added build method with working nesting

src/main/Main.java

@@ -1,7 +1,16 @@
 package main;
 
+import main.htmlTags.*;
+
 public class Main {
     public static void main(String[] args) {
-
+        moduleBody body = new moduleBody();
+        moduleDiv mainDiv = new moduleDiv();
+        moduleDiv thirdDiv = new moduleDiv();
+        moduleDiv secondDiv = new moduleDiv();
+        body.addModule(mainDiv);
+        mainDiv.addModule(thirdDiv);
+        body.addModule(secondDiv);
+        System.out.println(body.build());
     }
 }

src/main/htmlTags/moduleBody.java

@@ -0,0 +1,7 @@
+package main.htmlTags;
+
+public class moduleBody extends moduleGeneric{
+    public moduleBody(){
+        setTag("body");
+    }
+}

src/main/htmlTags/moduleDiv.java

@@ -2,7 +2,7 @@ package main.htmlTags;
 
 public class moduleDiv extends moduleGeneric {
 
-    public void tagDiv(){
-
+    public moduleDiv(){
+        setTag("div");
     }
 }

src/main/htmlTags/moduleGeneric.java

@@ -1,13 +1,16 @@
 package main.htmlTags;
 
+import java.util.ArrayList;
+
 public class moduleGeneric {
 
     String tag = "";
     int ID;
+    ArrayList<moduleGeneric> content = new ArrayList<>();
 
 
     public void addModule(moduleGeneric module){
-
+        content.add(module);
     }
 
     public void setID(int ID) {
@@ -25,4 +28,20 @@ public class moduleGeneric {
     public String getTag() {
         return tag;
     }
+
+    public StringBuilder build(){
+        StringBuilder buildString = new StringBuilder();
+        buildString
+                .append("\n<")
+                .append(tag)
+                .append(">");
+        for (main.htmlTags.moduleGeneric moduleGeneric : content) {
+            buildString.append(moduleGeneric.build());
+        }
+        buildString
+                .append("\n</")
+                .append(tag)
+                .append(">");
+        return buildString;
+    }
 }