BAEL-5921 | Guide to Java Priority Queue (#13087)

* BAEL-5921 | Guide to Java Priority Queue

* BAEL-5921 | Adressed comments

* BAEL-5921 | Fixed test

* BAEL-5921 | Fixed test

core-java-modules/core-java-collections-4/src/main/java/com/baeldung/collections/priorityqueue/ColoredNumber.java

@@ -0,0 +1,27 @@
+package com.baeldung.collections.priorityqueue;
+
+public final class ColoredNumber {
+    private int value;
+    private String color;
+
+    public ColoredNumber(int value, String color) {
+        this.value = value;
+        this.color = color;
+    }
+
+    public int getValue() {
+        return value;
+    }
+
+    public void setValue(int value) {
+        this.value = value;
+    }
+
+    public String getColor() {
+        return color;
+    }
+
+    public void setColor(String color) {
+        this.color = color;
+    }
+}

core-java-modules/core-java-collections-4/src/main/java/com/baeldung/collections/priorityqueue/ColoredNumberComparable.java

@@ -0,0 +1,44 @@
+package com.baeldung.collections.priorityqueue;
+
+public final class ColoredNumberComparable implements Comparable<ColoredNumberComparable> {
+    private int value;
+    private String color;
+
+    public ColoredNumberComparable(int value, String color) {
+        this.value = value;
+        this.color = color;
+    }
+
+    @Override
+    public int compareTo(ColoredNumberComparable o) {
+        // (both numbers are red) or (both numbers are not red)
+        if ((this.color.equals("red") && o.color.equals("red")) ||
+                (!this.color.equals("red") && !o.color.equals("red"))) {
+            return Integer.compare(this.value, o.value);
+        }
+        // only the first number is red
+        else if (this.color.equals("red")) {
+            return -1;
+        }
+        // only the second number is red
+        else {
+            return 1;
+        }
+    }
+
+    public int getValue() {
+        return value;
+    }
+
+    public void setValue(int value) {
+        this.value = value;
+    }
+
+    public String getColor() {
+        return color;
+    }
+
+    public void setColor(String color) {
+        this.color = color;
+    }
+}

core-java-modules/core-java-collections-4/src/test/java/com/baeldung/collections/priorityqueue/PriorityQueueComparatorUnitTest.java

@@ -0,0 +1,74 @@
+package com.baeldung.collections.priorityqueue;
+
+import org.junit.jupiter.api.Test;
+
+import java.util.Collections;
+import java.util.PriorityQueue;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
+
+public class PriorityQueueComparatorUnitTest {
+
+    @Test
+    void givenIntegerQueue_defaultComparator_followsNaturalOrdering() {
+        PriorityQueue<Integer> integerQueue = new PriorityQueue<>();
+        PriorityQueue<Integer> integerQueueWithComparator = new PriorityQueue<>((Integer c1, Integer c2) -> Integer.compare(c1, c2));
+
+        integerQueueWithComparator.add(3);
+        integerQueue.add(3);
+
+        integerQueueWithComparator.add(2);
+        integerQueue.add(2);
+
+        integerQueueWithComparator.add(1);
+        integerQueue.add(1);
+
+        assertThat(integerQueue.poll()).isEqualTo(1).isEqualTo(integerQueueWithComparator.poll());
+        assertThat(integerQueue.poll()).isEqualTo(2).isEqualTo(integerQueueWithComparator.poll());
+        assertThat(integerQueue.poll()).isEqualTo(3).isEqualTo(integerQueueWithComparator.poll());
+    }
+
+    @Test
+    void givenIntegerQueue_reverseOrderComparator_followsInverseNaturalOrdering() {
+        PriorityQueue<Integer> reversedQueue = new PriorityQueue<>(Collections.reverseOrder());
+
+        reversedQueue.add(1);
+        reversedQueue.add(2);
+        reversedQueue.add(3);
+
+        assertThat(reversedQueue.poll()).isEqualTo(3);
+        assertThat(reversedQueue.poll()).isEqualTo(2);
+        assertThat(reversedQueue.poll()).isEqualTo(1);
+    }
+
+    @Test
+    void givenNotComparableQueue_classCastException() {
+        assertThatThrownBy(() -> {
+            PriorityQueue<ColoredNumber> queue = new PriorityQueue<>();
+            queue.add(new ColoredNumber(3, "red"));
+            queue.add(new ColoredNumber(2, "blue"));
+        }).isInstanceOf(ClassCastException.class);
+    }
+
+    @Test
+    void givenCustomOrderingQueue_orderIsCorrect() {
+        PriorityQueue<ColoredNumberComparable> queue = new PriorityQueue<>();
+        queue.add(new ColoredNumberComparable(10, "red"));
+        queue.add(new ColoredNumberComparable(20, "red"));
+        queue.add(new ColoredNumberComparable(1, "blue"));
+        queue.add(new ColoredNumberComparable(2, "blue"));
+
+        ColoredNumberComparable first = queue.poll();
+        assertThat(first.getColor()).isEqualTo("red");
+        assertThat(first.getValue()).isEqualTo(10);
+
+        queue.poll();
+
+        ColoredNumberComparable third = queue.poll();
+        assertThat(third.getColor()).isEqualTo("blue");
+        assertThat(third.getValue()).isEqualTo(1);
+    }
+
+}
+