Fix iiitv#509: Add Queue [Java] (iiitv#516)

* Add Queue [Java]

* remove unnecessary constructor

* update README.md with Queue row and remove not needed implements statement

* fix wrong class name

* add main function that demonstrates the use of the queue implementation

* fix main test example

* apply google code style to Queue.java

README.md

@@ -58,6 +58,8 @@ Community (college) maintained list of Algorithms and Data Structures implementa
 | [Linked List](https://en.wikipedia.org/wiki/Linked_list) | [:white_check_mark:](linked_list/linkedList.c.c) | | [:white_check_mark:](linked_list/LinkedList.java) | [:white_check_mark:](linked_list/linked_list.py) | [:white_check_mark:](linked_list/linked_list.go) | [:white_check_mark:](linked_list/linkedList.js) | [:white_check_mark:](linked_list/LinkedList.cs) |
 | [Stack](https://en.wikipedia.org/wiki/Stack_(abstract_data_type)) | [:white_check_mark:](stack/stack.c) | | [:white_check_mark:](stack/Stack.java) | [:white_check_mark:](stack/stack.py) | [:white_check_mark:](stack/stack.go) | [:white_check_mark:](stack/stack.js) | [:white_check_mark:](stack/Stack.cs) |
 | [Trie](https://en.wikipedia.org/wiki/Trie) | | [:white_check_mark:](trie/trie.cpp) | [:white_check_mark:](trie/Trie.java) | | [:white_check_mark:](trie/trie.go) | | [:white_check_mark:](trie/Trie.cs) | |
+| [Queue](https://en.wikipedia.org/wiki/Queue_(abstract_data_type)) | | | [:white_check_mark:](queue/Queue.java) | | | | | |
+
 
 
 

queue/Queue.java

@@ -0,0 +1,143 @@
+import java.util.NoSuchElementException;
+
+/**
+ * Implements https://docs.oracle.com/javase/8/docs/api/java/util/Queue.html
+ */
+public class Queue<T> {
+
+    private int total;
+    // Linked Lists used to store the data
+    private Node first, last;
+
+    private class Node {
+
+        private T data;
+        private Node next;
+
+        public Node(T data) {
+            this.data = data;
+        }
+    }
+
+    /**
+     * Inserts the specified element into this queue if it is possible to do so immediately without
+     * violating capacity restrictions, returning true upon success and throwing an
+     * IllegalStateException if no space is currently available.
+     *
+     * @param element the element to add
+     * @return true upon success, throwing an IllegalStateException if no space is currently
+     * available.
+     */
+    public boolean add(T element) {
+        Node newNode = new Node(element);
+
+        Node current = last;
+        last = newNode;
+
+        if (total == 0) {
+            first = last;
+        } else {
+            current.next = last;
+        }
+        total++;
+
+        return true;
+    }
+
+    /**
+     * Inserts the specified element into this queue if it is possible to do so immediately without
+     * violating capacity restrictions.
+     *
+     * @param element the element to add
+     * @return true if the element was added to this queue, else false
+     */
+    public boolean offer(T element) {
+        try {
+            return add(element);
+        } catch (Exception ex) {
+            //failed to add, return false
+            return false;
+        }
+    }
+
+    /**
+     * Retrieves and removes the head of this queue.
+     * This method differs from poll only in that it throws an exception if this queue is empty.
+     *
+     * @return the head of this queue.
+     * @throws NoSuchElementException if this queue is empty
+     */
+    public T remove() throws NoSuchElementException {
+        if (total == 0) {
+            throw new NoSuchElementException();
+        }
+        T element = first.data;
+        first = first.next;
+        total--;
+        if (total == 0) {
+            last = null;
+        }
+        return element;
+    }
+
+    /**
+     * Retrieves and removes the head of this queue, or returns null if this queue is empty.
+     * This method differs from peek only in that it throws an exception if this queue is empty.
+     *
+     * @return the head of this queue, or returns null if this queue is empty.
+     */
+    public T poll() {
+        try {
+            return remove();
+        } catch (Exception ex) {
+            //failed to remove, return null
+            return null;
+        }
+    }
+
+    /**
+     * Retrieves, but does not remove, the head of this queue.
+     *
+     * @return the head of this queue
+     * @throws NoSuchElementException if this queue is empty
+     */
+    public T element() throws NoSuchElementException {
+        if (total == 0) {
+            throw new NoSuchElementException();
+        }
+        return first.data;
+    }
+
+    /**
+     * Retrieves, but does not remove, the head of this queue, or returns null if this queue is empty.
+     *
+     * @return the head of this queue, or returns null if this queue is empty.
+     */
+    public T peek() {
+        return total == 0 ? null : first.data;
+    }
+
+    @Override
+    public String toString() {
+        StringBuilder sb = new StringBuilder();
+        Node tmp = first;
+        while (tmp != null) {
+            sb.append(tmp.data).append(", ");
+            tmp = tmp.next;
+        }
+        return sb.toString();
+    }
+
+    public static void main(String[] args) {
+        Queue<Integer> queue = new Queue<>();
+        for (int i = 1; i <= 10; i++) { // Creates a dummy queue which contains integers from 1-10
+            queue.add(i);
+        }
+
+        System.out.println("Queue :");
+
+        while (queue.peek() != null) {
+            System.out.println(queue.poll());
+        }
+    }
+}