KotlinCS 124 LogoJava

Catching Exceptions

try {
int notANumber = Integer.parseInt("You are not alone!");
} catch (NumberFormatException e) {
System.out.println("That's not a number!");

We’ll spend the rest rest of the week learning about Java exceptions. Errors are a normal part of programming, and Java provides nice ways for handling them. Let’s learn more!

Exceptions: When Things Go Wrong
Exceptions: When Things Go Wrong

It’s natural to make mistakes when you write computer programs. But even well-designed programs may encounter errors!

Imagine the following scenario. You design an app that prompts the user to enter a number that you plan to use in a mathematical calculation. What you receive is a String, so you need to convert it to an Integer. What could go wrong? Let’s find out!

// Problems with Integer Parsing

What is happening here? Let’s examine the documentation for Integer.parseInt to find out.


In Java, when code that we write or call encounters an error, it can throw an Exception. Over the next few lessons we’ll explore Java’s error-handling mechanisms, including types of exceptions and how to design and throw them in our own code.

But let’s start at looking at how to handle exceptions that we might encounter. To do this we use a new Java programming construct: try-catch. Let’s see how that works!

// try-catch

try-catch consists of two or more code blocks. First, the try block, containing the code that might throw an exception. Second, one or more catch blocks that handle various kinds of exceptions that the code might throw. Let’s look at some code that can generate several kinds of exceptions and see how to handle them:

import java.util.Random
Random random = new Random();
int choice = random.nextInt()

Exception Control Flow
Exception Control Flow

One of the more difficult parts of exceptions is understanding how code flow changes when an exception is thrown. When an exception is thrown, Java jumps into the first enclosing catch block. This might be in that method, or in calling method, or even in the caller’s caller or higher up. Let’s look at an admittedly contrived example:

void foo1() {
void foo2() {
void foo3() {
void foo4() {
Object o = null;

Don’t worry if this doesn’t make perfect sense yet—we’ll get lots of practice with this over the next few days!

More Practice

Need more practice? Head over to the practice page.