it is done

core/src/mindustry/entities/traits/Entity.java

@@ -0,0 +1,42 @@
+package mindustry.entities.traits;
+
+import mindustry.entities.EntityGroup;
+
+public interface Entity extends MoveTrait{
+
+    int getID();
+
+    void resetID(int id);
+
+    default void update(){}
+
+    default void removed(){}
+
+    default void added(){}
+
+    EntityGroup targetGroup();
+
+    @SuppressWarnings("unchecked")
+    default void add(){
+        if(targetGroup() != null){
+            targetGroup().add(this);
+        }
+    }
+
+    @SuppressWarnings("unchecked")
+    default void remove(){
+        if(getGroup() != null){
+            getGroup().remove(this);
+        }
+
+        setGroup(null);
+    }
+
+    EntityGroup getGroup();
+
+    void setGroup(EntityGroup group);
+
+    default boolean isAdded(){
+        return getGroup() != null;
+    }
+}