BuffExample.java

import java.util.*;
import java.util.function.*;
import java.io.*;

class Main {
    public static void main(String args[]) { 
        BuffGroup group = new BuffGroup();
        group.add(new MobBonusXPBuff(10));
        
        Buff buff = group.get(MobBonusXPBuff.class);
        boolean correctType = buff instanceof MobBonusXPBuff;
        if(!correctType) {	
            String errorMessage = String.format("Type assertion failed. Expected[%s] Actual[%s]", MobBonusXPBuff.class, buff.getClass());
        	throw new IllegalStateException(errorMessage);
        }
        
        MobBonusXPBuff castedBuff = (MobBonusXPBuff) buff;
        boolean correctBonus = castedBuff.getXPBonus() == 10;      
        if(!correctBonus) {
        	String errorMessage = String.format("Bonus assertion failed. Expected[%d] Actual[%d]", 10, castedBuff.getXPBonus());
        	throw new IllegalStateException(errorMessage);
        }
        
        System.out.println("Passed");
    }
}

abstract class Buff { }

class MobBonusXPBuff extends Buff {
	private final int xpBonus;
	
	public MobBonusXPBuff(int xpBonus) {
		this.xpBonus = xpBonus;
	}
	
	public int getXPBonus() {
		return xpBonus;
	}
}

class BuffGroup {
	private Map<Class<? extends Buff>, Buff> buffs = new HashMap<>();
	
	public void add(Buff buff) {
		Class<? extends Buff> type = buff.getClass();
		if(buffs.containsKey(type))
			return;
		
		buffs.put(type, buff);
	}
	
	public Buff get(Class<? extends Buff> type) {
		return buffs.get(type);
	}
}