使用扫描仪读取Java中String的整行

Question

I have this code:

Set<String> uniquePairs = new HashSet<String>();
Scanner sc = new Scanner(System.in);

int t = sc.nextInt();
sc.useDelimiter(System.getProperty("line.separator"));

for(int i=0; i<t ;++i) {
    if(sc.hasNext()) {
        String element = sc.next();
        uniquePairs.add(element);
        System.out.println(uniquePairs.size());
    }
}

Input:

5
john tom
john mary
john tom
mary anna
mary anna

My Output (stdout)

1
2
3
3
4

Expected Output

1
2
2
3
3

Why it differs? Is it due to Scanner$nextLine(); ?

However, I get correct output if I perform following changes:

• Remove the line:

   sc.useDelimiter(System.getProperty("line.separator")); 

• Replace the line:

   String element = sc.next(); 

  with:

   String element = sc.next() + " " + scan.next()j 

Please clarify the same?

Answer 1

Here is the code which is giving you the wrong output:

    Set<String> uniquePairs = new HashSet<String>();
    Scanner sc = new Scanner(System.in);
    int t = sc.nextInt();
    sc.useDelimiter(System.getProperty("line.separator"));
    for(int i=0; i<t ;++i) {
      if(sc.hasNextLine()) {
        String element = sc.nextLine();
        uniquePairs.add(element);
        System.out.println(uniquePairs.size());
      }

Output:
1
2
3
3
4

---

The issue is once you read the int value, new line character is left behind and gets read in the loop and produces wrong result. You can read that new line character using a nextLine() call and ignore it. Then use the nextLine() method as per the requirement.

Here is the code producing the correct result.

    Set<String> uniquePairs = new HashSet<String>();
    Scanner sc = new Scanner(System.in);
    int t = sc.nextInt();
    sc.nextLine();    // Ignore the next line char.

    for(int i=0; i<t ;++i) {
      if(sc.hasNextLine()) {
        String element = sc.nextLine();
        uniquePairs.add(element);
        System.out.println(uniquePairs.size());
      }

Output:
1
2
2
3
3