Java polymorphism - getting child command from parent map
Java polymorphism - getting child command from parent map
I've made two classes - Building and Warehouse. Warehouse extends Building. I've created Building map (building_map), where I've put both Building objects and Warehouse objects.
I'd like to get Warehouse's method by using the map, but I can see only things that are from Building class.
Building
Warehouse
Warehouse
Building
Building
building_map
Building
Warehouse
Warehouse
Building
building_map.get(ware_id); /*--Warehouse_function--*/;
(It's one of my first posts here so sorry if I've put too little informations on my problem)
EDIT:
Here's how I'm adding objects into the map:
public static Map<String, Building> building_map = new HashMap<>();
Building bD_02 = new Building("bD_02", 2, "Duargian", 2.5, 180, 450, 100, ""
+ "i0_024!i0_029!i0_036?90!10!10!" //upgrade lv 1
+ "&"
+ "" //upgrade lv 2
+ "&"
+ "" //upgrade lv 3
+ "&");
Building bD_03 = new Warehouse("bD_03", 3, "Duargian", 2.8, 800, 2500, 1500, 2.35, 3000, ""
+ "i0_021!i0_024!i0_029?200!500!70!" //upgrade lv 1
+ "&"
+ "" //upgrade lv 2
+ "&"
+ "" //upgrade lv 3
+ "&");
building_map.put("bD_02", bD_02);
building_map.put("bD_03", bD_03);
EDIT2:
Thanks, TungstenX! It worked, I can close the topic now.
Please add code of how your building_map
– TungstenX
Sep 13 '18 at 10:52
I've updated the question with code of building_map.
– FumyaHero
Sep 13 '18 at 10:57
you will have to check the object's type with
instanceof and cast it to Warehouse– Jack Flamp
Sep 13 '18 at 10:58
instanceof
Warehouse
See also the official tutorial to get started: docs.oracle.com/javase/tutorial/java/IandI/polymorphism.html
– Hulk
Sep 13 '18 at 11:02
3 Answers
3
You can cast the result of the get to a warehouse assuming that you extended Warehouse with Building. I.e class Warehouse extends Building
class Warehouse extends Building
And your map looks like:Map<String, Building> buildingMap = new HashMap<>();
Then you can do
Map<String, Building> buildingMap = new HashMap<>();
Update: Safe casting
Building building = buildingMap.get("A");
if (building instanceof Warehouse)
((Warehouse)building).warehousMethod();
what if the value of
building_map.get("key") is a Building object?– Jack Flamp
Sep 13 '18 at 11:00
building_map.get("key")
Oh, it worked! Thank you for help, that's the thing I was looking for!
– FumyaHero
Sep 13 '18 at 11:01
@JackFlamp You are correct, I just gave an example ;-) Always check you objects
– TungstenX
Sep 13 '18 at 11:02
@FumyaHero Welcome to SO. Please select the solution/answer that suited you best.
– TungstenX
Sep 13 '18 at 11:04
Could you add safe casting to this post? Would be the cleanest.
– Wesley De Keirsmaeker
Sep 13 '18 at 11:47
If you a have a map of Buildings, not every object there has to be a Warehouse. It can also be a Building or other implementation.
Building
Warehouse
Building
If you're sure it's a warehouse, then you can cast it, like so:
Map<String, Building> map = new HashMap<>();
map.put("b", new Building());
map.put("w", new Warehouse());
Warehouse w = (Warehouse) map.get("w");
w.yourMethod();
but keep in mind that someone can put another instance of a child of Bulding, like so
Bulding
class OtherClass extends Building
map.put("b", new Building());
map.put("w", new Warehouse());
map.put("o", new OtherClass());
Warehouse w = (Warehouse) map.get("b"); // ClassCastException
Warehouse w = (Warehouse) map.get("o"); // ClassCastException
PS: As @Przemyslaw Kruglej suggested, if you want to validate an implementation of a class in a generic collection (like map) in runtime you can use instanceof before casting:
instanceof
Building possibleWarehouse = map.get("w");
if (possibleWarehouse instanceof Warehouse) // true
Warehouse warehouse = (Warehouse) possibleWarehouse;
+1, but it would be better if you provided some information about
instanceof usage in this case to prevent the ClassCastException.– Przemyslaw Kruglej
Sep 13 '18 at 11:58
instanceof
ClassCastException
@PrzemyslawKruglej Yes, I could but I didn't want to enourage such coding standards. It's better to design a better architecture, than to design it bad and then
instanceof around it :) You're right though.– Danon
Sep 13 '18 at 11:59
instanceof
Danon sure I understand, however,
instanceof is there if needed and even if it is better to use another design, it is still good to know there is something like instanceof ;)– Przemyslaw Kruglej
Sep 13 '18 at 12:03
instanceof
instanceof
Yes, I agree, that's right.
– Danon
Sep 13 '18 at 12:03
Values in map are Building type, so return type Object is expected to be building type. If as a developer you know that a specific key is binded to warehouse, than typecast it to warehouse. ((Warehouse) building_map.get("bD_03"))
take care of null also before typecasting.
– vermaji
Sep 13 '18 at 11:01
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Please read about java naming conventions: no _ char in names (unless constants). This isnt python ;-) And please read Minimal, Complete, and Verifiable example and enhance your question accordingly. That example you give here is simply insufficient, it is not clear what you are asking for.
– GhostCat
Sep 13 '18 at 10:52