不知道有没解释对SprinklerSystem类里面定义了soure这一变量 private WaterSource source = new WaterSource(); 也就是说source 这个变量属于WaterSource类 然后main程序中System.out.println(sprinklers)会调用 public String toString() { return "valve1 = " + valve1 + " " + "valve2 = " + valve2 + " " + "valve3 = " + valve3 + " " + "valve4 = " + valve4 + "\n" + "i = " + i + " " + "f = " + f + " " + "source = " + source; } 这里面"source = " + source;这一句又会调用source这一变量（属于WaterSource类）中的 public String toString() { return s; } 而s的初始值是 Constructed