title: How to create Map in LambdaJ where value is Collection?
body: <p>I know how to make <code>Map&lt;String, Car&gt;</code> but how to make <code>Map&lt;String, List&lt;Car&gt;&gt;</code> in lambdaj?</p>
  
  <p>This is code I want to write in LambdaJ:</p>
  
  <pre><code>Map&lt;String, List&lt;Car&gt;&gt; driverCarsMap = new HashMap&lt;String, List&lt;Car&gt;&gt;();
  for (Car car : cars)
  {
      String driver = car.getDriver();
      if (!driverCarsMap.containsKey(driver))
              driverCarsMap.put(driver, new ArrayList&lt;Car&gt;());
  
      driverCarsMap.get(driver).add(car);
  }
  </code></pre>
  
  <p>Unfortunately the code:</p>
  
  <pre><code>Map&lt;String, List&lt;Car&gt;&gt; driverCarsMap = index(cars, on(Car.class).getDriver());
  </code></pre>
  
  <p>creates the Map, but value is not being extended but overwritten. So, de facto we do note have a List but single object.</p>