Convert Field to HashMap

Question

I want to loop through the Fields of a class, like Class.getDeclaredFields() but now I want to check if one Field is a HashMap and if so it should remove one Value of it. How can I reach this? I am clueless, but trying

This is my code so far: (It makes use of the Spigot API)

@EventHandler public void onInvClose(InventoryCloseEvent e) { Player p = (Player) e.getPlayer(); List<RegisteredListener> listeners = HandlerList.getRegisteredListeners(Core.core); for(RegisteredListener l : listeners) { Field[] fields = l.getClass().getDeclaredFields(); for(Field field : fields) { // Check if field is a HashMap if(field.getType().isAssignableFrom( HashMap.class )) { // Remove Player p from field } } } }

Have a look at docs.oracle.com/javase/7/docs/api/java/lang/reflect/… — Michal
– Michal, Commented Jul 12, 2018 at 15:28
Possible duplicate of Java Reflection get the Instance from a Field — Michal
– Michal, Commented Jul 12, 2018 at 15:31

Max · Accepted Answer · 2018-07-12 15:34:06Z

1

var map = (Map<Player, YourValueObject>)field.get(l); Should work and then just remove by key with p. Though I cannot try that on my phone right now.

edited Jul 12, 2018 at 15:34

answered Jul 12, 2018 at 15:31

Max

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5 Comments

Michal Over a year ago

Unfortunately var map is not valid Java.

Max Over a year ago

It did not take the < > and what´s in it without formatting finding the right type of apostrophe is not that easy on phones. xD Also var is totally a thing in Java10.

Richee Nektar Over a year ago

But I could try Map<?,?> :P

Michal Over a year ago

Ok, I have overseen the var Java 10 construct. Thanks for pointing that out.

Max Over a year ago

If you dont have java 10 just use the same as you did in the cast in place of var.

Aman Chhabra · Accepted Answer · 2018-07-12 15:42:38Z

You can use field.get(objectInstance) to get particular field of particular instance and then remove specific entry from that map.

Detailed steps would be as follows:

Step 1: Take all the declared fields by calling getDeclaredFields()

Step 2: For each field returned compare the type with HashMap by using equals method.

Step 3: If field type matches with HashMap, then get the value by calling get(objectInstace)and remove the not required value from map.

Please refer the complete example below;

jshell> class Player{ ...> Object someObject; ...> HashMap<Integer,String> someMap; ...> { ...> someMap = new HashMap<>(); ...> someMap.put(1,"Player"); ...> } ...> } | modified class Player jshell> jshell> jshell> Player player = new Player(); player ==> Player@71423665 jshell> java.lang.reflect.Field[] fields = player.getClass().getDeclaredFields(); fields ==> Field[2] { java.lang.Object Player.someObject, ja ... l.HashMap Player.someMap } jshell> for(java.lang.reflect.Field field : fields) { ...> // Check if field is a HashMap ...> if(field.getType().equals( HashMap.class)) { ...> ...> HashMap<Integer,String> data = (HashMap<Integer,String>)(field.get(player)); ...> data.remove(1); ...> System.out.println("Data left in map is "+data.size()); ...> } ...> } Data left in map is 0

Could you describe the get(objectInstance) method in more detail? So I can remember it better :P
@RicheeNektar I have shared the example as well in my answer. You can call this function from field variable and pass the instance of whom you want the value to be retrieved, so in your case, objectInstance will be l

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