Added create and destroy functions

javaecs/src/main/java/nz/ac/massey/programming_project_159333_s1_2021/ECS.java

@@ -13,6 +13,7 @@ public class ECS {
     // Components are either primitive types or class/struct definitions, with an additional list associating
     // the entities the component is assigned to
     List<Object> components = new ArrayList<>();
+    List<List<Object>> componentAssociations = new ArrayList<>();
     // Systems are user-defined functions that are called by the game engine, usually at a regular interval such 
     // as between every render frame. It must be able to find all instances of a specific component, in order
     // to update/read its data. A component  
@@ -23,8 +24,35 @@ public class ECS {
     int componentIndex = 0;
     int systemIndex = 0;
 
+    /**
+     * Takes the next element off the list of unassigned entities
+     * @return the value of the new entity
+     */
     Integer createEntity(){
-        
+        int newEntity = entities.remove(0);
+        if (componentAssociations.size() <= newEntity){
+            for (int i = componentAssociations.size(); i < newEntity+1 - componentAssociations.size(); i++) {
+                componentAssociations.add(i, new ArrayList<>());
+            }
+        }
+        componentAssociations.add(newEntity, element);
+        return newEntity;
+    }
+
+    /**
+     * Adds the entity back to the list of unassigned entities,
+     * and empty all component associations
+     * @param entity
+     */
+    void destroyEntity(int entity){
+        entities.add(entity);
+        componentAssociations.set(entity, new ArrayList<>());
+    }
+
+    void init(int maxEntities){
+        for (int i = 0; i < maxEntities; i++) {
+            entities.add(i);
+        }
     }
 
 }