Throw and Throws

Video Materials

Beyond catching exceptions, there are a few other important concepts to learn related to exceptions. Let’s go over a couple of them.

Throws

When the Java compiler finds a checked exception, we must either surround that code in a Try-Catch statement, or we must declare the exception to be thrown by adding throws to our method declaration, followed by the exception type.

Here’s an example:

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

public class Throw{
  
  public static void main(String[] args){
    
    Scanner reader;
    
    reader = new Scanner(new File("input.txt"));
    int x = Integer.parseInt(reader.nextLine());
    System.out.println(x);
    
  }
}

The code above handles user inputs. However, it does not includes the throws keyword. So, when we try to compile this code, we’ll get the following error message.

8j-except/Throw.java:11: error: unreported exception FileNotFoundException; must be caught or declared to be thrown
      reader = new Scanner(new File(args[0]));
               ^
1 error

To allow this code to compile, we must simply add throws FileNotFoundException to the end of our method declaration to tell the compiler that we aren’t going to handle that exception in our code. We’ll also have to add import java.io.FileNotFoundException to the top of the file. Notice that we aren’t using throws Exception this time, since we now know more about how exceptions work. As we’ll discuss later in this chapter, it is always better to include a specific exception instead of a generic one whenever possible.

Of course, we may also want to use a Try-Catch statement to handle this error directly in our code. We’ll discuss those tradeoffs later as well.

Throw

We can also generate our own exceptions using the throw keyword. This allows us to create new exceptions whenever needed. Let’s look at an example:

import java.util.Scanner;
import java.io.File;
import java.lang.ArithmeticException;
import java.io.FileNotFoundException;

public class Throw{
  
  public static void main(String[] args) throws FileNotFoundException{
    
    Scanner reader;
    
    reader = new Scanner(new File("input.txt"));
    
    double x = Double.parseDouble(reader.nextLine());
    double y = Double.parseDouble(reader.nextLine());
    
    if(y == 0.0){
      throw new ArithmeticException("Divide by Zero!");
    }else{
      System.out.println(x / y);
    }
    
  }
}

In this code, we are asking the user to input two floating point numbers. Then, we will output the first number divided by the second. However, we want to avoid the situation where the second number, the divisor, is $0$. Since Java doesn’t generate an ArithmeticException when dividing by $0$ using floating point numbers, we can do that ourselves using an If-Then statement and the throw keyword.

Following the throw keyword, we must create a new Exception. In this case, we are creating a new ArithmeticException, but any of the exception types we’ve learned about so far will work the same way. Inside of the parentheses, we can provide a helpful error message to go along with this exception.

Try It!

Let’s see if we can use these keywords in another example. We’ll start with this code:

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

public class Throw{
  
  public static void main(String[] args){
    
    Scanner reader;
    
    reader = new Scanner(new File(args[0]));
   
    // *** ADD CODE HERE ***
    
  }
}

Place this code in Throw.java and modify it to do the following:

1. If the file given in args[0] does not exist, it should throw a FileNotFoundException. We’ll need to add the throws keyword and additional information in the correct place.
2. It should read a single string of input from the user. To make this simple, don’t forget to use the trim() method to remove any extra whitespace!
3. If the string does not begin with a capital letter, it should throw a new IllegalArgumentException that contains the error message “Proper Capitalization Required!”
4. Of course, if the string is empty, it should throw a StringIndexOutOfBoundsException when it tries to access the first character of the string. (Hint: Will the String.charAt() method throw this exception for us? We can check the Java Documentation to find out)
5. Otherwise, it should print the string to the terminal.

java Throw input.txt

java Throw input.txt output.txt