Add shuffle test

src/Card.java

@@ -8,7 +8,6 @@ public class Card {
         DIAMONDS
     };
 
-
     public Card(int number, Suit suit) {
         this.number = number;
         this.suit = suit;

src/Pile.java

@@ -27,13 +27,17 @@ public class Pile extends ArrayList<Card> {
      * Fisher-Yates shuffle()
      * https://stackoverflow.com/questions/1519736/random-shuffling-of-an-array
      */
-    public void shuffle() {
-        Random random = new Random();
+    public void shuffle(Random random) {
         for (int i = this.size() - 1; i > 0; i--) {
             Collections.swap(this, i, random.nextInt(i + 1));
         }
     }
 
+    public void shuffle() {
+        Random random = new Random();
+        shuffle(random);
+    }
+
     public boolean uniqueMaximum(Card max, int maxIndex) {
         for (int i = 0; i < this.size(); i++) {
             if (maxIndex != i && max.number == this.get(i).number) {

src/PileTest.java

@@ -0,0 +1,44 @@
+import org.junit.jupiter.api.Test;
+
+import java.util.Random;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+class PileTest {
+
+    @Test
+    void shuffleEmptyPile() {
+        Pile pile = new Pile();
+        pile.shuffle();
+        assertEquals(0, pile.size());
+    }
+
+    @Test
+    void shuffleOneCard() {
+        Pile pile = new Pile();
+        pile.add(new Card(1, Card.Suit.CLUBS));
+        pile.shuffle();
+        assertEquals(1, pile.size());
+        assertEquals(1, pile.size());
+    }
+
+    @Test
+    void shuffleCards() {
+        Pile pile = new Pile();
+        pile.add(new Card(1, Card.Suit.CLUBS));
+        pile.add(new Card(2, Card.Suit.DIAMONDS));
+        pile.add(new Card(3, Card.Suit.HEARTS));
+        pile.add(new Card(4, Card.Suit.SPADES));
+        Random random = new Random(1);
+        pile.shuffle(random);
+        assertEquals(4, pile.get(0).number);
+        assertEquals(Card.Suit.SPADES, pile.get(0).suit);
+        assertEquals(1, pile.get(1).number);
+        assertEquals(Card.Suit.CLUBS, pile.get(1).suit);
+        assertEquals(2, pile.get(2).number);
+        assertEquals(Card.Suit.DIAMONDS, pile.get(2).suit);
+        assertEquals(3, pile.get(3).number);
+        assertEquals(Card.Suit.HEARTS, pile.get(3).suit);
+        assertEquals(4, pile.size());
+    }
+}