toString(）方法细节

toString()，每一个非基本类型的对象都有一个toString()方法，当编译器需要一个Sting,而你只有一个对象时，该方法会自动调用。

class WaterSource {
    private String s;
    WaterSource(){
        System.out.println("WaterSource()");
        s = "Constructed";
    }
    public String toString(){return s;}
}
public class FuYong01{
    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 + " 
" +
                "1 = " + i + " " + "f = " + f + " " +
                "source = " + source;　　//自动调用toSting方法
    }
public static void main2(String[] args){
    FuYong01 fy = new FuYong01();
    System.out.println(fy);　　//就相当于System.out.println(fy.toSting());
    
    }
}

运行结果：
WaterSource()
valve1 = null valve2 = null valve3 = null valve4 = null 
1 = 0 f = 0.0 source = Constructed

注意：

域中为基本类型是能够被初始化为0，但是对象引用会被初始化为null；看上边输出结果的红色null，就是因为引用的对象。

每一个类中都重写了一个toSting方法；我还以为同一个文件中只能有一个toSring方法了