Fix Java project structure.

assets/src/main/Main.java

@@ -1,6 +1,12 @@
+package main;
+
 public class Main {
 
     public static void main(String[] args) {
         System.out.println("Hello, world!");
     }
+
+    public static int add(int a, int b) {
+        return a + b;
+    }
 }

assets/src/test/Main.java

@@ -0,0 +1,12 @@
+package test;
+
+public class Main {
+
+    public static void main(String[] args) {
+        testAdd();
+    }
+
+    public static void testAdd() {
+        assert main.Main.add(2, 2) == 4;
+    }
+}