About Question enthuware.ocajp.i.v8.2.1059 :

[mj.anjuthan]

Can you explain this?

Code: Select all

public static void main(String[] args){
for(;;)
System.out.println("Hi");
}

The above code compiles and result in an infinite loop when run.

Code: Select all

public static void main(String[] args){
for(;;);
System.out.println("Hi");
}

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?

[admin]

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.

[shownofear]

i can't get it

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

How excalty i can use .05 value?

[admin]

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

[dxie1154]

I am a little bit confused on this part:

Code: Select all

 for(;;Math.random()){
System.out.println("true");
}

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?

[admin]

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.

[Kevin30]

Code: Select all

for(;;Math.random()){
    System.out.println("true");
}

is valid, while

Code: Select all

for(;;Math.random()<0.5){
    System.out.println("true");
}

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?

[admin]

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