BAEL-5699 (#12674)

* BAEL-5699

First draft of the article example

* BAEL-5699

Removed unused class FifoFixedSizeQueueRemoveOldestInOffer.java
Removed unused UnitTest for the removed class
Added some tests to check count attribute

core-java-modules/core-java-collections-4/src/main/java/com/baeldung/collections/fixedsizequeues/FifoFixedSizeQueue.java

@@ -0,0 +1,79 @@
+package com.baeldung.collections.fixedsizequeues;
+
+import java.util.AbstractQueue;
+import java.util.ArrayList;
+import java.util.Iterator;
+import java.util.List;
+
+public class FifoFixedSizeQueue<E> extends AbstractQueue<E> {
+
+    /** The queued items */
+    final Object[] items;
+
+    /** Number of elements in the queue */
+    int count;
+
+    public FifoFixedSizeQueue(int capacity) {
+        super();
+
+        items = new Object[capacity];
+        count = 0;
+    }
+
+    @Override
+    public boolean offer(E e) {
+        if (e == null) {
+            throw new NullPointerException("Queue doesn't allow nulls");
+        }
+        if (count == items.length) {
+            this.poll();
+        }
+        this.items[count] = e;
+        count++;
+        return true;
+    }
+
+    @Override
+    public E poll() {
+        if (count <= 0) {
+            return null;
+        }
+        E item = (E) items[0];
+        shiftLeft();
+        count--;
+        return item;
+    }
+
+    private void shiftLeft() {
+        int i = 1;
+        while (i < items.length) {
+            if (items[i] == null) {
+                break;
+            }
+            items[i - 1] = items[i];
+            i++;
+        }
+    }
+
+    @Override
+    public E peek() {
+        if (count <= 0) {
+            return null;
+        }
+        return (E) items[0];
+    }
+
+    @Override
+    public int size() {
+        return count;
+    }
+
+    @Override
+    public Iterator<E> iterator() {
+        List<E> list = new ArrayList<>(count);
+        for (int i = 0; i < count; i++) {
+            list.add((E) items[i]);
+        }
+        return list.iterator();
+    }
+}

core-java-modules/core-java-collections-4/src/test/java/com/baeldung/collections/fixedsizequeues/FifoFixedSizeQueueUnitTest.java

@@ -0,0 +1,133 @@
+package com.baeldung.collections.fixedsizequeues;
+
+import java.util.Iterator;
+
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertNull;
+
+public class FifoFixedSizeQueueUnitTest {
+
+    @Test
+    void givenEmptyQueue_whenIterating_thenHasNextIsFalse() {
+        FifoFixedSizeQueue<String> toTest = new FifoFixedSizeQueue<>(10);
+        Iterator<String> i = toTest.iterator();
+        assertFalse(i.hasNext());
+    }
+
+    @Test
+    void givenEmptyQueue_whenOffer_thenCountIsOne() {
+        FifoFixedSizeQueue<String> toTest = new FifoFixedSizeQueue<>(10);
+        toTest.offer("1");
+        assertEquals(1, toTest.size());
+    }
+
+    @Test
+    void givenEmptyQueue_whenPeek_thenNull() {
+        FifoFixedSizeQueue<String> toTest = new FifoFixedSizeQueue<>(10);
+        String tail = toTest.peek();
+        assertNull(tail);
+    }
+
+    @Test
+    void givenEmptyQueue_whenPoll_thenNull() {
+        FifoFixedSizeQueue<String> toTest = new FifoFixedSizeQueue<>(10);
+        String dequeued = toTest.poll();
+        assertNull(dequeued);
+    }
+
+    @Test
+    void givenNonEmptyQueue_whenIterating_thenHasNextIsTrue() {
+        FifoFixedSizeQueue<String> toTest = new FifoFixedSizeQueue<>(10);
+        Iterator<String> i = toTest.iterator();
+        assertFalse(i.hasNext());
+    }
+
+    @Test
+    void givenNonEmptyQueue_whenOffer_thenCountMatchesQueueElements() {
+        FifoFixedSizeQueue<String> toTest = new FifoFixedSizeQueue<>(10);
+        toTest.offer("1");
+        toTest.offer("2");
+        assertEquals(2, toTest.size());
+    }
+
+    @Test
+    void givenNonEmptyQueue_whenPeek_thenFirstElementMustBeReturned() {
+        String expectedElement = "1";
+
+        FifoFixedSizeQueue<String> toTest = new FifoFixedSizeQueue<>(10);
+        toTest.offer(expectedElement);
+        toTest.offer("2");
+
+        assertEquals(expectedElement, toTest.peek());
+        assertEquals(expectedElement, toTest.peek());
+    }
+
+    @Test
+    void givenNonEmptyQueue_whenPoll_thenFirstElementMustBeReturnedAndRemoved() {
+        String expectedPoll = "1";
+        String expectedTailAfterPoll = "2";
+
+        FifoFixedSizeQueue<String> toTest = new FifoFixedSizeQueue<>(10);
+        toTest.offer(expectedPoll);
+        toTest.offer(expectedTailAfterPoll);
+
+        assertEquals(expectedPoll, toTest.poll());
+        assertEquals(expectedTailAfterPoll, toTest.peek());
+    }
+
+    @Test
+    void givenFullQueue_whenOffer_thenRemovedOldestFromTailAndOffer() {
+        FifoFixedSizeQueue<String> toTest = new FifoFixedSizeQueue<>(2);
+        toTest.offer("1");
+        toTest.offer("2");
+        toTest.offer("3");
+
+        Iterator<String> i = toTest.iterator();
+        assertEquals("2", i.next());
+        assertEquals("3", i.next());
+        assertFalse(i.hasNext());
+    }
+
+    @Test
+    void whenPoll_thenSizeDecreases() {
+        FifoFixedSizeQueue<String> toTest = new FifoFixedSizeQueue<>(2);
+        toTest.offer("1");
+
+        toTest.poll();
+        assertEquals(0, toTest.size());
+    }
+
+    @Test
+    void whenOffer_thenSizeIncreases() {
+        FifoFixedSizeQueue<String> toTest = new FifoFixedSizeQueue<>(2);
+        toTest.offer("1");
+        assertEquals(1, toTest.size());
+    }
+
+    @Test
+    void givenFullQueue_whenOffer_thenElementIsInsertedAndSizeEqualsCapacity() {
+        FifoFixedSizeQueue<String> toTest = new FifoFixedSizeQueue<>(2);
+        toTest.offer("1");
+        toTest.offer("2");
+        toTest.offer("3");
+        assertEquals(2, toTest.size());
+        assertEquals("2", toTest.peek());
+    }
+
+    @Test
+    void givenEmptyQueue_whenSizeRequest_thenZero() {
+        FifoFixedSizeQueue<String> toTest = new FifoFixedSizeQueue<>(2);
+        assertEquals(0, toTest.size());
+    }
+
+    @Test
+    void givenEmptyQueue_whenPoll_thenReturnNullAndSizeIsZero() {
+        FifoFixedSizeQueue<String> toTest = new FifoFixedSizeQueue<>(2);
+        assertNull(toTest.poll());
+        assertEquals(0, toTest.size());
+    }
+
+}