### 異常處理
Clojure代碼里面拋出來的異常都是運行時異常。當然從Clojure代碼里面調用的java代碼還是可能拋出那種需要檢查的異常的。 `try` , `catch` , `finally` 以及 `throw` 提供了和java里面類似的功能:
```
(defn collection? [obj]
(println "obj is a" (class obj))
; Clojure collections implement clojure.lang.IPersistentCollection.
(or (coll? obj) ; Clojure collection?
(instance? java.util.Collection obj))) ; Java collection?
(defn average [coll]
(when-not (collection? coll)
(throw (IllegalArgumentException. "expected a collection")))
(when (empty? coll)
(throw (IllegalArgumentException. "collection is empty")))
; Apply the + function to all the items in coll,
; then divide by the number of items in it.
(let [sum (apply + coll)]
(/ sum (count coll))))
(try
(println "list average =" (average '(2 3))) ; result is a clojure.lang.Ratio object
(println "vector average =" (average [2 3])) ; same
(println "set average =" (average #{2 3})) ; same
(let [al (java.util.ArrayList.)]
(doto al (.add 2) (.add 3))
(println "ArrayList average =" (average al))) ; same
(println "string average =" (average "1 2 3 4")) ; illegal argument
(catch IllegalArgumentException e
(println e)
;(.printStackTrace e) ; if a stack trace is desired
)
(finally
(println "in finally")))
```
上面代碼的輸出是這樣的：
```
obj is a clojure.lang.PersistentList
list average = 5/2
obj is a clojure.lang.LazilyPersistentVector
vector average = 5/2
obj is a clojure.lang.PersistentHashSet
set average = 5/2
obj is a java.util.ArrayList
ArrayList average = 5/2
obj is a java.lang.String
#<IllegalArgumentException java.lang.IllegalArgumentException:
expected a collection>
in finally
```
