LoadBalancers.java

package com.interview.misc;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class LoadBalancers {

    private List<String> servers = new ArrayList<String>();
    Map<String,Integer> index = new HashMap<String,Integer>();
    
    public void addServer(String name){
        servers.add(name);
        index.put(name, servers.size()-1);
    }
    
    public void removeServer(String name){
        Integer pos = index.get(name);
        if(pos == null){
            throw new IllegalArgumentException();
        }
        servers.set(pos, servers.get(servers.size()-1));
        servers.remove(servers.size()-1);
    }
    
    public String getRandom(){
        int r = (int)(Math.random()*1000);
        return servers.get(r%servers.size());
    }
    
    public static void main(String args[]){
        LoadBalancers lb = new LoadBalancers();
        lb.addServer("1");
        lb.addServer("2");
        lb.addServer("3");
        lb.addServer("4");
        lb.addServer("5");
        
        System.out.print(lb.getRandom());
        System.out.print(lb.getRandom());
        System.out.print(lb.getRandom());
        System.out.print(lb.getRandom());
            
        lb.removeServer("3");
        lb.removeServer("1");
        System.out.print(lb.getRandom());
        System.out.print(lb.getRandom());
        System.out.print(lb.getRandom());
        System.out.print(lb.getRandom());
        
    }
}