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。

 1 class  WaterSource{
 2     private String s;
 3     WaterSource(){
 4         System.out.println("WaterSource()
");
 5         s = "Constructed";
 6     }
 7     public String toString(){
 8         return s;
 9     };
10     
11 }
12 public class zuheyufa {
13 
14     /**
15      * @param args
16      */
17     private String value1,value2,value3,value4;
18     private WaterSource source = new WaterSource();
19     private int i;
20     private float f;
21     public String toString(){
22         return "value1 =" + value1 + " " +
23                "value2 =" + value2 + " " +
24                "value3 =" + value3 + " " +
25                "value4 =" + value4 +"
" +
26                "i =" + i + " " +
27                "f =" + f + " " + 
28                "source = " + source;
29     }
30   
31     public static void main(String[] args) {
32         // TODO Auto-generated method stub
33         zuheyufa zuhe  = new zuheyufa();
34         System.out.println(zuhe);
35     }
36 
37 }

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。

 1 class  WaterSource{
 2     private String s ;
 3     WaterSource(String str){
 4         System.out.println("WaterSource()
");
 5         s = "Constructed";
 6         s = str;
 7     }
 8     public String toString(){
 9         return s;
10     };
11     
12 }
13 public class zuheyufa {
14 
15     /**
16      * @param args
17      */
18     private String value1,value2,value3,value4;
19     private WaterSource source = new WaterSource("jk");
20     private int i;
21     private float f;
22     public String toString(){
23         return "value1 =" + value1 + " " +
24                "value2 =" + value2 + " " +
25                "value3 =" + value3 + " " +
26                "value4 =" + value4 +"
" +
27                "i =" + i + " " +
28                "f =" + f + " " + 
29                "source = " + source;
30     }
31   
32     public static void main(String[] args) {
33         // TODO Auto-generated method stub
34         zuheyufa zuhe  = new zuheyufa();
35         System.out.println(zuhe);
36     }
37 
38 }

结果为：

WaterSource()

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