Question

What is the mistake in the following code?

 while ((char t==(char) System.in.read())!='0')

Answer 1

You can not declare a new variable in a while loop.

    while (boolean always = true) {
    } // DOES NOT COMPILE!!!

You’d have to declare the variable before and outside of the loop, so perhaps something like this:

    boolean always = true;
    while (always) {
        break;
    } // compiles fine!

    // always is still in scope after the loop!
    always = !always;

In this sense, for loop is unique: you can in fact declare a new local variable whose scope is limited to that loop:

    for (boolean always = true; always; ) {
        break;
    } // compiles fine!

    // always is no longer declared after the loop!
    always = !always; // DOES NOT COMPILE!

That said, looking at what you’re doing, you may want to look at java.util.Scanner. I suspect that it will serve your need much better.

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Example

Here’s an example of using Scanner to read numbers from standard input, terminating at 0. It then prints the sum of those numbers. It handles invalid input gracefully using hasNextInt() instead of Integer.parseInt/NumberFormatException.

    Scanner sc = new Scanner(System.in);
    System.out.println("Enter numbers (0 to end):");
    int sum = 0;
    int number;
    do {
        while (!sc.hasNextInt()) {
            System.out.println("I'm sorry, that's not a number! Try again!");
            sc.next();
        }
        number = sc.nextInt();
        sum += number;
    } while (number != 0);
    System.out.println("The sum of those numbers is " + sum);

Here’s an example session:

Enter numbers (0 to end):
1
3
-1
five
I'm sorry, that's not a number! Try again!
2
0
The sum of those numbers is 5