Effective Java Item10-总是覆盖toString方法，item10-tostring

Effective Java Item10-总是覆盖toString方法，item10-tostring

Effective Java 2^nd Edition Reading Notes

Item10: Always override toString

总是覆盖toString方法

 

java.lang.Object的toString方法的说明为：

Returns a string representation of the object. In general, the toString method returns a string that "textually represents" this object. The result should be a concise but informative representation that is easy for a person to read. It is recommended that all subclasses override this method.

The toString method for class Object returns a string consisting of the name of the class of which the object is an instance, the at-sign character `@', and the unsigned hexadecimal representation of the hash code of the object. In other words, this method returns a string equal to the value of:

 getClass().getName() + '@' + Integer.toHexString(hashCode())

 

java.lang.Object的toString方法返回的是类的名字，加上@符号，然后是一个无符号的16进制数(对象的hash code值)，这样的toString方法显然传达的信息还不够。而toString方法的说明中包含了“建议所有的子类都覆盖这个方法”。

 

toString方法在println，printf，字符串连接，以及assert断言中，以及在调试器中，都被调用。

 

toString方法应该包含所有对象的重要信息。

 

另外一个问题是：是否需要对toString返回的值进行指定格式。

指定格式的好处在于它可以做为一个标准，不会带有歧义，并且可读性好。这个返回值可以用于输入或者输出，持久化的可读对象(xml文件)。如果指定格式的话，最好再提供一个工厂类用于根据String构造实例。

指定格式的缺点在于它是和接口一样，一旦指定了，就不能改变，因为可能有外部使用它，一旦它改变了，外部代码将无法使用。

一旦需要指定格式，那么要准确的定义并进行文档注释：

/**

 * Returns the string representation of this phone number.

 * The string consists of fourteen characters whose format

 * is "(XXX) YYY-ZZZZ", where XXX is the area code, YYY is

 * the prefix, and ZZZZ is the line number.  (Each of the

 * capital letters represents a single decimal digit.)

 *

 * If any of the three parts of this phone number is too small

 * to fill up its field, the field is padded with leading zeros.

 * For example, if the value of the line number is 123, the last

 * four characters of the string representation will be "0123".

 *

 * Note that there is a single space separating the closing

 * parenthesis after the area code from the first digit of the

 * prefix.

 */

@Override public String toString() {

return String.format("(%03d) %03d-%04d",

areaCode, prefix, lineNumber);

}