我在第一个Clojure程序中苦于使用NPE,并希望得到一些帮助。Clojure中的NullPointerException减少方法
call-elem
是包含结构: -
:id
,像some.packagename.SomeClass.someMethod
:calls
(孩子)的字符串,
定义如下其它call-elem
结构的向量:
(defstruct call-struct :id :calls)
(defn- class-name [call]
(let [id (call :id)]
(.substring id 0 (.lastIndexOf id "."))))
(defn- method-name [call]
(let [id (call :id)]
(.substring id (inc (.lastIndexOf id ".")))))
我也有一个方法foo
需要class-map
和call-elem
作为参数。 class-map
是class-name
到method-map
的地图。 method-map
是method-name
到variation
的列表的地图。每个variation
是具有密钥:class-name
,:method-name
和:calls
的地图。 :calls
是其他变体的列表。所述foo
函数应返回一个矢量具有两个元素: -
- 第一元素是一个类的地图与对应于
call-elem
新项,并且其孩子 - 第二元件是对应于参数的变化图
call-elem
下面是代码:
(declare foo' add-new-variation)
(defn foo [class-map call-elem]
(let [children (call-elem :calls)
class-name (class-name call-elem)
method-name (method-name call-elem)]
(if (empty? children)
(let [new-variation {:class-name class-name
:method-name method-name
:calls []}
new-class-map (add-new-variation class-map
class-name method-name
new-variation)]
[new-class-map new-variation])
(let [[new-class-map child-variations-list]
(reduce #(foo' %1 %2) (class-map '()) children)
new-variation {:class-name class-name
:method-name method-name
:calls child-variations-list}
new-class-map' (add-new-variation new-class-map
class-name method-name
new-variation)]
[new-class-map' new-variation]))))
(defn foo' [[class-map variations-list] call-elem]
(let [[new-class-map new-variation] (foo class-map call-elem)]
[new-class-map (cons new-variation variations-list)]))
(defn add-new-variation [class-map class-name method-name variation]
(let [method-map (if (contains? class-map class-name)
(class-map class-name) {})
variations-list (if (contains? method-map method-name)
(method-map method-name) [])
new-variations-list (conj variations-list variation)
new-method-map (assoc method-map method-name new-variations-list)]
(assoc class-map class-name new-method-map)))
当我尝试运行FO llowing代码:
(try
(let [call-elem
(struct call-struct "Class1.method1"
[(struct call-struct "Class2.method1" [])])]
(second (foo {} call-elem)))
(catch Exception ex
(.printStackTrace ex)))
我得到以下结果:
{:class-name "Class1",
:method-name "method1",
:calls ({:class-name "Class2", :method-name "method1", :calls []})}
但是当我尝试运行下面的代码,增加了通话更深一层,我得到了NullPointerException
:
(try
(let [call-elem
(struct call-struct "Class1.method1"
[(struct call-struct "Class2.method1"
[(struct call-struct "Class3.method1" [])])])]
(second (foo {} call-elem)))
(catch Exception ex
(.printStackTrace ex)))
以下是堆栈跟踪:
java.lang.NullPointerException
at first.simple$foo.invoke(NO_SOURCE_FILE:46)
at first.simple$foo_SINGLEQUOTE_.invoke(NO_SOURCE_FILE:55)
at first.simple$foo$fn__1986.invoke(NO_SOURCE_FILE:46)
at clojure.lang.ArrayChunk.reduce(ArrayChunk.java:58)
at clojure.core.protocols$fn__5565.invoke(protocols.clj:30)
at clojure.core.protocols$fn__5543$G__5538__5552.invoke(protocols.clj:11)
at clojure.core$reduce.invoke(core.clj:5995)
at first.simple$foo.invoke(NO_SOURCE_FILE:46)
at first.simple$eval2010.invoke(NO_SOURCE_FILE:6)
at clojure.lang.Compiler.eval(Compiler.java:6465)
at clojure.lang.Compiler.eval(Compiler.java:6431)
at clojure.core$eval.invoke(core.clj:2795)
at clooj.repl$create_clojure_repl$repl_thread_fn__578$fn__589.invoke(repl.clj:147)
at clojure.main$repl$read_eval_print__5967.invoke(main.clj:244)
at clojure.main$repl$fn__5972.invoke(main.clj:265)
at clojure.main$repl.doInvoke(main.clj:265)
at clojure.lang.RestFn.invoke(RestFn.java:1523)
at clooj.repl$create_clojure_repl$repl_thread_fn__578.invoke(repl.clj:145)
at clojure.lang.AFn.run(AFn.java:24)
at java.lang.Thread.run(Thread.java:680)
现货!谢谢!你对改进代码有什么建议吗?正如我所说,这是我第一次尝试Clojure,所以我们欢迎任何建议。 :) – 2011-12-27 20:09:18