IntelliJ IDEA 2019.3 Help

Debugging Your First Java Application

You have created and run your Java application. Let's imagine you have discovered that it functions not the way you expected. For example, it returns a wrong value or crashes with an exception. Seems like you have errors in your code and it’s time to debug it.

What Is Debugging?

Broadly, debugging is the process of detecting and correcting errors in a program.

There are different kinds of errors, which you are going to deal with. Some of them are easy to catch, like syntax errors, because they are taken care of by the compiler. Another easy case is when the error can be quickly identified by looking at the stack trace, which helps you figure out where the error occurred.

However, there are errors which can be very tricky and take really long to find and fix. For example, a subtle logic error, which happened early in the program may not manifest itself until very late, and sometimes it is a real challenge to sort things out.

This is where the debugger is useful. The debugger is a powerful tool, which lets you find bugs a lot faster by providing an insight into the internal operations of a program. This is possible by pausing the execution and analyzing the state of the program by thorough examination of variables and how they are changed line by line. While debugging, you are in full control of the things. In this manual we are covering a basic debugging scenario to get you started.

Examine the code

Let's try a simple debugging case. Imagine we have the following application:

public class AverageFinder { public static void main(String[] args) { System.out.println("Average finder v0.1"); double avg = findAverage(args); System.out.println("The average is " + avg); } private static double findAverage(String[] input) { double result = 0; for (String s : input) { result += Integer.parseInt(s); } return result; } }

The program is supposed to calculate the average of all values passed as command-line arguments.

It compiles and runs without issues, however the result is not what one would expect. For instance, when we pass 1 2 3 as the input, the result is 6.0.

First of all, you need to think about where the suspected error might be coming from.

We can assume the problem is not in the print statements. Most likely, unexpected results are coming from our findAverage method. In order to find the cause, let's examine its behavior at runtime.

Set breakpoints

Breakpoints indicate the lines of code where the program will be suspended for you to examine its state.

  • Click the gutter at the line where the findAverage method is called.

    A breakpoint is set at the line that calls the findAverage method

Run the program in debug mode

Now let's start the program in debug mode.

Since we are going to pass arguments for running and debugging the program, make sure the run/debug configuration has these arguments in place.

  1. From the main menu, select Run | Edit Configurations.

  2. Enter arguments in the Program arguments field.

    Arguments are entered in the Program arguments field
  3. Click the Run button near the main method or its containing class. From the menu, select Debug.

    After you click a Run button in the gutter, a menu with run/debug options appears.

Analyze the program state

After the debugger session has started, the program runs normally until a breakpoint is hit. When this happens, the line where the program paused gets highlighted and the Debug tool window appears.

Debugger tool window appears. The line with the breakpoint is highlighted

The highlighted line has not been executed yet. The program now waits for further instructions from you. The suspended state lets you examine variables, which hold the state of the program.

As the findAverage method has not been called yet, all its local variables like result are not yet in scope, however, we can examine the contents of the args array (args is in scope because we are still in the main method). The contents of args are displayed inline where args is used:

Inline debugging shows variable values right at the line where the respective variables are used

You can also get information about all variables that are currently in scope in the Variables panel.

Variable values are shown in the Variables panel

Step through the program

Now that we are comfortable with the Debug tool window, it's time to step into the findAverage method and find out what is happening inside it.

  1. To step into a method, click the Step Into button or press F7.

    Step into button located in the top part of the Debug tool window
  2. Let's keep stepping and see how a local variable result is declared and how it is changed with each iteration of the loop.

    Inline debugging helps us get information about the variable values

    Right now the variable s contains the value "3". It is going to be converted to an Integer and be added to result, which currently has the value of 3.0. No errors so far. The sum is calculated correctly.

  3. Two more steps take us to the return statement and we see where the omission was. We forgot to divide the sum by the number of values. This was the cause of incorrect method return.

    The value of result is returned as is, without dividing it by the number of arguments.
  4. Let's correct the error.

    return result/input.length;

Stop and rerun the debugger session

In order to check that the program works fine, let's stop the debugger session and rerun the program.

  1. Click the Stop button or press Ctrl+F2.

    Debugger session is stopped using the Stop button located in the left-hand part of the Debug tool window
  2. Click the Run button near the main method or its containing class. From the menu, select Run.

    After you click a Run button in the gutter, a menu with run/debug options appears.
  3. Verify that the program works correctly now.

    The program outputs 2.0 now
Last modified: 9 December 2019