// Introduced in Chapter 3/** A light bulb. */public class Light {  /** Whether the Light is on. */  private boolean on;  /** A Light is off by default. */  public Light() {    on = false;  }  /** Return true if the Light is on. */  public boolean isOn() {    return on;  }  /** Set whether the Light is on. */  public void setOn(boolean on) {	this.on = on;  }  public String toString() {    if (on) {      return "O";    } else {      return ".";    }  }}