Debugging in Java
Assumed Knowledge
Learning Outcomes
- Be able to identify input/output pairs that are good for debugging.
- Be able to debug Java programs with appropriate tools.
- Be able to do paper traces of Java code.
Author: Gaurav Gupta
Debugging
For any given problem, we design a solution and then implement it.
As an example, let’s say that we are writing a program that gives you the number of digits in an integer. We expect the following input-output mappings.
| Input | Output |
|---|---|
| 1729 | 4 |
| 1234567890 | 10 |
| 0 | 0 |
| -888 | 3 |
Now, it’s possible that the actual outputs you get from your code are as follows,
| Input | Output |
|---|---|
| 1729 | 4 |
| 1234567890 | 10 |
| 0 | 1 |
| -888 | 4 |
We need to find out why do some inputs have incorrect outputs. So we go through our design and implementation looking for possible bugs. A logical way to do this is to trace every variable at every stage and see where does the program deviate from the expected.
Example 1
Consider the following code that is supposed to return the product of all integers from 1 to n (n >= 1).
The input-output mappings are -
| Input | Expected Output | Actual Output |
|---|---|---|
| 4 | 24 | 0 |
| 6 | 720 | 0 |
| 7 | 5040 | 0 |
| 1 | 1 | 0 |
If you trace the program, you’ll see that the loop executes when i=1 and result becomes result (0) * i (1) = 0. And every subsequent time, result becomes 0 * i = 0. A trace for n=4 using logic table is provided below,
| i | i < 4 | result |
|---|---|---|
| 1 | true | 0*1 = 0 |
| 2 | true | 0*2 = 0 |
| 3 | true | 0*3 = 0 |
| 4 | false |
Thus, the first bug is result should be initalized to 1 and not 0.
Our partially fixed code:
The new input-output mappings are -
| Input | Expected Output | Actual Output |
|---|---|---|
| 4 | 24 | 6 |
| 6 | 720 | 120 |
| 7 | 5040 | 720 |
| 1 | 1 | 1 |
A trace for n=4 using logic table is provided below,
| i | i < 4 | result |
|---|---|---|
| 1 | true | 1*1 = 1 |
| 2 | true | 1*2 = 2 |
| 3 | true | 2*3 = 6 (not the expected output) |
| 4 | false |
It can now be seen that the loop should execute for i=4 and multiply it into the result but it doesn’t. By changing i < n to i <= n, we fix this problem.
To confirm, we trace once more for i=4.
| i | i < 4 | result |
|---|---|---|
| 1 | true | 1*1 = 1 |
| 2 | true | 1*2 = 2 |
| 3 | true | 2*3 = 6 |
| 4 | true | 6*4 = 24 (expected output) |
| 5 | false |
Debug the following method for which the expected input-output mappings are provided in the javadoc (comment above the method). CRITICAL STEP!!! Write down the actual input-output mappings after every iteration of debugging
solution
Performing Debugging in Popular IDEs
Most of the modern Integrated Development Environments (IDEs) have a comprehensive debugging feature that lets you trace the variables as your program executes.
The debugger relies of placing breakpoints that are like pitstops in a car race. The program runs till the next breakpoint where you can see the values of all the variables and when you hit resume, it goes to the next breakpoint.
Visual Studio Code
Eclipse
Pen and paper methodology
At this point, it becomes tempting to throw away your notebook, you have everything you need in the programming environment right?
No.
Debugging is still mostly done in your head or on paper, so it is worth revisiting our programming tracing skills. What follows is a set of exercises for you to get familiar with tracing Java code. You should do all these exercises with just a pen/pencil and paper as the skill you are training is to be able to trace a program without the aid of a computer.
Trace the flow of the following program and determine the value of result at the end of it.
solution
a < b is true
b % a == 0 is true
a % 2 == 0 is false
The expression becomes true && true && false
This is false
Hence, the else block executes and result becomes b (10).
Trace the flow of the following program and determine the value of result at the end of it.
solution
a == b is false,
else` executes
b decreases by 5, becomes 5
a == b is true.
if block executes and result becomes b (5).
Trace the flow of the following program and determine the value of result at the end of it.
solution
| i | i<=7 | i%2 | i%2==1 | result | | — | — | — | — | — | | 1 | true | 1 | true | -3+1 = -2 | | 2 | true | 0 | false | | | 3 | true | 1 | true | -2+3 = 1 | | 4 | true | 0 | false | | | 5 | true | 1 | true | 1+5 = 6 | | 6 | true | 0 | false | | | 7 | true | 1 | true | 6+7 = 13 | | 8 | false | | | |
Trace the flow of the following code -
solution
Solution
At the end of the code,
a = 5, b = 10, c = 2, d = false, result = 10.
Explanation -