BAEL2526 queue interface code (#6115)

* BAEL2526 queue interface code

* renamed test class to end with 'UnitTest', removed camel case from package name

core-java-collections/src/main/java/com/baeldung/queueInterface/CustomBaeldungQueue.java

@@ -0,0 +1,48 @@
+package com.baeldung.queueinterface;
+
+import java.util.AbstractQueue;
+import java.util.Iterator;
+import java.util.LinkedList;
+
+public class CustomBaeldungQueue<T> extends AbstractQueue<T> {
+
+  private LinkedList<T> elements;
+
+  public CustomBaeldungQueue() {
+    this.elements = new LinkedList<T>();
+  }
+
+  @Override
+  public Iterator<T> iterator() {
+    return elements.iterator();
+  }
+
+  @Override
+  public int size() {
+    return elements.size();
+  }
+
+  @Override
+  public boolean offer(T t) {
+    if(t == null) return false;
+    elements.add(t);
+    return true;
+  }
+
+  @Override
+  public T poll() {
+
+    Iterator<T> iter = elements.iterator();
+    T t = iter.next();
+    if(t != null){
+      iter.remove();
+      return t;
+    }
+    return null;
+  }
+
+  @Override
+  public T peek() {
+    return elements.getFirst();
+  }
+}

core-java-collections/src/test/java/com/baeldung/queueInterface/PriorityQueueUnitTest.java

@@ -0,0 +1,53 @@
+package com.baeldung.queueinterface;
+
+import org.junit.Before;
+import org.junit.Test;
+
+import java.util.PriorityQueue;
+
+import static org.junit.Assert.assertEquals;
+
+public class PriorityQueueUnitTest {
+
+
+
+  @Test
+  public void givenIntegerQueue_whenIntegersOutOfOrder_checkRetrievalOrderIsNatural() {
+
+    PriorityQueue<Integer> integerQueue = new PriorityQueue<>();
+
+    integerQueue.add(9);
+    integerQueue.add(2);
+    integerQueue.add(4);
+
+    int first = integerQueue.poll();
+    int second = integerQueue.poll();
+    int third = integerQueue.poll();
+
+    assertEquals(2, first);
+    assertEquals(4, second);
+    assertEquals(9, third);
+
+
+  }
+
+  @Test
+  public void givenStringQueue_whenStringsAddedOutOfNaturalOrder_checkRetrievalOrderNatural() {
+
+    PriorityQueue<String> stringQueue = new PriorityQueue<>();
+
+    stringQueue.add("banana");
+    stringQueue.add("apple");
+    stringQueue.add("cherry");
+
+    String first = stringQueue.poll();
+    String second = stringQueue.poll();
+    String third = stringQueue.poll();
+
+    assertEquals("apple", first);
+    assertEquals("banana", second);
+    assertEquals("cherry", third);
+
+
+  }
+}

core-java-collections/src/test/java/com/baeldung/queueinterface/CustomBaeldungQueueUnitTest.java

@@ -0,0 +1,30 @@
+package com.baeldung.queueinterface;
+
+import org.junit.Before;
+import org.junit.Test;
+
+import static org.junit.Assert.*;
+
+public class CustomBaeldungQueueUnitTest {
+
+  private CustomBaeldungQueue<Integer> customQueue;
+
+  @Before
+  public void setUp() throws Exception {
+    customQueue = new CustomBaeldungQueue<>();
+  }
+
+  @Test
+  public void givenQueueWithTwoElements_whenElementsRetrieved_checkRetrievalCorrect() {
+
+    customQueue.add(7);
+    customQueue.add(5);
+
+    int first = customQueue.poll();
+    int second = customQueue.poll();
+
+    assertEquals(7, first);
+    assertEquals(5, second);
+
+  }
+}