Why does a Try/Catch block create new variable scope?

by

Why are Objects created within the try/catch block not in scope with the rest of the method?

They are. Variables declared within the try/catch block are not in scope in the containing block, for the same reason that all other variable declarations are local to the scope in which they occur: That’s how the specification defines it. 🙂 (More below, including a reply to your comment.)

Here’s an object created within a try/catch which is accessible outside of it:

SomeObject someObject = null;
try
{
    someObject = new SomeObject();
    someObject.dangerousMethod();
}
catch(Exception e)
{
}
someObject.anotherMethod(); // This is fine -- unless the SomeObject
                            // constructor threw the exception, in which
                            // case someObject will be null

Note the difference. Where the variable is declared defines the scope in which it exists, not where the object was created.

But based on the method names and such above, the more useful structure for that would be:

SomeObject someObject = new SomeObject();
try
{
    someObject.dangerousMethod();
}
catch(Exception e)
{
}
someObject.anotherMethod();

Re your comment:

I guess I’m confused as to why another scope has even been created for a try/catch block.

In Java, all blocks create scope. The body of an if, the body of an else, of a while, etc. — they all create a new, nested variable scope:

if (foo) {
    SomeObject bar = new SomeObject();
}
bar.doSomething(); // <== Compilation error, `bar` is not defined

(In fact, even a block without any control structure creates one.)

And if you think about it, it makes sense: Some blocks are conditional, like the one defining the body of an if or while. In the above if, bar may or may not have been declared (depending on the value of foo), which makes no sense because of course the compiler has no concept of the runtime value of foo. So probably for consistency, the designers of Java went with having all blocks create a new nested scope. (The designer of JavaScript went the other way — there is no block scope at all, yet, though it’s being added — and that approach also confuses people.)

More Related Contents:

  1. Why can't we declare variables in if statement?
  2. Whats Happening in this for loop? [closed]
  3. Catching java.lang.OutOfMemoryError?
  4. Can I catch multiple Java exceptions in the same catch clause?
  5. Error: ‘else’ without ‘if’
  6. In Java, are enum types inside a class static?
  7. Spring Java Config: how do you create a prototype-scoped @Bean with runtime arguments?
  8. What is the difference between a local variable, an instance field, an input parameter, and a class field?
  9. Java, “Variable name” cannot be resolved to a variable
  10. How do I create a static local variable in Java?
  11. What do curly braces in Java mean by themselves?
  12. Problems with local variable scope. How to solve it?
  13. Java if vs. try/catch overhead
  14. Java try/catch/finally best practices while acquiring/closing resources
  15. Java io ugly try-finally block
  16. EOFException – how to handle?
  17. Accessing a static variable via an object reference in Java
  18. Try-with-resources equivalent in Java 1.6
  19. Java try/catch performance, is it recommended to keep what is inside the try clause to a minimum?
  20. I can pass a variable from a JSP scriptlet to JSTL but not from JSTL to a JSP scriptlet without an error
  21. What is the default scope of a method in Java?
  22. throws Exception in finally blocks
  23. Why can’t variables be declared in an if statement?
  24. Catching an exception that is nested into another exception
  25. Java local variable visibility in anonymous inner classes – why is ‘final’ keyword required?
  26. Java InputMismatchException
  27. How can I catch all exceptions thrown through reading / writing a file?
  28. What is the difference between @ApplicationScoped and @Singleton scopes in CDI?
  29. try-catch for division by zero
  30. Servlet redirect to same page with error message

Leave a Comment