Scope of variable inside do-while loop

问题

I'm trying to loop through an entire program if the user inputs "y" or "Y" when they are prompted at the end of the program, but I'm getting a "cannot find symbol - variable response" error since the variable I declared is inside the do-while loop, therefore I suppose I can't use it in the condition of the do-while loop. Is there any way to get around this problem and still be able to loop through the program if the user inputs a "y" or "Y"? This is all in the main method and getFileRunStats is the method that contains all other methods from the program.

Here's the relevant code:

public static void main(String[] args) throws IOException {
    System.out.println("A program to analyze home field advantage in sports.");
    System.out.println();        
    Scanner keyboard = new Scanner(System.in);

do{   
    getFileRunStats(keyboard);

    System.out.println("Do you want to check another data set?");
    System.out.print("Enter Y or y for to analyze another file, anything else to quit: ");
    String response = keyboard.nextLine();
}while(response == "Y" || response == "y");


}

回答1:

You can just remove the statement, and add it to the loop condition.

do {
} while (keyboard.nextLine().equalsIgnoreCase("y"));

回答2:

Answer by @ergonaut is the best solution for your exact problem:

You can just remove the statement, and add it to the loop condition.

do {

} while (keyboard.nextLine().equalsIgnoreCase("y"));

Notice that the code also fixed the error you had by using an equals method, not the == operator.

However, a more general solution for how to control a do-while loop using valued defined inside the loop, you have two choices:

// Define variable outside, no need to initialize it
String response;
do {
    // code here
    response = keyboard.nextLine();
} while (response.equals("Y") || response.equals("y"));

// Use break inside an infinite loop
for (;;) { // or   while (true) { ... }   or   do { ... } while (true)
    // code here
    String response = keyboard.nextLine();
    if (! response.equals("Y") && ! response.equals("y"))
        break;
}

回答3:

just providing a more extensible approach to your problem because it allows you to dynamically add more options to your program in future.

public static void main(String[] args) {
    System.out.println("A program to analyze home field advantage in sports.");
    System.out.println();
    Scanner keyboard = new Scanner(System.in);

    boolean quit = false;
    do{
        System.out.println("Do you want to check another data set?");
        System.out.print("Enter Y or y for to analyze another file ");
        System.out.print("Enter X to do X");
        System.out.print("Enter Z to do Z");
        String response = getUserInput(keyboard);

        if (response.equalsIgnoreCase("X"))
            doX();
        else if (response.equalsIgnoreCase("Y"))
            doY();
        else if (response.equalsIgnoreCase("Z"))
            doZ();
        else
            quit = true;

    }while(!quit);
}

private static String getUserInput(Scanner scanner) {
    return scanner.nextLine();
}

private static void doX() {
    System.out.println("X");
}

private static void doY() {
    System.out.println("Y");
}

private static void doZ() {
    System.out.println("Z");
}

}

回答4:

Just declare the "response" varialbe outside of do-while loop, like this:

String reponse = null;
do{   
    getFileRunStats(keyboard);

    System.out.println("Do you want to check another data set?");
    System.out.print("Enter Y or y for to analyze another file, anything else to quit: ");
    response = keyboard.nextLine();
}while(response == "Y" || response == "y");


}

来源：https://stackoverflow.com/questions/33324653/scope-of-variable-inside-do-while-loop

标签

java

if-statement

while-loop

scope

do-while