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