Java variable inside a loop

Question

my first question, I'm a beginner.

Please refer to the below code, I tried to define j twice and java compiler give me an error, I totally understand.

My question here is if I cannot define a variable twice, why the "char c = ..." inside the loop is working. from the logic, when the loop body execute first time, the char c variable is defined, when the loop body execute the second time, because the char c is already defined, it should throw an error, but it didn't. Why?

public class test{
    public static void main(String[] args){
        int j=1;
        for (int i=0; i<10; ++i){
            char c = (char)(Math.random()*26+97);
            System.out.println(i+1+" = "+c);
        }
        int j=2;
    }
}

Thanks

Your `char c` has its scope limited to the _current_ iteration of the loop, and will be dismissed afterwards. There is no duplicate declaration in this case. — Arnaud, Jul 13 '17 at 12:53
You declare variables at *compile* time. It does not matter how often the code is *executed*. — Timothy Truckle, Jul 13 '17 at 12:54
You could declare your variable as `final`, too, inside a loop. Those declarations live just inside the block, that is, every iteration has to recreate them... — Usagi Miyamoto, Jul 13 '17 at 12:54
[Found it](https://stackoverflow.com/questions/26692795/why-can-i-create-multiple-instances-with-the-same-name-in-a-loop), added it to the one RealSkeptic flagged. — T.J. Crowder, Jul 13 '17 at 13:02

score 3 · Accepted Answer · edited Jul 13 '17 at 12:57

3

why the "char c = ..." inside the loop is working

Because the for loop has its own scope, so there will be a different variable in each iteration.

So in total your code will create 10 different variables.

edited Jul 13 '17 at 12:57

T.J. Crowder

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answered Jul 13 '17 at 12:54

cнŝdk

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score 1 · Answer 2 · answered Jul 13 '17 at 12:55

1

The variable c lives only in the scope of the for loop. When the iteration ends the variable dies, so it can be redeclared in the next iteration.

answered Jul 13 '17 at 12:55

Guy

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score 0 · Answer 3 · answered Jul 13 '17 at 12:54

0

Declarations are checked by the compiler. They are executed during, well, execution.

The compiler sees that the variable is declared inside a scope, and that's all it cares about. It doesn't check how often the code inside that scope is executed.

answered Jul 13 '17 at 12:54

S.L. Barth is on codidact.com

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score 0 · Answer 4 · answered Jul 13 '17 at 13:20

Whenever you declare a variable within braces that variable exists only inside those braces, that is, any code outside the braces cannot access the variable.

if(true) {
    char c = 'x';
}
 c = 'y';    // Error

This is because once outside the scope, the variable no longer exists.

This is what is happening in your code

for (int  i=0; i < 5; i++)
{
   char c = 'x'; // variable created on each iteration hence no error
}
 c = 'y'; // error since variable gets destroyed

Java variable inside a loop

4 Answers4