在 java 中编写线程同步代码时，我遇到了以下错误

Question

So I am a beginner in java.

Yesterday while writing a code for thread synchronization I came across an error java.util.NoSuchElementException . Code and the error are specified below. Here the shared variable among threads is a . From what I can guess from the error the variable b is having problems in taking the second value.

        class Sum
        {
            synchronized void addition(int a)
            {
                Scanner sc = new Scanner(System.in);
                System.out.println("enter vale of b: ");
                int b = sc.nextInt();
                sc.close();
                int c = a + b;
                System.out.println(c);
            }
        }
        class ThreadP1 extends Thread
        {
            Sum s1;
            ThreadP1(Sum s1)
            {
                this.s1 = s1;
            }
            public void run()
            {
                Scanner sc1 = new Scanner(System.in);
                System.out.println("enter element a: ");
                s1.addition(sc1.nextInt());
                sc1.close();
            }
        }
        class ThreadP2 extends Thread
        {
            Sum s1;
            ThreadP2(Sum s1)
            {
                this.s1 = s1;
            }
            public void run()
            {
                Scanner sc2 = new Scanner(System.in);
                System.out.println("enter element a: ");
                s1.addition(sc2.nextInt());
                sc2.close();
            }
        }
    
    
    
        public class Synchronisation1 
        {
            public static void main(String args[])
            {
                Sum obj3 = new Sum();
                ThreadP1 t12 = new ThreadP1(obj3);
                t12.start();
                ThreadP2 t13 = new ThreadP2(obj3);
                t13.start();
            }
        }

Error

>Thread-1 java.util.NoSuchElementException  
        >>at java.base/java.util.Scanner.throwFor(Scanner.java:937)
        >>at java.base/java.util.Scanner.next(Scanner.java:1594)   
        >>at java.base/java.util.Scanner.nextInt(Scanner.java:2258)
        >>at java.base/java.util.Scanner.nextInt(Scanner.java:2212)
        >>at Sum.addition(Synchronisation1.java:8)
        >>at ThreadP2.run(Synchronisation1.java:40)

Answer 1

As commented, by closing the Scanner , you close the System.in . The latter scanner will get an exception when it realizes System.in is closed.

One way to fix it will be not to close the scanner.

By the way, ThreadP1 , ThreadP2 seem to do the same thing. Consider creating 2 instances of the same class instead.