Why do declarations following conditions of control structures need to be in a block?

Question

When trying to compile the following

public class Test {
    public void method(String foo) {
        // This compiles if the curly braces are uncommented
        if(foo instanceof Object) // {
            Object bar = (Object) foo;
        // }
    }
}

I get the following errors

javac -Xlint:all Test.java
Test.java:5: error: not a statement
            Object bar = foo;
            ^
Test.java:5: error: ';' expected
            Object bar = foo;
                  ^
2 errors

Why does Object bar = (Object) foo; need to be in a block for the code to compile?

Answer 1

Why does Object bar = (Object) foo; need to be in a block for the code to compile?

Because it's pointless to declare a variable when that's the only statement in the block. The declaration is meaningless, as you won't be able to refer to the variable in any subsequent code. (The scope of the variable would be just the declaration.)

Basically, the compiler is stopping you from doing something pointless.

In terms of the specification, this is the production you're trying to use (section 14.9 of the JLS):

IfThenStatement:
    if ( Expression ) Statement    

Now the Statement production is defined by section 14.5.

Statement:
    StatementWithoutTrailingSubstatement
    LabeledStatement
    IfThenStatement
    IfThenElseStatement
    WhileStatement
    ForStatement

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

StatementNoShortIf:
    StatementWithoutTrailingSubstatement
    LabeledStatementNoShortIf
    IfThenElseStatementNoShortIf
    WhileStatementNoShortIf
    ForStatementNoShortIf

Note there's no LocalVariableDeclarationStatement there. That only occurs in the BlockStatement production, defined in section 14.4 of the JLS.

Answer 2

You cannot declare local scope variables in order to directly use it in parent scopes.

When you don't place curly braces, it leads to a single interpreted statement for the condition.

Thus, if this statement is an assignment, clearly this assignment wouldn't be able to be used afterwards since only parent scope (outside curly braces) is the unique way to manipulate it.

Therefore, compiler complains about this situation.

In order to avoid that and without putting curly braces, you have to do as follows:

public void method(String foo) {
        Object bar;
        if(foo instanceof Object)   //redundant here by the way...
          bar = (Object) foo;
          //...
}