why loop is not breaking here?

Question

class hashmaps{
public static void main(String args[]){
    Scanner s = new Scanner(System.in);
    LinkedHashMap<String,Integer> hm = new LinkedHashMap<String,Integer>();
    while(true){
        String a=s.next();
        if(a.equals("") || a==null){
            break;
        }
        else if(!hm.containsKey(a)){
            hm.put(a,1);
        }
        else{
            hm.put(a,hm.get(a)+1);
        }
    }
    System.out.println(hm);
}
}

I'm trying to take infinite values from the user and trying to print the values stored in the hashmap when the user enters an empty value or string but the loop is not breaking when I enter an empty value in the console.

azro · Accepted Answer · 2020-09-06T10:04:58.777

You need to use nextLine(); instead of next() which waits to get non-blank content, so it doesn't stop when it sees a newline unlike nextLine. Also

the null check is useless
you can use isEmpty
improve the increment with merge

Scanner s = new Scanner(System.in);
LinkedHashMap<String, Integer> hm = new LinkedHashMap<>();
while (true) {
    System.out.print("Give a word: ");
    String a = s.nextLine();
    if (a.isEmpty()) {
        break;
    }
    hm.merge(a, 1, Integer::sum);
}
System.out.println(hm);

The hm.merge(a, 1, Integer::sum); means

for key a
put the value 1
if a value already exists, apply Integer::sum, same as (prevVal, value) -> prevVal + value

score 1 · Answer 2 · answered Sep 06 '20 at 09:55

You need to use s.nextline() instead of s.next(), check the modified code below:

public class hashmaps{
public static void main(String args[]){
    Scanner s = new Scanner(System.in);
    LinkedHashMap<String,Integer> hm = new LinkedHashMap<String,Integer>();
    while(true){
        String a=s.nextLine();
        if(a==null || a.equals("")){
            break;
        }
        else if(!hm.containsKey(a)){
            hm.put(a,1);
        }
        else{
            hm.put(a,hm.get(a)+1);
        }
    }
    System.out.println(hm);
} }

score 0 · Answer 3 · answered Sep 06 '20 at 11:15

One of the main problem is that you're using s.next() instead of s.nextLine(): the difference between the two method is that Scanner.next(), I quote Java documentation,

Finds and returns the next complete token from this scanner. A complete token is preceded and followed by input that matches the delimiter pattern. This method may block while waiting for input to scan, even if a previous invocation of hasNext() returned true.

Scanner.nextLine(), instead,

Advances this scanner past the current line and returns the input that was skipped. This method returns the rest of the current line, excluding any line separator at the end. The position is set to the beginning of the next line.

Beyond that, when you create a scanner, you have to remind to always close it when you finished, because it cause a memory leak if you aren't using it anymore. Also, I strongly suggest to use, whenever you have to compare two strings, the equals() method, but in this specific case you can also use the more practical isEmpty(). I suggest you delve into the subject by reading this awesome answer on the topic and the String documentation. In conclusion the code should be something like this:


public static void main(String args[]){
        Scanner s = new Scanner(System.in);
        LinkedHashMap<String,Integer> hm = new LinkedHashMap<String,Integer>();
        while(true){
            String a=s.nextLine();
            if(a.isEmpty())
                break;
            else if(!hm.containsKey(a))
                hm.put(a,1);
            else
                hm.put(a,hm.get(a)+1);
        }
        s.close();
        System.out.println(hm);
    }

why loop is not breaking here?

3 Answers3