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A Conversation with Ken Arnold, Part IV
by Bill Venners
September 30, 2002

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Bill Venners: Why are fields in entries public?

Ken Arnold: We could have used typical accessors, such as get and set methods. In any pair of get and set methods, such as in a JavaBean, there's a contract. The documentation of the JavaBean says that setting this value will result in the following behavior. One option, for example, could define get as "get next," where the returned value monotonically increases. set could mean "set the starting point,"—reset where the value returned by get monotonically increases from. That is a legitimate get and set contract. In random number generators, for example, you set the seed and get the next random number.

The contract for get and set methods of entries would essentially be: if you call set with a given value, and then return later and call get with no intervening set, the value would be the same—it would be unmodified. Furthermore, remember the matching is exact. When you call set on your template to set a particular value to 17, you are asking for an entry where the value is 17. When you receive an entry and call its get method, you better get 17, not 18. Incrementing is not OK.

So if you examine the contract description for an entry's get and set methods, you would see it describes a field. get and set would have to act exactly like a field. Therefore, we asked ourselves, why should we have get and set methods whose behavior is exactly like this other language construct called a field? Why not just make it a field? If we make it a field, it will have the correct behavior. Nobody can accidentally screw up their get and set methods. Making it a field eliminates a source of error.

Now this sometimes makes people uncomfortable because they've been told not to have public fields; that public fields are bad. And often, people interpret those things religiously. But we're not a very religious bunch. Rules have reasons. And the reason for the private data rule doesn't apply in this particular case. It is a rare exception to the rule. I also tell people not to put public fields in their objects, but exceptions exist. This is an exception to the rule, because it is simpler and safer to just say it is a field. We sat back and asked: Why is the rule thus? Does it apply? In this case it doesn't.

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