Flesh out JUnit (#400)

* Flesh out JUnit

* Remove 'assertions'

* Re-add 'public'

* Alphabetize imports

content/languages/java/junit.md

@@ -2,3 +2,61 @@
 title: JUnit
 kind: reference
 ---
+
+You can use JUnit [4](https://junit.org/junit4/) or [5](https://junit.org/junit5/) in Codewars. 5 offers nesting, pretty name display and multiple test classes.
+
+Note that JUnit assertions use `(expected, actual)` parameter ordering rather than the typical `(actual, expected)`.
+
+## Basic Setup
+
+### Example solution
+
+```java
+public class Adder {
+    public static int add(int a, int b) {
+        return a + b;
+    }
+}
+```
+
+### JUnit 4
+
+```java
+import org.junit.Test;
+import static org.junit.Assert.assertEquals;
+
+public class AdderTests {
+    @Test
+    public void testAdd() {
+        assertEquals(3, Adder.add(1, 2));
+    }
+}
+```
+
+### JUnit 5
+
+```java
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Nested;
+import org.junit.jupiter.api.Test;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+@DisplayName("Testing Adder")
+class AdderTests {
+    @Test
+    @DisplayName("Adder.add(1, 1) returns 2")
+    void testPositives() {
+        assertEquals(2, Adder.add(1, 1), "1 + 1 should equal 2");
+    }
+
+    @Nested
+    @DisplayName("Negative Integers")
+    class NegativeTests {
+        @Test
+        @DisplayName("Adder.add(-1, -1) returns -2")
+        void testNegatives() {
+            assertEquals(-2, Adder.add(-1, -1), "-1 + -1 should equal -2");
+        }
+    }
+}
+```