Use the shortcut ctrl+shft+o in Eclipse. It automatically imports whatever classes needed and suggests you with possible imports if conflict occurs.

About your "bug":
From experience i can tell you that eclipse has its special magical moments when something seems wrong to it and then out of a sudden it accepts it. This is because sometimes the background processes of eclipse are a little slow and it takes some time until it realizes what is actually happening. My recommendation here is for imports use Strg+O or autocompletion Strg+Space. This way you tell eclipse to search for what you need and choose from its offers or it just auto picks it if it sees only one possible fit. Then eclipse knows straight away what you are using and it doesn't have to wait for you to stop typing and run a background process to find out what you are trying to do.

About your programm:
For reading/writing from/to the console java uses InputStreams and OutputStreams. System.in points to the InputStream used to read from the console. System.out points to the OutputStream used to write on the console.

System.in alone would already be enough to read from the console. However a simple InputStream has very basic methods to read from the stream. It is more comfortable to use a wrapper class like a Scanner or a BufferedReader.

Scanner provides you with multiple methods to read from an InputStream, of which you use nextLine(). This method blocks the process until a linebreak (enter) was typed on the console. Then it returns everything that was typed as a String.

This String you can use now for multiple things. You can print them back on the console like you do at the moment with System.out.println(...) or you can use them for further operations. For that i would recommend you to save the returned value in a variable

Scanner scan = new Scanner(System.in);
String readLine = scan.nextLine();
scan.close();

With the value you then can do further operations like converting it to an Integer for example

Integer convertedNumber = Integer.parseInt(readLine);

Which you then could use to call your function isPrime(...)

Scanner is a Java class. All classes in Java should start with a capital letter.

Change

java.util.scanner;

to

java.util.Scanner;

I guess you are using an IDE (like Netbeans or eclipse) which allows you to run the code even if certain classes are not compilable. During the application's runtime, if you access this class it would lead to this exception.

Solution :- Simply Clean Your Project and Build and Run Then Again.

You're outputting the wrong variable. Try switching userinput (Scanner object) with x (First line of the input). Like this:

System.out.print("Hello " + x);

Your probably not getting focus on the console. AFter you run the java program switch to the console.