binary-search: use generics

The binary-search tests force the practitioner to generify the class
but only one type (integer) was actually being used in the tests.
This should be changed so that more than one type is actually used, to
make it more clear that generics are necessary and to demonstrate how
generics is used.

As discussed in #230, move linked-list to one of the last difficulty 6
exercises, so that binary-seach is the first exercise that introduces
generics.

Add a hint to the binary-search exercise to explain generics as this
is now the first exercise to require it.

Fixes #230.

config.json

       "uuid": "76d28d97-75d3-47eb-bb77-3d347b76f1b6"
     },
     {
-      "core": true,
-      "difficulty": 6,
-      "slug": "linked-list",
-      "topics": null,
-      "unlocked_by": null,
-      "uuid": "7ba5084d-3b75-4406-a0d7-87c26387f9c0"
-    },
-    {
       "core": false,
       "difficulty": 6,
       "slug": "grade-school",
       "uuid": "4b3f7771-c642-4278-a3d9-2fb958f26361"
     },
     {
+      "core": true,
+      "difficulty": 6,
+      "slug": "linked-list",
+      "topics": null,
+      "unlocked_by": null,
+      "uuid": "7ba5084d-3b75-4406-a0d7-87c26387f9c0"
+    },
+    {
       "core": false,
       "difficulty": 7,
       "slug": "anagram",

exercises/binary-search/.meta/hints.md

+This exercise introduces [generics](https://docs.oracle.com/javase/tutorial/java/generics/index.html).
+To make the tests pass you need to construct your class such that it accepts any type of input, e.g. `Integer` or `String`.
+
+Generics are useful because they allow you to write more general and reusable code.
+The Java [List](https://docs.oracle.com/javase/8/docs/api/java/util/List.html) and [Map](https://docs.oracle.com/javase/8/docs/api/java/util/Map.html) implementations are both examples of classes that use generics.
+By using them you can construct a `List` containing `Integers` or a list containing `Strings` or any other type.
+
+There are a few constraints on the types used in generics.
+One of them is that once you've constructed a `List` containing `Integers`, you can't put `Strings` into it.
+You have to specify which type you want to put into the class when you construct it, and that instance can then only be used with that type.
+
+For example you could construct a list of `Integers`:
+
+`List<Integer> someList = new LinkedList();`
+
+Now `someList` can only contain `Integers`. You could also do:
+
+`List<String> someOtherList = new LinkedList()`
+
+Now `someOtherList` can only contain `Strings`.
+
+Another constraint is that any type used with generics cannot be a [primitive type](https://docs.oracle.com/javase/tutorial/java/nutsandbolts/datatypes.html), such as `int` or `long`.
+
+It can help to look at an [example use case of generics](https://docs.oracle.com/javase/tutorial/java/generics/types.html) to get you started.

exercises/binary-search/src/test/java/BinarySearchTest.java

 
     @Test
     public void findsAValueInAnArrayWithOneElement() {
-        List<Integer> listOfUnitLength = Collections.singletonList(6);
+        List<Character> listOfUnitLength = Collections.singletonList('6');
 
-        BinarySearch<Integer> search = new BinarySearch<>(listOfUnitLength);
+        BinarySearch<Character> search = new BinarySearch<>(listOfUnitLength);
 
-        assertEquals(0, search.indexOf(6));
+        assertEquals(0, search.indexOf('6'));
     }
 
     @Ignore("Remove to run test")
     @Test
     public void findsAValueInTheMiddleOfAnArray() {
-        List<Integer> sortedList = Collections.unmodifiableList(
-                Arrays.asList(1, 3, 4, 6, 8, 9, 11)
+        List<String> sortedList = Collections.unmodifiableList(
+                Arrays.asList("1", "3", "4", "6", "8", "9", "11")
         );
 
-        BinarySearch<Integer> search = new BinarySearch<>(sortedList);
+        BinarySearch<String> search = new BinarySearch<>(sortedList);
 
-        assertEquals(3, search.indexOf(6));
+        assertEquals(3, search.indexOf("6"));
     }
 
     @Ignore("Remove to run test")
     @Ignore("Remove to run test")
     @Test
     public void identifiesThatAValueIsNotIncludedInTheArray() {
-        List<Integer> sortedList = Collections.unmodifiableList(
-                Arrays.asList(1, 3, 4, 6, 8, 9, 11)
+        List<String> sortedList = Collections.unmodifiableList(
+                Arrays.asList("1", "3", "4", "6", "8", "9", "11")
         );
 
-        BinarySearch<Integer> search = new BinarySearch<>(sortedList);
+        BinarySearch<String> search = new BinarySearch<>(sortedList);
 
-        assertEquals(-1, search.indexOf(7));
+        assertEquals(-1, search.indexOf("7"));
     }
 
     @Ignore("Remove to run test")