Ruby语言中存在着许多表达式，这些表达式用法不尽相同，实现的功能也不同。熟练的掌握这些表达式的用法，可以有助于我们编程的方便性。今天看到有人在Groovy的邮件列表上问Groovy能不能支持Ruby case when表达式：#t#

Ruby case when表达式代码示例：

1. car="Patriot"
2. manufacturer=casecar
3. when"Focus":"Ford"
4. when"Navigator":"Lincoln"
5. when"Camry":"Toyota"
6. when"Civic":"Honda"
7. when"Patriot":"Jeep"
8. when"Jetta":"VW"
9. when"Ceyene":"Porsche"
10. when"Outback":"Subaru"
11. when"520i":"BMW"
12. when"Tundra":"Nissan"
13. else"Unknown"
14. end
15. puts"The"+car+"ismadeby
    "+manufacturer
16. car="Patriot"
17. manufacturer=casecar
18. when"Focus":"Ford"
19. when"Navigator":"Lincoln"
20. when"Camry":"Toyota"
21. when"Civic":"Honda"
22. when"Patriot":"Jeep"
23. when"Jetta":"VW"
24. when"Ceyene":"Porsche"
25. when"Outback":"Subaru"
26. when"520i":"BMW"
27. when"Tundra":"Nissan"
28. else"Unknown"
29. end
30. puts"The"+car+"ismadeby
    "+manufacturer

然后Guillaume给出了这么一段Ruby case when表达式代码：

defcar="Patriot" 
defmanufacturer=match(car){ 
when"Focus","Ford" 
when"Navigator","Lincoln" 
when"Camry","Toyota" 
when"Civic","Honda" 
when"Patriot","Jeep" 
when"Jetta","VW" 
when"Ceyene","Porsche" 
when"Outback","Subaru" 
when"520i","BMW" 
when"Tundra","Nissan" 
otherwise"Unknown" 
} 
println"The$carismadeby
$manufacturer" 
defmatch(obj,closure){ 
closure.subject=obj 
closure.when={value,result-> 
if(value==subject) 
thrownewMatchResultException
(result:result) 
} 
closure.otherwise={returnit} 
closure.resolveStrategy=
Closure.DELEGATE_FIRST 
try{ 
closure() 
closure.otherwise() 
}catch(MatchResultExceptionr){ 
r.result 
} 
} 
classMatchResultException
extendsRuntimeException{ 
defresult 
} 
defcar="Patriot"
defmanufacturer=match(car){ 
when"Focus","Ford" 
when"Navigator","Lincoln" 
when"Camry","Toyota" 
when"Civic","Honda" 
when"Patriot","Jeep" 
when"Jetta","VW" 
when"Ceyene","Porsche" 
when"Outback","Subaru" 
when"520i","BMW" 
when"Tundra","Nissan" 
otherwise"Unknown" 
} 
println"The$carismade
by$manufacturer" 
defmatch(obj,closure){ 
closure.subject=obj
closure.when={value,result->
if(value==subject) 
thrownewMatchResultException
(result:result) 
} 
closure.otherwise={returnit} 
closure.resolveStrategy=
Closure.DELEGATE_FIRST 
try{ 
closure() 
closure.otherwise() 
}catch(MatchResultExceptionr){ 
r.result 
} 
} 
classMatchResultException
extendsRuntimeException{ 
defresult 
}

我不是很喜欢里面用异常来控制程序的流程，而且觉得“when "Focus", "Ford"”中间的逗号不够直观，因此就在上面的Ruby case when表达式代码的基础上做了一些修改：

defmatch(subject,closure){ 
defwhenMap=[:],otherwise=null 
closure.when={map->whenMap.putAll(map)} 
closure.otherwise={otherwise=it} 
closure.resolveStrategy=Closure.DELEGATE_FIRST 
closure() 
defresult=whenMap.find{condition,
value->subjectincondition} 
returnresult?result.value:otherwise 
} 
defmanufacturer(car){ 
match(car){ 
when"Focus":"Ford" 
when"Navigator":"Lincoln" 
when"Camry":"Toyota" 
when"Civic":"Honda" 
when"Patriot":"Jeep" 
when"Jetta":"VW" 
when"Ceyene":"Porsche" 
when"Outback":"Subaru" 
when"520i":"BMW" 
when"Tundra":"Nissan" 
otherwise"Unknown" 
} 
} 
println"ThePatriotismade
by${manufacturer('Patriot')}" 
println"TheQQismadeby$
{manufacturer('QQ')}" 
defmatch(subject,closure){ 
defwhenMap=[:],otherwise=null
closure.when={map->whenMap.putAll(map)} 
closure.otherwise={otherwise=it} 
closure.resolveStrategy=Closure.
DELEGATE_FIRST 
closure() 
defresult=whenMap.find{condition,
value->subjectincondition} 
returnresult?result.value:otherwise 
} 
defmanufacturer(car){ 
match(car){ 
when"Focus":"Ford" 
when"Navigator":"Lincoln" 
when"Camry":"Toyota" 
when"Civic":"Honda" 
when"Patriot":"Jeep" 
when"Jetta":"VW" 
when"Ceyene":"Porsche" 
when"Outback":"Subaru" 
when"520i":"BMW" 
when"Tundra":"Nissan" 
otherwise"Unknown" 
} 
} 
println"ThePatriotismade
by${manufacturer('Patriot')}" 
println"TheQQismadeby$
{manufacturer('QQ')}"

以上Ruby case when表达式代码在Groovy 1.6下编译通过。