Code Tracing and Debugging

By the end of this lesson, you will be able to trace code execution manually to determine output, identify common programming errors, and apply debugging strategies.

Code Tracing

Code tracing is manually executing code step-by-step to determine what it does without running it. This is a critical skill for:

• AP exam multiple-choice and FRQ
• Understanding code you didn't write
• Debugging problems

The trace table method: for each step of execution, write down the current value of every variable.

Trace Example

int x = 5;
int y = 2;
int z = 0;

while (x > y) {
    z = z + x;
    x--;
}
System.out.println(z);

Step	x	y	z	Condition (x > y)
Start	5	2	0	—
Iteration 1	4	2	5	5 > 2 → true
Iteration 2	3	2	9	4 > 2 → true
Iteration 3	2	2	12	3 > 2 → true
Check	2	2	12	2 > 2 → false → EXIT

Output: 12

Predict It: Trace the Loop

Tracing is the whole skill this lesson is about, so here it is as a question rather than as a worked example you read.

Walk the lines one at a time and keep a note of every variable as it changes.

Predict It: The Off-by-One

The same loop with one character changed. Trace it again rather than assuming.

Common Programming Errors

Syntax errors: invalid Java code that won't compile.

• Missing semicolons
• Mismatched braces or parentheses
• Misspelled variable names
• Wrong data type assignment

Runtime errors (exceptions): the code compiles but crashes during execution.

• ArrayIndexOutOfBoundsException: accessing index ≥ length or < 0
• NullPointerException: calling methods on a null reference
• ArithmeticException: dividing by zero (integer division)
• : invalid type cast