Exceptions Handling in Java

Exceptions are interruptions in the normal flow of program thanks to an error and other exceptional issue. If you don't handle it properly, it can cause your entire program to terminate and output an error.

Examples of when exceptions can occur:

• when a divide by zero operation was attempted
• a network connection being interrupted
• a user entering incorrect data

In this lesson, we'll learn how to safely and gracefully handle an exception so that your program can continue running properly.

Try Catch Block

Exceptions in Java are handled using the try-catch block. The try block is where the code that could throw an exception is run, whereas the catch block is the code that you want to run if an exception is indeed thrown.

This is the format for catching exceptions:

try {
    // code to try
} catch(Exception e) {
    // code to handle exception
}

Now let's look at a basic example:

public class HelloWorld {

    public static void main(String[] args) {
        try {
            int test = 1 / 0;
        } catch(ArithmeticException e) {
            System.out.println(e);
        }
    }
}

java.lang.ArithmeticException: / by zero

Finally Block

When you want to run code no matter if an exception was thrown or not, you can use the finally block:

public class Main {

    public static void main(String[] args) {
        try {
            int test = 1 / 0;
        } catch(ArithmeticException e) {
            System.out.println("An ArithmeticException was thrown!");
        } finally {
            System.out.println("Moving on!");
        }
    }
}

An ArithmeticException was thrown!
Moving on!

Multiple Exceptions

You can handle multiple exceptions by chaining the catch blocks:

public class HelloWorld {

    public static void main(String[] args) {
        try {
            String str = null;
            str.length();
        } catch(ArithmeticException e) {
            System.out.println("An ArithmeticException was thrown!");
        } catch(NullPointerException e) {
            System.out.println("A NullPointerException was thrown!");
        } finally {
            System.out.println("Moving on!");
        }
    }
}

A NullPointerException was thrown!
Moving on!

In the above code, we have defined multiple catch blocks and the correct one was ran when that exception was thrown.

Throwing an Exception

You can throw an exception by using the throw keyword:

public class HelloWorld {

    public static void main(String[] args) {
        try {
            throw new NullPointerException("NULL");
        } catch(NullPointerException e) {
            System.out.println("A NullPointerException was thrown!");
        } finally {
            System.out.println("Moving on!");
        }
    }
}

A NullPointerException was thrown!
Moving on!

We can throw our own exceptions! Now if you're wondering why this is useful, well, you can use exceptions to ensure that things in your program are progressing correctly, like this:

public class HelloWorld {

    static void checkMoney(int money) {
        if (money < 200) {
            throw new ArithmeticException("You must have at least $200.");
        } else {
            System.out.println("You have enough money!");
        }
    }

    public static void main(String[] args) {
        int money = 100;

        checkMoney(money);
    }
}

Exception in thread "main" java.lang.ArithmeticException: You must have at least $200.
    at Main.checkMoney(Main.java:5)
    at Main.main(Main.java:14)

Now that the method checkMoney can throw an exception of its own, you can choose to handle it like any other risky piece of code:

public class HelloWorld {

    static void checkMoney(int money) {
        if (money < 200) {
            throw new ArithmeticException("You must have at least $200.");
        } else {
            System.out.println("You have enough money!");
        }
    }

    public static void main(String[] args) {
        int money = 100;

        try {
            checkMoney(money);
        } catch(ArithmeticException e) {
            System.out.println("You must have at least $200.");
        }
    }
}

You must have at least $200.

Not only is that a much cleaner output, but you can fully control it entirely, and even run more code if needed.