Scanner.nextLine() 返回 null

Question

I had written this code :

class PerfectPair {
    public static void main(String args[]) throws IOException{
        Scanner scan = new Scanner(System.in);
        int test=scan.nextInt();
        StringBuffer b=null;
        String a=null;
        while(test!=0){
            a=scan.nextLine();
            b=new StringBuffer(scan.nextLine());
            System.out.println(a);
            String reverse=b.reverse().toString();
            if((a.length()!=b.length()) || !(reverse.equals(a))){
                System.out.println("No");
            } 
            else 
            {   
                if((a.length()==b.length()) && (reverse.equals(a))) System.out.println("Yes");
            }

            --test;
        }
    }
}

Input which is entered:

1
aa
ab

but the value of variable a is null ..WHY?? Please explain .Also please correct the code so that it reads full input.

Answer 1

That's because you are entered 1 followed by an enter . So your nextLine method call just reads return key while nextInt just reads integer value ignoring the return key. To avoid this issue:

Just after reading input, you call something like:

int test=scan.nextInt();
scan.nextLine();//to read the return key.

If you want to avoid that as well, then i would suggest, you read the whole line and then convert it to integer. Some thing like:

int test=Integer.parseInt(scan.nexLine());

Answer 2

when you use

int test=scan.nextInt();

the carriage return that you push after entering the Integer does not get read off the input stream, so what you can do is

int test=scan.nextInt();
scan.nextLine();