String-Conditionals in a Java While Loop

Question

I'm trying to prompt the user to give me one of three strings: "Amsterdam," "Lexington," and "Madison." If the user doesn't enter one of those strings, they should be repeatedly prompted until they do so.

When I type a string that's supposed to be acceptable, like "Lexington," I still receive "Please enter a valid city."

Can anyone tell me how the While loop is being run even when I'm negating the conditionals in it?

    public String readCity() {
        String x = keyboard.next();
        while (!x.equals("Amsterdam") || !x.equals("Lexington") || !x.equals("Madison")) {
            System.out.println("Please enter a valid city.");
            x = keyboard.next();
        }
    return x;
    }

You could also store the cities in an ArrayList and just do an if (!cityArray.contains(x)). This would also shorten your if-statement, especially if more cities are added later — Ascalonian
– Ascalonian, Commented Feb 3, 2015 at 20:53
your condition says that if userInput is not equal to A or B or C, please enter valid city. Problem is if you give valid input (eg. A), still it is not equal to B/C and hence still 'or' condition holds true. Change 'or' to 'and' — Rusheel Jain
– Rusheel Jain, Commented Feb 3, 2015 at 20:55
Think about it - if x contains "Amsterdam", then what does !x.equals("Amsterdam") return? What does !x.equals("Lexington") return? What does !x.equals("Madison") return? What happens when you || those things together? — Stack Exchange Broke The Law
– Stack Exchange Broke The Law, Commented Feb 3, 2015 at 21:05

Maroun · Accepted Answer · 2015-02-03 20:46:49Z

9

Refer to De-Morgan's laws:

(NOT a) OR (NOT b)

is actually

NOT (a AND b)

You need to have && instead of ||.

answered Feb 3, 2015 at 20:46

Maroun

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1 Comment

Rohit Jain Over a year ago

It's actually NOT (a AND b)

roeygol · Accepted Answer · 2015-02-03 20:48:21Z

1

You should use AND instead of OR like this:

String x = keyboard.next();
while (!x.equals("Amsterdam") && !x.equals("Lexington") && !x.equals("Madison")) {
    System.out.println("Please enter a valid city.");
    x = keyboard.next();
}

answered Feb 3, 2015 at 20:48

roeygol

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Comments

manzur · Accepted Answer · 2015-02-03 20:51:51Z

0

If you use Java 7 or above I would prefer the following code:

public String readCity() {
    while (true) {
        String x = keyboard.next();

        switch(x) {
            case "Amsterdam":
            case "Lexington":
            case "Madison":
                return x;
            default:
                System.out.println("Please enter a valid city.");
         }
    }
}

answered Feb 3, 2015 at 20:51

manzur

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