This tutorial is a little more advanced. To use this with a scanner, you have to use also a new part of the theory. I will give a short explanation about it:

First I will show you how to use it then add the last part of this page/article I will explain the “new part”, that is the try and catch statement.

Alright, to get input from a file, you first have to import:

• class scanner
• class File
  • We use this as a tool to give the content of the file to the scanner.

Oke,

You will give everything in your file to your scanner, therefore you need to use the Class File. First, save everything of the file in the class File. This is the way to do this:

• package P;
  
  import java.io.File;
  
  public class ReadFile {
  
  	void start() {
  		File file = new File("/Users/username/Downloads/hoi.txt");
  	}
  
  	public static void main(String[] args) {
  		new ReadFile().start();
  	}
  }

• So you name your file: “file”, and give as input the location(User/username/Downloads/) and the name of the file (hoi.txt)

Now you can give this variable “file” to your scanner

package P;

import java.io.File;
import java.util.Scanner;

public class ReadFile {

	void start() {
		File file = new File("/Users/username/Downloads/hoi.txt");
		Scanner in = new Scanner(file);
	}

	public static void main(String[] args) {
		new ReadFile().start();
	}
}

We will get an exception at this point because as you should see will your program give an error, and when you click on it you see two options: “add throws declaration” or “surround with try/catch”.

Try and catch

In short, both options will do more or less the same thing. It tries your commands in the method and if there is an error the program catches the error. You will use this when you:

• don’t want your program to crash
• when you want your program to crash but want to control which error the program gives as output

For now, it is not really important. I will give three options for now:

1. just print the standard error

package P;

import java.io.File;
import java.util.Scanner;

public class ReadFile {

	void start() throws FileNotFoundException {
		File file = new File("/Users/username/Downloads/hoi.txt");
		Scanner in = new Scanner(file);
	}

	public static void main(String[] args) throws FileNotFoundException {
		new ReadFile().start();
	}
}

add after every method: throws FileNotFoundException

2. print a personalised error

package P;

import java.io.File;
import java.util.Scanner;

public class ReadFile {

	void start() {
		File file = new File("/Users/username/Downloads/hoi.txt");
		try {
			Scanner in = new Scanner(file);
		} catch (Exception e) {
			throw new ArithmeticException("custom error");
		}
	}
	public static void main(String[] args) {
		new ReadFile().start();
	}
}

3. print an error but don’t let the program crash

package P;

import java.io.File;
import java.io.PrintStream;
import java.util.Scanner;

public class ReadFile {
	
	PrintStream out;
	
	ReadFile() {
		out = new PrintStream(System.out);
	}

	void start() {
		readFile();
		calculateSum();
	}

	void readFile() {
		File file = new File("/Users/username/Downloads/hoi.txt");
		try {
			Scanner in = new Scanner(file);
		} catch (Exception e) {
			out.println("File doesn't exist");
		}
	}
	
	void calculateSum() {
		int sum = 1 + 2;
		out.println(sum);
	}

	public static void main(String[] args) {
		new ReadFile().start();
	}
}

This is a bigger program as the programs of example 1 and 2. Now you could see that if you give a wrong file as input the rest of the program will still continue.