CODEWITHSUNDEEP
javasyntax
Ubiux
Ubiux

Reputation: 127

Statement with braces is not valid

Can someone tell me why this is valid in Java:

for (int i= 1; i<11; i++){
       Movie movie = randomMovie();
}

and this is not?

for (int i= 1; i<11; i++)
      Movie movie = randomMovie();

I do not get it because to me it seems to be the exact same thing, but when I put the curly braces, it suddenly turns correct.

enter image description here

Upvotes: 1

Views: 70

Answers (2)

Stephen C
Stephen C

Reputation: 718966

Because the Java grammar forbids the latter. The relevant grammar rules are:

BasicForStatement:
    for ( [ForInit] ; [Expression] ; [ForUpdate] ) Statement 

Statement:
    StatementWithoutTrailingSubstatement
    LabeledStatement
    IfThenStatement
    IfThenElseStatement
    WhileStatement
    ForStatement

StatementWithoutTrailingSubstatement:
    Block
    EmptyStatement
    ExpressionStatement
    AssertStatement
    SwitchStatement
    DoStatement
    BreakStatement
    ContinueStatement
    ReturnStatement
    SynchronizedStatement
    ThrowStatement
    TryStatement

Note that the above do not include LocalVariableDeclarationStatement.

You can find the complete syntactic grammar for Java in JLS Chapter 19.

Hypothetical question for you. Supposing that the following code is valid Java:

for (int i = 1; i < limit; i++)
     Movie movie = randomMovie();
System.out.println("The last movie is " + movie);

what is the scope of the movie variable?

The answer is that is must go out of scope immediately1. Which makes it a useless declaration, and the usage in the println call is a compilation error.

I believe that this is the reason that they specified the Java syntax to disallow this. The declaration is useless and most likely a mistake, and should be brought to the programmer's attention via a compilation error.

(By the same argument the declaration in 

for (int i = 1; i < 11; i++){
     Movie movie = randomMovie();
}

is also a mistake. But doing something about this in the language grammar would be difficult.)

1 - The alternative is not tenable. In a statically checked language like Java, you cannot have a situation where an in-scope variable only exists depending on an earlier execution path. That would the value of movie be if limit was zero?

Upvotes: 1

NaveenBharadwaj
NaveenBharadwaj

Reputation: 1333

Check if this works.

Movie movie = null;
for (int i= 1; i<11; i++)
      movie = randomMovie();

Upvotes: 1

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