Loop not breaking when using user input

Question

public static void main(String[] args) {
    Scanner input = new Scanner(System.in);
    Queue que = new Queue(50);

    System.out.println("Noki's Calculator");
    System.out.println("Choose an operation (*, /, +, -)");
    String inputOp = input.nextLine();
    System.out.println(" ");

    System.out.println("Now please enter your numbers. When finished, simply press enter.");
    boolean cont = true;
    while(true){
        System.out.print(" ::  ");
        String j = input.next();
        if(j != null) {
            int numb = Integer.parseInt(j);
            que.insert(numb);
        } else if (j==null) {
            input.close();
            break;
        }
    }

    que.view(); // PROBLEM: never executes this line

}

My code won't break out of the while loop, instead still asks for user input. When I press enter, it doesn't print "::" again, but never executes que.view()

then debug the code. j can never be null. When you just press enter, j is "". — f1sh
– f1sh, Commented Aug 28, 2018 at 19:30
Check your assumptions. Fire up your debugger when the code fails to meet your expectations. — duffymo
– duffymo, Commented Aug 28, 2018 at 19:34
Please do not edit your question to an extent that invalidates all already given answers. Instead, post a new question. — Zabuzard
– Zabuzard, Commented Aug 28, 2018 at 20:28

Pallav Kabra · Accepted Answer · 2018-08-31 18:25:55Z

2

Use nextLine() instead of next() like below,

String j = input.nextLine();

edited Aug 31, 2018 at 18:25

answered Aug 28, 2018 at 19:46

Pallav Kabra

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Comments

Shehan Charuka · Accepted Answer · 2018-08-28 20:31:13Z

1

According to your implementation 'j' will never be null. When you hit enter "" will be assigned to j. You may need to change your logic as follows; Use input.nextLine() instead of input.next()

if(!(j.isEmpty())) {
    int numb = Integer.parseInt(j);
    que.insert(numb);
} else {
    input.close();
    break;
}

edited Aug 28, 2018 at 20:31

answered Aug 28, 2018 at 19:39

Shehan Charuka

261 silver badge7 bronze badges

4 Comments

Andy Turner Over a year ago

"j' will never be null" so why check if it is null, then?

Ben McCalmon Over a year ago

I updated my code; the problem still persists. Is there something I can't catch?

Shehan Charuka Over a year ago

@Ben Use input.nextLine() instead of input.next()

Zabuzard Over a year ago

@ShehanCharuka Note that you can use j.isEmpty() instead of checking against "" and the length.

cs.spathis · Accepted Answer · 2018-08-29 20:09:54Z

0

input.next() does not return "null" when you just press "enter".

change

else if (j==null)

to

else if (j.equals(""))

Edit: this doesn't exactly answer the question but it has already been answered so i am leaving it as is.

edited Aug 29, 2018 at 20:09

answered Aug 28, 2018 at 19:38

cs.spathis

13 bronze badges

4 Comments

Poger Over a year ago

You are right about input.next() never returning null. However, please pay attention that while having if(j != null) the else if will never be executed. Please, consider updating your answer.

Ben McCalmon Over a year ago

I updated my code; the problem still persists. Is there something I can't catch?

Zabuzard Over a year ago

@cs.spathis Note that there is also String#isEmpty. May be more readable.

cs.spathis Over a year ago

@Poger You are right i should have read the question more carefully

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