CODEWITHSUNDEEP
javafor-loop
Ali Akber
Ali Akber

Reputation: 3800

What's the error in 'for' loop?

Why do the following raise an error?

for(; 0   ;)     System.out.println("guess");  // or
for(;false;)     System.out.println("guess");  // or
for(; 1   ;)     System.out.println("guess");

But the following runs okay (infinitely):

for(;true;)      System.out.println("guess");

Why does it work for true but not for false?

Upvotes: 2

Views: 231

Answers (3)

Tim Hallyburton
Tim Hallyburton

Reputation: 2929

Java requires a boolean as second parameter in your loop header, it evaluates the statement and if the statement returns true the jvm will run the code of the loop-body, not the body will be skipped.

0 and 1 are obviously no booleans nor do they define a statement which could be evaluated (like x < y) and since java is a static and strong typed language (unlike Python or Perl) it cannot cast an int to a boolean, so it crashes.

Edit: If you provide "false" as statement the JVM will notice that the loop-body never can be reached, this will cause a runtime-error.

Upvotes: 1

Nick Louloudakis
Nick Louloudakis

Reputation: 6005

Unlike C, in Java, true and false correspond to boolean type values, where 1 and 0 to int (in fact, in C there is no boolean declarative type, and boolean checks are done based on integer comparisons. In Java, things are distinct).

Upvotes: 0

arshajii
arshajii

Reputation: 129497

The condition (i.e. the bit between the ;s) must be a boolean, so this immediately rules out the first and third variants in your first snippet.

Now, the second variant, in which you have used a boolean, doesn't compile because the compiler realizes that the loop will never be entered and hence issues an error:

Untitled.java:3: error: unreachable statement
        for(;false;)     System.out.println("guess");
                         ^
1 error

Note that the JLS mandates that errors be issued for unreachable statements (see §14.21):

It is a compile-time error if a statement cannot be executed because it is unreachable.

...

The contained statement is reachable iff the for statement is reachable and the condition expression is not a constant expression whose value is false.

Upvotes: 8

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