WeightedRandom1.java

import java.util.ArrayList;
import java.util.Random;

class WeightedRandom1{

	public static void main(String args[]){
		ArrayList<Factor> fact = new ArrayList<>();
		ArrayList<Integer> result = new ArrayList<>();
		fact.add(new Factor("★5",2));
		fact.add(new Factor("★4",12));
		fact.add(new Factor("★3",86));
		int r = getRandom(fact);

		System.out.println("result:"+fact.get(r).name);
	}

	private static int sum(ArrayList<Factor> f){
		int n=0;
		int total=0;
		while(n<f.size()){
			total += f.get(n).ratio;
			n++;
		}
		return total;
	}

	private static int getRandom(ArrayList<Factor> f){
		int sumWeight = sum(f);
		Random rnd = new Random();
		int val = rnd.nextInt(sumWeight); 
	
		int n = -1;
		do{
			n++;
			val -= f.get(n).ratio;
		}while(val>=0 && f.size()>n);
	
		return n;
	}
	
}

class Factor{
	String name;
	int ratio;
	Factor(String name,int ratio){
		this.name = name;
		this.ratio = ratio;
	}
}