Add tests

build.gradle

@@ -8,3 +8,11 @@ version '1.0-SNAPSHOT'
 repositories {
     mavenCentral()
 }
+
+dependencies {
+    implementation 'org.junit.jupiter:junit-jupiter-api:5.8.1'
+}
+
+test {
+    useJUnitPlatform()
+}

src/test/java/HashSetTest.java

@@ -0,0 +1,31 @@
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+public class HashSetTest {
+
+    @Test
+    public void testIntegersNoCollisions() {
+        HashSet<Integer> set = new HashSet<>(10);
+        set.add(1);
+        set.add(2);
+        assertTrue(set.contains(1));
+        assertTrue(set.contains(2));
+        assertFalse(set.contains(0));
+        // hashCode() of an int is that int
+        assertEquals(0, set.getNumCollisions());
+    }
+
+    @Test
+    public void testIntegersWithCollisions() {
+        HashSet<Integer> set = new HashSet<>(10);
+        set.add(1);
+        set.add(2);
+        set.add(11);
+        assertTrue(set.contains(1));
+        assertTrue(set.contains(2));
+        assertTrue(set.contains(11)); // collides with 1
+        assertFalse(set.contains(0));
+        assertEquals(1, set.getNumCollisions());
+    }
+}