do while loop question

[prateek shaw]

Hi All,

I have one small doubt about do while loop.
As all of know that we can use do while like below

do
System.out.println("doo");
while(false);

It is not mandatory to have {} with do if you are having one statement.

do 
int p=10;
while(false);

But above code does not compile.it will give below compile error.
error: variable declaration not allowed here
int p=10;

Before checking this point my understanding was if do have one and only one statement then we can omit the {}. But now i need to remember this as below
If do block has only one statement and this statement should not declare new variable then we can omit the {}.
However l liked this rule , because it does not make any sense do declare new variable if you can not able to use that.

Is my understanding is correct ?

[Roel De Nijs]

prateek shaw wrote:Is my understanding is correct ?

Yes, you are correct!

Please note that this rule is not restricted to a do while statement. It also applies to other flow control statements like if, for, while,... too. Here are a few code snippets which don't compile

if (1 < 2)
  int p1=10;

for (int i<0 ; i<10 ; i++)
  int p2=10;

if (true)
  System.out.println("true");
else
  int p3=10;

for (String s : Arrays.asList("a", "b", "c"))
  int p4=10;

Hope it helps!
Kind regards,
Roel

[Campbell Ritchie]

You need to look up the grammar to understand that. Look in the Java® Language Specification (=JLS), and it says:-

DoStatement:
    do Statement while ( Expression ) ;

That means that whatever goes after do and before while must fulfil the requirments to be a statement. Something like
int p = 10;
is not a statement, but a declaration and initialisation. Please work out what you can do with the declaration, and you shoul‍d find an explanation why the grammar doesn't allow declarations there. I think you will find the same if you look up if or while statements in the JLS, too.

[prateek shaw]

Thanks for answering my question. However i know java specification document is there but i feel it is tough to understand.

[Campbell Ritchie]

That's a pleasure I am afraid the JLS can be very difficult to understand, yes.