Fixes #647 use typed empty lists

exercises/custom-set/src/example/java/CustomSet.java

     private Set<T> set;
 
     public CustomSet() {
-        this(Collections.EMPTY_LIST);
+        this(Collections.emptyList());
     }
 
     public CustomSet(Collection<T> data) {

exercises/custom-set/src/test/java/CustomSetTest.java

     @Test
     public void setsWithNoElementsAreEmpty() {
         final boolean actual
-                = new CustomSet<>(Collections.EMPTY_LIST)
+                = new CustomSet<>(Collections.emptyList())
                         .isEmpty();
         assertTrue(actual);
     }
     @Test
     public void nothingIsContainedInAnEmptySet() {
         final boolean actual
-                = new CustomSet<>(Collections.EMPTY_LIST)
+                = new CustomSet<>(Collections.emptyList())
                         .contains(1);
 
         assertFalse(actual);
     @Test
     public void emptySetIsASubsetOfAnotherEmptySet() {
         final boolean actual
-                = new CustomSet<>(Collections.EMPTY_LIST)
+                = new CustomSet<>(Collections.emptyList())
                         .isSubset(
-                                new CustomSet<>(Collections.EMPTY_LIST)
+                                new CustomSet<>(Collections.emptyList())
                         );
 
         assertTrue(actual);
         final boolean actual
                 = new CustomSet<>(Collections.singletonList(1))
                         .isSubset(
-                                new CustomSet<>(Collections.EMPTY_LIST)
+                                new CustomSet<>(Collections.emptyList())
                         );
 
         assertTrue(actual);
     @Test
     public void nonEmptySetIsNotASubsetOfEmptySet() {
         final boolean actual
-                = new CustomSet<>(Collections.EMPTY_LIST)
+                = new CustomSet<>(Collections.emptyList())
                         .isSubset(
                                 new CustomSet<>(Collections.singletonList(1))
                         );
     @Test
     public void theEmptySetIsDisjointWithItself() {
         final boolean actual
-                = new CustomSet<>(Collections.EMPTY_LIST)
+                = new CustomSet<>(Collections.emptyList())
                         .isDisjoint(
-                                new CustomSet<>(Collections.EMPTY_LIST)
+                                new CustomSet<>(Collections.emptyList())
                         );
 
         assertTrue(actual);
     @Test
     public void emptySetIsDisjointWithNonEmptySet() {
         final boolean actual
-                = new CustomSet<>(Collections.EMPTY_LIST)
+                = new CustomSet<>(Collections.emptyList())
                         .isDisjoint(
                                 new CustomSet<>(Collections.singletonList(1))
                         );
         final boolean actual
                 = new CustomSet<>(Collections.singletonList(1))
                         .isDisjoint(
-                                new CustomSet<>(Collections.EMPTY_LIST)
+                                new CustomSet<>(Collections.emptyList())
                         );
 
         assertTrue(actual);
     @Test
     public void emptySetsAreEqual() {
         final boolean actual
-                = new CustomSet<>(Collections.EMPTY_LIST)
+                = new CustomSet<>(Collections.emptyList())
                         .equals(
-                                new CustomSet<>(Collections.EMPTY_LIST)
+                                new CustomSet<>(Collections.emptyList())
                         );
 
         assertTrue(actual);
     @Test
     public void emptySetIsNotEqualToNonEmptySet() {
         final boolean actual
-                = new CustomSet<>(Collections.EMPTY_LIST)
+                = new CustomSet<>(Collections.emptyList())
                         .equals(
                                 new CustomSet<>(Arrays.asList(1, 2, 3))
                         );
         final boolean actual
                 = new CustomSet<>(Arrays.asList(1, 2, 3))
                         .equals(
-                                new CustomSet<>(Collections.EMPTY_LIST)
+                                new CustomSet<>(Collections.emptyList())
                         );
 
         assertFalse(actual);
                         Collections.unmodifiableList(Collections.singletonList(element))
                 );
         final CustomSet<Integer> actual
-                = new CustomSet<>(Collections.EMPTY_LIST);
+                = new CustomSet<>(Collections.emptyList());
 
         actual.add(element);
 
     @Test
     public void intersectionOfTwoEmptySetsIsAnEmptySet() {
         final CustomSet<Integer> actual
-                = new CustomSet<>(Collections.EMPTY_LIST)
+                = new CustomSet<Integer>(Collections.emptyList())
                         .getIntersection(
-                                new CustomSet<>(Collections.EMPTY_LIST)
+                                new CustomSet<>(Collections.emptyList())
                         );
 
         assertNotNull(actual);
     @Test
     public void intersectionOfAnEmptySetAndNonEmptySetIsAnEmptySet() {
         final CustomSet<Integer> actual
-                = new CustomSet<>(Collections.EMPTY_LIST)
+                = new CustomSet<Integer>(Collections.emptyList())
                         .getIntersection(
                                 new CustomSet<>(Arrays.asList(3, 2, 5))
                         );
         final CustomSet<Integer> actual
                 = new CustomSet<>(Arrays.asList(1, 2, 3, 4))
                         .getIntersection(
-                                new CustomSet<>(Collections.EMPTY_LIST)
+                                new CustomSet<>(Collections.emptyList())
                         );
 
         assertNotNull(actual);
     @Test
     public void differenceOfTwoEmptySetsIsAnEmptySet() {
         final CustomSet<Integer> actual
-                = new CustomSet<>(Collections.EMPTY_LIST)
+                = new CustomSet<Integer>(Collections.emptyList())
                         .getDifference(
-                                new CustomSet<>(Collections.EMPTY_LIST)
+                                new CustomSet<>(Collections.emptyList())
                         );
 
         assertNotNull(actual);
     @Test
     public void differenceOfAnEmptySetAndNonEmptySetIsAnEmptySet() {
         final CustomSet<Integer> actual
-                = new CustomSet<>(Collections.EMPTY_LIST)
+                = new CustomSet<Integer>(Collections.emptyList())
                         .getDifference(
                                 new CustomSet<>(Arrays.asList(3, 2, 5))
                         );
         final CustomSet<Integer> actual
                 = new CustomSet<>(Arrays.asList(1, 2, 3, 4))
                         .getDifference(
-                                new CustomSet<>(Collections.EMPTY_LIST)
+                                new CustomSet<>(Collections.emptyList())
                         );
 
         assertNotNull(actual);
     @Test
     public void unionOfTwoEmptySetsIsAnEmptySet() {
         final CustomSet<Integer> actual
-                = new CustomSet<>(Collections.EMPTY_LIST)
+                = new CustomSet<Integer>(Collections.emptyList())
                         .getUnion(
-                                new CustomSet<>(Collections.EMPTY_LIST)
+                                new CustomSet<>(Collections.emptyList())
                         );
 
         assertNotNull(actual);
                         Collections.unmodifiableList(Collections.singletonList(2))
                 );
         final CustomSet<Integer> actual
-                = new CustomSet<>(Collections.EMPTY_LIST)
+                = new CustomSet<Integer>(Collections.emptyList())
                         .getUnion(
                                 new CustomSet<>(Collections.singletonList(2))
                         );
         final CustomSet<Integer> actual
                 = new CustomSet<>(Arrays.asList(1, 3))
                         .getUnion(
-                                new CustomSet<>(Collections.EMPTY_LIST)
+                                new CustomSet<>(Collections.emptyList())
                         );
 
         assertNotNull(actual);