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    引言： Implementing equalsDiscussion:A

 

 

覆盖equals【引自http://www.javapractices.com/Topic17.cjp】
implementing equalsdiscussion:all objects have both identity (the objects location in memory) and state (the objects data). the == operator always compares identity. the default implementation of equals compares identity as well.

sometimes the default implementation of equals has the desired behaviour (as in a type-safe enumeration, for example), but  equals should usually compare state, not identity. this is particularly true for "data-centric" classes which map to database records.

if you override equals, you must override hashcode hashcode must generate equal values for equal objects objects placed in a list , set, or map (as either a key or value) should have an appropriate definition of equals. (see, for example, the javadoc for java.sun.com/j2se/1.5.0/docs/api/java/util/collection.html#contains(java.lang.object)">collection.contains , java.sun.com/j2se/1.5.0/docs/api/java/util/map.html#containskey(java.lang.object)">map.containskey, and java.sun.com/j2se/1.5.0/docs/api/java/util/map.html#containsvalue(java.lang.object)">map.containsvalue .)

if you extend a concrete class, and add a new field which contributes to equals, then it is not possible to write a perfectly correct equals method for the new class. instead, you should use composition instead of inheritance. 【程序编程相关:Mysql对文件操作的封装】

hashcode and equals are closely related : 【推荐阅读:Hibernate 3新增XML关系持久】

here is an implementation of equals for a data-centric class. it demonstrates how different types of fields are treated: 【扩展信息:使用Middlegen简化Hiberna】

example

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