小弟目前刚刚上手java, 看到think in java里有这一段code，如下：

//: reusing/SprinklerSystem.java

// Composition for code reuse.

class WaterSource {

private String s;

WaterSource() {

System.out.println("WaterSource()");

s = "Constructed";

}

public String toString() { return s; }

}

public class SprinklerSystem {

private String valve1, valve2, valve3, valve4;

private WaterSource source = new WaterSource();

private int i;

private float f;

public String toString() {

return

"valve1 = " + valve1 + " " +

"valve2 = " + valve2 + " " +

"valve3 = " + valve3 + " " +

"valve4 = " + valve4 + "\n" +

"i = " + i + " " + "f = " + f + " " +

"source = " + source;

}

public static void main(String[] args) {

SprinklerSystem sprinklers = new SprinklerSystem();

System.out.println(sprinklers);

}

}

/* Output:

WaterSource()

valve1 = null valve2 = null valve3 = null valve4 = null

i = 0 f = 0.0 source = Constructed

*///:

实在不懂为什么最后output里的source=Constructed. 特地前来请教，在线等。