Flash cards
Review the key moves
What is the main idea behind Java Debugging?
Lesson checks
Practice each idea before moving on
Short Mimo-style checks built from this lesson's code, terms, and sequence.
Which statement best captures the main point of this lesson?
Complete the missing token from the example code.
___.out.println("Before division"); // Debug outputPut the learning moves in the order that makes the concept easiest to apply.
After learning about common errors, the next step is understanding how to debug your Java code - that is, how to find and fix those errors effectively.
This page introduces simple debugging techniques that are useful for beginners and helpful even for experienced developers.
What is Debugging?
Debugging is the process of identifying and fixing errors or bugs in your code.
It often involves
- Reading error messages
- Tracing variable values step by step
- Testing small pieces of code independently
Tip
Debugging is a skill that improves with practice. The more you debug, the better you get at spotting problems quickly.
Print Statements for Debugging
The most basic (and often most effective) way to debug Java code is to use System.out.println() to print values and check the flow of the program.
In this example, the first line "Before division" will print, but the second line is never reached because the program crashes due to division by zero:
Example
int x = 10;
int y = 0;
System.out.println("Before division"); // Debug output
int result = x / y; // Crashes
System.out.println("Result: " + result); // Never runsTip
Add print statements before and after key lines of code to find out where things go wrong.
Check Variable Values
If something unexpected happens, print out the values of your variables:
Example
int age = 17;
System.out.println("Age: " + age);
if (age >= 18) {
System.out.println("Access granted");
} else {
System.out.println("Access denied");
}Tip
This is a good way to test whether a condition is working correctly - try changing age to 18 or 19 and observe the output!
Debugging with IDEs
Modern IDEs like IntelliJ IDEA , Eclipse , and NetBeans come with built-in debugging tools.
- Set breakpoints to pause the program at specific lines
- Step through code line by line
- Inspect variable values in real time
Tip
Use your IDE's debugger to find errors faster - it's more powerful than print statements alone!
Debugging Checklist
- Read the full error message, it often tells you exactly what's wrong
- Check if all variables are initialized before use
- Print variable values to trace the problem
- Watch for off-by-one errors in loops and arrays
- Comment out sections of code to find bugs
In the next chapter, you will learn about how to handle errors gracefully in your programs with Java Exceptions .