Compiler error when declaring a variable inside if condition and no curly braces
为什么这个第一if编译阱和第二失败？
if(proceed) {int i;} // This compiles fine.
if(proceed) int i;// This gives an error. (Syntax error on token ")", { expected after this token)
Because the language spec says so:
http://docs.oracle.com/javase/specs/jls/se7/html/jls-6.html
A declaration introduces an entity into a program and includes an identifier (§3.8) that can be used in a name to refer to this entity. A declared entity is one of the following:
...
A local variable, one of the following:
* A local variable declared in a block (§14.4)
* A local variable declared in a for statement (§14.14)
Your first example is declaring i inside a block (denoted by curly braces). Your second isn't, nor is it a for statement.
Edited to add: Which just makes commons sense. If it were allowed, it would be useless. It would immediately fall out of scope.
From the Java Language Spec.
Block:
{ BlockStatementsopt }
BlockStatements:
BlockStatement
BlockStatements BlockStatement
BlockStatement:
LocalVariableDeclarationStatement
ClassDeclaration
Statement
and
IfThenStatement:
if ( Expression ) Statement
It seems that int i is a LocalVariableDeclarationStatement , not a Statement . So it doesn't work.
This is because it would not be useful code. If you have an if-statement without curly braces ({}), only the first line / statement after the if is executed. So if you only declare a local variable, it cannot be used anywhere else. So declaring it is absolutely superfluous.
if(proceed){
int i= 0;
// variable i can be used here
//...
}
if (proceed) int i; // i can not be used anywhere as it is a local variable
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