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Groovy闭包
闭包是一种表示可执行代码块的方法。闭包也是对象，可以像方法一样传递参数。由于闭包是代码块，因此也可以在需要时执行。像方法一样，在定义的过程中，闭包也可以使用一个或者多个参数。
一、inject方法
inject方法可用于遍历集合，首先将需要传递的值和集合项目传给闭包，此时其传递的值将作为处理结果，然后再和下一个集合项目一起传给闭包，依此类推。方法原型为：
Object inject(Object value,Closure closure)
例：求5的阶乘值
－－－－－－－－－－－
def
?factorial?
=
?[
2
,
3
,
4
,
5
].inject(
1
){previous,?element?
->
?previous?
*
?element}
println?
"
Factorial(5):?${factorial}
"
－－－－－－－－－－－
结果：
Factorial(5): 120
－－－－－－－－－－－
def?fact?
=
?
1
[
2
,
3
,
4
,
5
]
.
each
{number?
->
?fact?
*=
?number}
println?
"
fact:?${fact}
"
－－－－－－－－－－－
结果：
fact: 120
－－－－－－－－－－－
def
?list?
=
?[
2
,
5
]
factorial?
=
?list.inject(
1
){previous,?element?
->
?previous
*
element}
println?
"
Factorial(5):?${factorial}
"
－－－－－－－－－－－
结果：
Factorial(5): 120
－－－－－－－－－－－
list?
=
?[
2
,
5
]
def
?closure?
=
?{previous,element?
->
?previous
*
element}
factorial?
=
?list.inject(
1
,?closure)
println?
"
Factorial(5):?${factorial}
"
－－－－－－－－－－－
结果：
Factorial(5): 120
由于闭包也是一个对象，因此它可以作为方法的参数。
例：闭包作为方法的参数
－－－－－－－－－－－
//
Find?those?items?that?qualify
def
?filter(list,?predicate){
????
return
?list.findAll(predicate)
}
//
Two?predicate?closure
def
?isEven?
=
?{x?
->
?
return
?(x
%
2
?
==
?0)}
def
?isOdd?
=
?{x?
->
?
return
?!isEven(x)}
def
?table?
=
?[
11
,
12
,
13
,
14
]
//
Apply?filter
def
?evens?
=
?filter(table,?isEven)
println?
"
evens:?${evens}
"
def
?odds?
=
?filter(table,?isOdd)
println?
"
odds:?${odds}
"
－－－－－－－－－－－
结果：
evens: [12,14]
odds: [11,13]
例：闭包作为另一个闭包的参数
－－－－－－－－－－－
//
Find?initial?list?that?conforms?to?predicate
def
?takeWhile?
=
?{predicate,list?
->
????
def
?result?
=
?[]
????
for
(element?
in
?list){
????????
if
(predicate(element)){
????????????result
<<
element
????????}?
else
{
????????????
return
?result
????????}
????}
????
return
?result
}
//
Two?predicate?closures
def
?isEven?
=
?{x?
->
?
return
?x
%
2
?
==
?0}
def
?isOdd?
=
?{x?
->
?
return
?!isEven(x)}
def
?table1?
=
?[
12
,
14
,
15
,
18
]
def
?table2?
=
?[
11
,
16
,
18
]
//
Apply?takeWhile
def
?evens?
=
?takeWhile.call(isEven,?table1)
println?
"
evens:?${evens}
"
def
?odds?
=
?takeWhile.call(isOdd,?table2)
println?
"
odds:?${odds}
"
－－－－－－－－－－－－
结果：
evens: [12,13,15]
例：闭包作为返回值
－－－－－－－－－－－－
def
?multiply(x){
????
return
?{y?
->
?
return
?x
*
y}
}
def
?twice?
=
?multiply(
2
)
println?
"
twice(4):?${twice(4)}
"
//
Closure?returning?a?closure
def
?multiplication?
=
?{x?
->
?
return
?{y?
->
?
return
?x
*
y}}
def
?quadruple?
=
?multiplication(
4
)
println?
"
quadruple(3):?${quadruple(3)}
"
－－－－－－－－－－－－
结果：
twice(4): 8
quadruple(3): 12
例：选择性排序
－－－－－－－－－－－－
def
?selectionSort?
=
?{list?
->
????
def
?swap?
=
?{sList,p,q?
->
?
????????
def
?temp?
=
?sList[p]
????????sList[p]?
=
?sList[q]
????????sList[q]?
=
?temp
????}
????
def
?minimumPosition?
=
?{pList,
from
?
->
????????
def
?mPos?
=
?
from
????????
def
?nextFrom?
=
?
1
?
+
?
from
????????
for
(j?
in
?nextFrom..
<
pList.size()){
????????????
if
(pList[j]?
<
?pList[mPos])
????????????????mPos?
=
?j
????????}
????????
return
?mPos
????}
????
def
?size?
=
?list.size()?
-
?
1
????
for
(k?
in
?0..size){
????????
def
?minPos?
=
?minimumPosition(list,?k)
????????swap(list,?minPos,?k)
????}
????
return
?list
}
def
?table?
=
?[
13
,
11
,
14
]
def
?sorted?
=
?selectionSort(table)
println?
"
sorted:?${sorted}
"
－－－－－－－－－－－－结果：sorted: [11,12,14,14]
（编辑：李大同）
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