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An inner class is a non-static nested class. Java also allows us to create a static nested class that can be used when an inner class does not require access to non-static fields and methods of the enclosing class. Here is an example (the static keyword is added to the Engine class):

public class Vehicle {
  private Engine engine;
  public Vehicle(int weightPounds, int horsePower) {
    this.engine = new Engine(horsePower, weightPounds)
  }
  public double getSpeedMph(double timeSec){
    return this.engine.getSpeedMph(timeSec);
  }
  private static class Engine {
    private int horsePower;
    private int weightPounds;
    private Engine(int horsePower, int weightPounds) {
      this.horsePower = horsePower;
    }
    private double getSpeedMph(double timeSec){
      double v = 2.0 * this.horsePower * 746 * 
                       timeSec * 32.17 / this.weightPounds;
      return Math.round(Math.sqrt(v) * 0.68);
    }
  }
}

Because a static class couldn't access a non-static member, we were forced to pass the weight value to the Engine class during its construction, and we removed the getWeightPounds() method as it's no longer needed.