Added sum with recursion to 03_recursion, java folder

.gitignore

@@ -28,6 +28,7 @@ highlights/atom-language-perl6/
 .DS_store
 highlights/package-lock.json
 zig-cache
+assignments
 
 # IDE specific
 .scala_dependencies

03_recursion/java/04_sum/src/Sum.java

@@ -0,0 +1,20 @@
+import java.util.*;
+
+public class Sum {
+    public static int sum(ArrayList<Integer> num_list) {
+
+        if (num_list.size() == 0) {
+            return 0;
+        } else {
+            int num = num_list.get(0);
+            num_list.remove(0);
+            return num + sum(num_list);
+        }
+
+    }
+
+    public static void main(String[] args) {
+        int total = sum(new ArrayList<Integer>(Arrays.asList(2, 4, 6)));
+        System.out.println(total);
+    }
+}