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String时，该对象却没有时，toString方法就会被调用。

class  WaterSource{
    private String s;
    WaterSource(){
        System.out.println("WaterSource()
");
        s = "Constructed";
    }
    public String toString(){
        return s;
    };
    
}
public class zuheyufa {

    /**
     * @param args
     */
    private String value1,value2,value3,value4;
    private WaterSource source = new WaterSource();
    private int i;
    private float f;
    public String toString(){
        return "value1 =" + value1 + " " +
               "value2 =" + value2 + " " +
               "value3 =" + value3 + " " +
               "value4 =" + value4 +"
" +
               "i =" + i + " " +
               "f =" + f + " " + 
               "source = " + source;
    }
  
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        zuheyufa zuhe  = new zuheyufa();
        System.out.println(zuhe);
    }

}

结果为：

WaterSource()

value1 =null value2 =null value3 =null value4 =null
i =0 f =0.0 source = Constructed

上例所示，当主函数需要输出zuheyufa的对象时，该对象没有String，因此会调用toString方法。

如果稍稍修改一下WaterSource类，为该类中的构造函数传递一个String参数，那么结果将发生变化了。

class  WaterSource{
    private String s ;
    WaterSource(String str){
        System.out.println("WaterSource()
");
        s = "Constructed";
        s = str;
    }
    public String toString(){
        return s;
    };
    
}
public class zuheyufa {

    /**
     * @param args
     */
    private String value1,value2,value3,value4;
    private WaterSource source = new WaterSource("jk");
    private int i;
    private float f;
    public String toString(){
        return "value1 =" + value1 + " " +
               "value2 =" + value2 + " " +
               "value3 =" + value3 + " " +
               "value4 =" + value4 +"
" +
               "i =" + i + " " +
               "f =" + f + " " + 
               "source = " + source;
    }
  
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        zuheyufa zuhe  = new zuheyufa();
        System.out.println(zuhe);
    }

}

结果为：

WaterSource()

value1 =null value2 =null value3 =null value4 =null
i =0 f =0.0 source = jk