Welcome to the website of Davis Fiore

java.util.Scanner offers an handy way to read the user input, but it's just as easy to use it incorrectly.

This is the first error:

	Scanner sc = new Scanner(System.in);
			
	// Bad way of reading the input
	System.out.print("Type something:");
	String line = sc.nextLine(); // The program waits here...		
	System.out.println("You typed : " + line);			
		
	sc.close();  

This method doesn't allow to call hasNextInt() or hasNextDouble(), so we can't check for a particular input format, unless we use regular expressions.

In the attempt of fixing it, we can do a second error:

	Scanner sc = new Scanner(System.in);
		
	// Bad way of reading the input
	System.out.print("Type a number:");
	if(sc.hasNextInt()) { // Waits here...
		System.out.println("You typed : " + sc.nextInt());
	}					
	
	sc.close();

This method considers spaces as a delimiter, instead of considering as a single input what we type before pressing enter.

Finally we can do a third error:

	Scanner sc = new Scanner(System.in);
		
	// Bad way of reading the input
	int num = 0;
	System.out.print("Type a number:");
	if(sc.hasNextLine()) { // Waits here...
		num = Integer.parseInt(sc.nextLine());
		System.out.println("You typed : " + num);
	}					

	sc.close();

Here we get very easily a NumberFormatException.

The best way to read a user input is by using hasNextLine() followed by a type check with hasNextInt() or hasNextDouble(), in that way:

	Scanner sc = new Scanner(System.in);
		
	//  Correct way of reading the input
	System.out.print("Type a number:");
	if(sc.hasNextLine()) { // Waits here...
		if (sc.hasNextInt()) // Don't wait here
			System.out.println("You typed the int : " + sc.nextInt());
		else if (sc.hasNextDouble()) // Don't wait here
			System.out.println("You typed the double : " + sc.nextDouble());
		else
			System.out.println("You typed the string : " + sc.next());			
	}
		
	sc.close();	

Note also that you need to check for integers before than doubles.