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Ace the AP Computer Science Exam for High Schoolers

Debugging in Java

Explore essential Java debugging methods to understand and resolve common programming errors such as syntax, runtime, and logical bugs. Learn how to reproduce issues, simplify inputs, and use debugging tools effectively. This lesson equips beginners with the skills needed to identify and fix defects in their Java code, improving overall programming proficiency and software reliability.

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❗ Note: This section introduces an important software development phase. It’s not a part of the AP CS A exam but is useful to know. The purpose is only to familiarize the students with the process.

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In computer programming, the process of finding and resolving an error or a bug is called debugging. A bug is just another word for defects or problems that prevent correct operation of the program. There are some common types of errors:

  • Syntax error
  • Runtime error
  • Logical error
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Syntax errors arise due to syntax (grammatical) mistakes. For example, not adding the type when declaring a variable or missing a semicolon at the end of the statement. The compiler doesn’t even let you run a program if it’s not free from syntax errors.

Runtime errors occur when the system is running; these errors can stop someone from doing what they need to do. Suppose we make a calculator program for students. A student wants to perform division. He gave two numbers: 123123 (numerator) and 00 (denominator). The program will crash because we can’t divide a number by 0. This is a very naive example. However, there exist some large scale runtime errors that affect performance.

Logical errors are the hardest to track down. Everything looks like it is working. Technically, the program is correct, but the expected results aren’t achieved. A famous example happened in 19991999 when NASA lost a spacecraft due to miscalculations between English and American units. The software was coded one way but needed to work another.

Before launching any application, it’s important to test for any errors and to debug. Normally, the first step in debugging is to reproduce the problem. After the bug is reproduced, the input of the program may need to be simplified to make it easier to debug. For example, a bug in a compiler can make it crash when given a huge number.

If there’s nothing wrong with the input, the process of execution probably needs to be traced. A programmer can use a debugger tool to examine program states (values of variables at different points) and track down the origin of the problem(s). In simple cases, tracing is just a few print statements that output the values of variables at certain points of program execution.