Type of Exception in catch block

Question

I am learning how to create file and directory in java using this code. On the ERROR LINE I am getting error as "IOException is never thrown in this block".
So how do I know which function is throwing what type of Exception? Or if I am not sure I should use generic Exception in every catch block.

public class FileTest {
    public static void main(String[] args) {
        //file creation
        boolean flag = false;
        File file = new File("/IdeaProjects/JavaCode/jstest.txt");
        try {
            flag  = file.createNewFile();
        }catch (IOException e){
            e.printStackTrace();
        }
        System.out.println("file path is : "  + file.getPath());

        //dir creation
        boolean dirFlag = false;
        File fileDir  = new File("/IdeaProjects/JavaCode/js");
        try{
            dirFlag = fileDir.mkdir();
        }catch (IOException e){//ERROR LINE
            e.printStackTrace();
        }
        if(dirFlag)
            System.out.println("created");
        else
            System.out.println("exist");
    }
}

You only have one method call in your try block. Read the Javadoc of that method to see what it may throw. — Eran, Jun 09 '16 at 10:33

score 1 · Accepted Answer · edited May 23 '17 at 11:52

1

The java.io.File#mkdir method only declares to throw SecurityException - see API.

java.lang.SecurityException is a RuntimeException and doesn't require being caught, although you may want to, depending on the context (again, see API).

Catching general java.lang.Exception in every catch block is absolutely not a recommended practice, although you may sometimes have to (not in your present case though).

See here for some SO literature on the matter.

edited May 23 '17 at 11:52

Community

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answered Jun 09 '16 at 10:39

Mena

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It means I must see type of Exception thrown by a function from java API and write accordingly. – js_248 Jun 09 '16 at 10:43
@user3747720 well yes, consuming an API implies understanding what it does or is supposed to do. This is valid in general, for any programming language and any given context. – Mena Jun 09 '16 at 11:04

score 0 · Answer 2 · answered Jun 09 '16 at 10:43

Remember what methods throw exceptions and which exceptions they are.
Check the documentation if you think a method may throw an exception.
Just attempt to compile the code and fix the errors the compiler throws (They will tell you what exceptions are thrown by what method if the try-catch block is missing).

The method in question (File.mkdir()) throws a SecurityException which doesn't need to be caught (you can if need be) as it is an unchecked RuntimeException.

java.io.File: https://docs.oracle.com/javase/7/docs/api/java/io/File.html#mkdir()

SecurityException: https://docs.oracle.com/javase/7/docs/api/java/lang/SecurityException.html

score 0 · Answer 3 · answered Jun 09 '16 at 11:11

Quoting JLS Section 11.2:

It is a compile-time error if a catch clause can catch checked exception class E1 and it is not the case that the try block corresponding to the catch clause can throw a checked exception class that is a subclass or superclass of E1, unless E1 is Exception or a superclass of Exception.

If no method you invoke in the try block declares that it throws IOException (and you don't throw new IOException(..) directly either), it is a compile-time error if you try to catch an IOException.

Type of Exception in catch block

3 Answers3