open:exception-handling [EL]

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# Exception Handling

Exception is broadly classified into the following categories

  - [[Checked Exception]]
    - The classes that extend Throwable class except [[RuntimeException]] and [[Error]] are known as checked exceptions. E.g. [[IOException]], [[SQLException]], etc. Checked exceptions are checked at compile-time.
  - [[Unchecked Exception]]
    - The classes that extend [[RuntimeException]] are known as unchecked exceptions. For example, [[ArithmeticException]], [[NullPointerException]], [[ArrayIndexOutOfBoundsException]], etc. Unchecked exceptions are not checked at compile-time rather they are checked at runtime.



### Example

<code>
(ns clojure.examples.example
   (:gen-class))

;; This program displays Hello World
(defn Example []
   (def string1 (slurp "Example.txt"))
   (println string1))
(Example)
</code>

#### Output

<code>
Caused by: java.io.FileNotFoundException: Example.txt (No such file or
directory)
at java.io.FileInputStream.open0(Native Method)
at java.io.FileInputStream.open(FileInputStream.java:195)
at java.io.FileInputStream.<init>(FileInputStream.java:138)
at clojure.java.io$fn__9185.invoke(io.clj:229)
at clojure.java.io$fn__9098$G__9091__9105.invoke(io.clj:69)
at clojure.java.io$fn__9197.invoke(io.clj:258)
at clojure.java.io$fn__9098$G__9091__9105.invoke(io.clj:69)
</code>

### Example

<code>
(ns clojure.examples.example
   (:gen-class))
(defn Example []
   (try
      (aget (int-array [1 2 3]) 5)
      (catch Exception e (println (str "caught exception: " (.toString e))))
      (finally (println "This is our final block")))
   (println "Let's move on"))
(Example)
</code>

#### Output

<code>
caught exception: java.lang.ArrayIndexOutOfBoundsException: 5
This is our final block
Let's move on
</code>

## Error

{{ https://www.tutorialspoint.com/clojure/images/exceptions_in_clojure.jpg }}

### Catching Exceptions

<code>
(try
   (//Protected code)
   catch Exception e1)
(//Catch block)
</code>

### Example

<code>
(ns clojure.examples.example
   (:gen-class))
(defn Example []
   (try
      (def string1 (slurp "Example.txt"))
      (println string1)
      (catch Exception e (println (str "caught exception: " (.getMessage e))))))
(Example)
</code>

#### Output

<code>
caught exception: Example.txt (No such file or directory)
</code>

## Multiple Catch Blocks

### Example

<code>
(ns clojure.examples.example
   (:gen-class))
(defn Example []
   (try
      (def string1 (slurp "Example.txt"))
      (println string1)
      
      (catch java.io.FileNotFoundException e (println (str "caught file
         exception: " (.getMessage e))))
      
      (catch Exception e (println (str "caught exception: " (.getMessage e)))))
   (println "Let's move on"))
(Example)
</code>

#### Output

<code>
caught file exception: Example.txt (No such file or directory)
Let's move on
</code>

## Finally Block

<code>
(try
   (//Protected code)
   catch Exception e1)
(//Catch block)
(finally
   //Cleanup code)
</code>

### Example

<code>
(ns clojure.examples.example
   (:gen-class))
(defn Example []
   (try
      (def string1 (slurp "Example.txt"))
      (println string1)
      
      (catch java.io.FileNotFoundException e (println (str "caught file
         exception: " (.getMessage e))))
      
      (catch Exception e (println (str "caught exception: " (.getMessage e))))
      (finally (println "This is our final block")))
   (println "Let's move on"))
(Example)
</code>

#### Output

<code>
caught file exception: Example.txt (No such file or directory)
This is our final block
Let's move on
</code>


### Example

<code>
(ns clojure.examples.example
   (:gen-class))
(defn Example []
   (try
      (def string1 (slurp "Example.txt"))
      (println string1)
      
      (catch java.io.FileNotFoundException e (println (str "caught file
         exception: " (.toString e))))
      
      (catch Exception e (println (str "caught exception: " (.toString e))))
   (finally (println "This is our final block")))
   (println "Let's move on"))
(Example)
</code>

#### Output

<code>
caught file exception: java.io.FileNotFoundException: Example.txt (No such file
or directory)
This is our final block
Let's move on
</code>



## Refs

- https://www.tutorialspoint.com/clojure/clojure_exception_handling.htm