Java question - for (;;)

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tchart

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Topic # 150548 26-Jul-2014 13:02

Im looking at some Java code. Im no Java expert.

There is a for loop that has this syntax;

for (;;)
{
//Do some stuff
}

What does this achieve? Surely it doesn't loop (ie it just runs the code once)?

gzt

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Reply # 1096094 26-Jul-2014 13:05

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Infinite loop. Within the loop there will be a condition that breaks the loop.

zaptor

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Reply # 1096123 26-Jul-2014 13:59

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I would not recommend using a for construct for infinite looping. I realize it's used quite often - sadly.

Of the 3 general looping constructs used in procedural programming (for, while, do-while), I'd recommend while as a better option. I don't like using For loops for indeterminate (non-finite) loops. It just makes the code harder to read. All my own opinion of course.

Getsited

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Reply # 1096151 26-Jul-2014 14:57

I can't remember the syntax but there will probably be a "break" there somewhere to exit the loop after some condition is met.

tchart

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Reply # 1096266 26-Jul-2014 17:50

Ah right, thanks guys. I wouldnt have thought it was an infinite loop.

Yes, there is a return statement in the loop with no condition. The code is very odd.

for (;;)
{
sendCommandToRemote(9, str2);
sendPureDatToRemote(str1.getBytes());
return;
str1 = "-1";
int i = str1.length();
str2 = "bnsrv_alarm mtk_bnapk -1 0 " + String.valueOf(i) + " ";
}

Behodar

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Reply # 1096269 26-Jul-2014 18:11

I don't know any Java, but at a glance it looks like it just calls the two "send" methods then leaves the loop... so why have a loop at all? *confused*

tchart

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Reply # 1096309 26-Jul-2014 20:34

Behodar: I don't know any Java, but at a glance it looks like it just calls the two "send" methods then leaves the loop... so why have a loop at all? *confused*

Exactly! So why would there be code after the return statement in the same loop. Very odd syntax.

redmouse

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Reply # 1096336 26-Jul-2014 20:56

I'm pretty sure that will not compile. Similar...

package example;

public class Dingo
{
public void thing()
{
for (;;)
{
System.out.println("hello");
return;
System.out.println("goodbye");
}
}

public static void main(String args[])
{
new Dingo().thing();
}
}

Compile...
javac example/Dingo.java

Yields...
example/Dingo.java:11: error: unreachable statement System.out.println("goodbye");
^ 1 error

zaptor

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Reply # 1096349 26-Jul-2014 21:24

tchart: The code is very odd.

for (;;)
{
sendCommandToRemote(9, str2);
sendPureDatToRemote(str1.getBytes());
return;
str1 = "-1";
int i = str1.length();
str2 = "bnsrv_alarm mtk_bnapk -1 0 " + String.valueOf(i) + " ";
}

Yep. That is indeed some bizarre code.

gzt

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Reply # 1096413 27-Jul-2014 00:13

To state the obvious:

a) 'Break' has not been used in this loop.
b) Technically 'Return' does not (simply) break the loop. It returns control to the calling method.
c) The statements after 'return;' are not executed

And a mildly insane possibility: There is recursion somewhere else in the code which refers to this structure in a very bizarre way.

Some less obvious possibilities: The code may originally have been complied with a not very standard compiler or compiled with some or many error and warning suppress flags.

gzt

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Reply # 1096414 27-Jul-2014 00:14

Questions:

d) So is this actual running code?
e) Does this compile on your system without warnings?
f) Or are you a student and this is some crazy example code you are supposed to sort out for a project task? : )