222, 333, 444, 555, 666, 777, 888, 999], capacity=10, head=6, tail=10}
QueueBasedArray{values=[000, 111, 222, 333, 444, 555, 666, 777, 888, 999], capacity=10, head=7, tail=10}
QueueBasedArray{values=[000, 111, 222, 333, 444, 555, 666, 777, 888, 999], capacity=10, head=8, tail=10}
QueueBasedArray{values=[000, 111, 222, 333, 444, 555, 666, 777, 888, 999], capacity=10, head=9, tail=10}
QueueBasedArray{values=[000, 111, 222, 333, 444, 555, 666, 777, 888, 999], capacity=10, head=10, tail=10}
QueueBasedArray{values=[111111, 121212, 131313, 141414, 151515, 161616, 171717, 181818, 191919, 999], capacity=10, head=0, tail=9}

链式队列

代码如下:

public class QueueBasedLinkedList implements QueueInterface {
    private Node head;
    private Node tail;

    /**
     * 入队
     *
     * @param value
     * @return
     */
    @Override
    public Boolean enqueue(String value) {
        Node newNode = new Node(value, null);

        // tail为null,表示队列中没有数据
        if (null == tail) {
            head = newNode;
            tail = newNode;
        } else {
            tail.next = newNode;
            tail = newNode;
        }

        return true;
    }

    /**
     * 出队
     *
     * @return
     */
    @Override
    public String dequeue() {
        // head == null,表示队列为空。
        if (null == head) {
            return null;
        }

        // 获取数据
        String value = head.getItem();
        // 移除头结点,让head指向下一个结点。
        head = head.next;
        // 如果此时的头结点指向null,说明队列已空,需要将tail指向null.
        if (null == head) {
            tail = null;
        }

        return value;
    }

    @Override
    public String toString() {
        return "QueueBasedLinkedList{" +
                "head=" + head +
                ", tail=" + tail +
                '}';
    }

    private static class Node {
        String item;
        private Node next;

        public Node(String item, Node next) {
            this.item = item;
            this.next = next;
        }

        public String getItem() {
            return item;
        }

        @Override
        public String toString() {
            return "Node{" +
                    "item='" + item + '\'' +
                    ", next=" + next +
                    '}';
        }
    }
}

测试代码:

    // 空队列
    QueueBasedLinkedList qbll = new QueueBasedLinkedList();
    System.out.println("空队列 " + qbll);
    System.out.println();

    // 入队一个数据
    System.out.println("数据入队是否成功:" + qbll.enqueue("0000"));
    System.out.println("入队一个数据后:" + qbll);
    System.out.println();

    // 出队一个数据
    System.out.println("出队的数据是:" + qbll.dequeue());
    System.out.println("出队一个数据后:" + qbll);
    System.out.println();

    // 异常测试:从空队列中出队,看结果
    System.out.println("出队的数据是1:" + qbll.dequeue());
    System.out.println("出队一个数据后1:" + qbll);
    System.out.println();

    // 入队十条数据
    for (int i = 0; i < 10; i++) {
        System.out.println("数据入队是否成功:" + qbll.enqueue("" + i + i + i));
        System.out.println(qbll);
    }

输出结果:符合预期

空队列 QueueBasedLinkedList{head=null, tail=null}

数据入队是否成功:true
入队一个数据后:QueueBasedLinkedList{head=Node{item='0000', next=null}, tail=Node{item='0000', next=null}}

出队的数据是:0000
出队一个数据后:QueueBasedLinkedList{head=null, tail=null}

出队的数据是1:null
出队一个数据后1:QueueBasedLinkedList{head=null, tail=null}

数据入队是否成功:true
QueueBasedLinkedList{head=Node{item='000', next=null}, tail=Node{item='000', next=null}}
数据入队是否成功:true
QueueBasedLinkedList{head=Node{item='000', next=Node{item='111', next=null}}, tail=Node{item='111', next=null}}
数据入队是否成功:true
QueueBasedLinkedList{head=Node{item='000', next=Node{item='111', next=Node{item='222', next=null}}}, tail=Node{item='222', next=null}}
数据入队是否成功:true
QueueBasedLinkedList{head=Node{item='000', next=Node{item='111', next=Node{item='222', next=Node{item='333', next=null}}}}, tail=Node{item='333', next=null}}
数据入队是否成功:true
QueueBasedLinkedList{head=Node{item='000', next=Node{item='111', next=Node{item='222', next=Node{item='333', next=Node{item='444', next=null}}}}}, tail=Node{item='444', next=null}}
数据入队是否成功:true
QueueBasedLinkedList{head=Node{item='000', next=Node{item='111', next=Node{item='222', next=Node{item='333', next=Node{item='444', next=Node{item='555', next=null}}}}}}, tail=Node{item='555', next=null}}
数据入队是否成功:true
QueueBasedLinkedList{head=Node{item='000', next=Node{item='111', next=Node{item='222', next=Node{item='333', next=Node{item='444', next=Node{item='555', next=Node{item='666', next=null}}}}}}}, tail=Node{item='666', next=null}}
数据入队是否成功:true
QueueBasedLinkedList{head=Node{item='000', next=Node{item='111', next=Node{item='222', next=Node{item='333', next=Node{item='444', next=Node{item='555', next=Node{item='666', next=Node{item='777', next=null}}}}}}}}, tail=Node{item='777', next=null}}
数据入队是否成功:true
QueueBasedLinkedList{head=Node{item='000', next=Node{item='111', next=Node{item='222', next=Node{item='333', next=Node{item='444', next=Node{item='555', next=Node{item='666', next=Node{item='777', next=Node{item='888', next=null}}}}}}}}}, tail=Node{item='888', next=null}}
数据入队是否成功:true
QueueBasedLinkedList{head=Node{item='000', next=Node{item='111', next=Node{item='222', next=Node{item='333', next=Node{item='444', next=Node{item='555', next=Node{item='666', next=Node{item='777', next=Node{item='888', next=Node{item='999', next=null}}}}}}}}}}, tail=Node{item='999', next=null}}

完整代码请查看

项目中搜索SingleLinkedList即可。

github传送门 https://github.com/tinyvampirepudge/DataStructureDemo

gitee传送门 https://gitee.com/tinytongtong/DataStructureDemo

参考:
队列:队列在线程池等有限资源池中的应用

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