Question

When I need to close a Scanner by method close(), I have a code as follows, could any one explain why I need to check if(input != null) please ?

Scanner input = new Scanner(new File("System.txt"));

...

Public void closeFile() {
   if(input != null) {
          input.close();
   }
}

Answer 1

could any one explain why I need to check if(input != null) please ?

its just a null check if input is null it’d throw NullPointerException.

consider below code:

    Scanner input = new Scanner(new File("System.txt"));
    public void someMethod(){
      input = doSomething(); //say doSomething() returns null
    }
    Public void closeFile()
   {
   if(input != null)
      input.close();
    }

now, as input is null, if you try to call close() it’d throw NPE>