Enthuware

Can you explain this?
The above code compiles and result in an infinite loop when run.
But when a semi-colon is added at the end of 'for loop', then the code fails to compile.

error: unreachable statement
print("hi");
^

How does the semi-colon affects?

That is just how the compiler parses the code. A semicolon is considered an empty statement but it is still a statement. Since the for in this case does not have a code block i.e. { }, the first statement after the for declaration becomes the part of the for block. Thus, for(;;); is equivalent to :
for(;;){
;
}

Now, since this for loop never ends, any statement after the for block i.e. System.out.println("Hi") becomes unreachable. Without the semicolon, the print statement becomes the part of the for loop.

HTH,
Paul.

i can't get it

for(;;){     if(Math.random()<.05) break; }

How excalty i can use .05 value?

Math.random returns a double, so the expression is only really comparing two doubles.

I am a little bit confused on this part: I had thought the third spot of a for loop, or in this case where Math.random() currently is at, should have some sort of increment/decreasing operation going on, like i++.
But in this case, it is just returning a double, not increase or decreasing anything.
How is this valid?

The third spot is commonly used for incrementing the loop interation variable but this is not a must. Technically, you can have any expression (and even multiple expressions separated by a comma) that is given in the list below.

Assignment
PreIncrementExpression
PreDecrementExpression
PostIncrementExpression
PostDecrementExpression
MethodInvocation
ClassInstanceCreationExpression

For example, the following examples are valid -

for(int i=0; i<10; System.out.println(i),System.out.println("aaa")); //This will keep looping for ever because i is not being incremented at all.

for(int i=0; i<10; i=20, System.out.println(i),System.out.println("aaa")); //This will loop just once.


HTH,
Paul.

is valid, while is not valid.

The explanation for this is that in the third part of a for loop only "Assignment, PreIncrementExpression, PreDecrementExpression, PostIncrementExpression, PostDecrementExpression, MethodInvocation, and ClassInstanceExpression" are valid.

Therefore, Math.random<0.5 which is a boolean, is not valid.

However, the result of a MethodInvocation can be a boolean also.

So, why is a boolean (Math.random<0.5) not valid for the third part of a for loop, while a MethodInvocation (which could result in a boolean) is valid?

Because language designers decided to allow method invocation but not an expression

I think the way the question was written is a little ambiguous.
What is a "valid" for loop? One that compiles? Or one that does not loop infinitely?

I read it the way that it will compile, in which case option 4 would be true.

The problem statement has now been updated to:
"Which of the following code fragments compile without any error?"

thank you for your feedback!

I find it great how quickly you guys react and adapt. Good work. Thanks.