Using the Java Debugger

Debuggers are very useful tools for analyzing the execution of your program. Debuggers allow you to pause your program while it is running, and slowly step through the code line by line to see the flow of execution. You can also print out the values of each variable at each step of your program.

The CodeHS IDE allows you to pause and step through your Java programs by entering "Debug Mode" in your editor settings:

Let's try out the debugger on a real program! Press "Run" to start the debugger, and "Step" to start stepping through the code

Debugger Commands

The debugger provides a few options for stepping through the code:

1. Step - steps to the very next instruction, regardless of which method the instruction is located in
2. Next - skips to the next instruction in the current method. Next will not step into any method calls, it will always go to the next command within the current method.
3. Continue - continues to run the program without pausing

Your Turn

Try using Step and Next in this program!

1. Run the program to start debugging
2. From here try using only Step to finish the program. It should take 13 Steps to finish every instruction.
3. Now try again, this time using only Next to finish the program. It should only take 5 Nexts to finish, since Next does not step into any method calls!

Printing Variables

While debugging your program, you can use the print and dump commands to print out the values of the variables in your code:

• print x - prints out the value of the variable x. For Objects, this command will print out the result of calling x.toString()
• dump x - For Objects, prints out the value of each field defined within the object. For primitives like int and double, dump is identical to print

Your Turn

Step to line 10 of this program, then:

• Type print firstStudent into the debugger console to see the result of calling firstStudent.toString()
• Type dump firstStudent into the debugger console to see all the fields defined inside the firstStudent object.

Now step to line 16 of this program, then:

• Type print i to see the value of the for loop counter

Breakpoints

What if you want to pause your program on a specific line of code? This is where breakpoints come in handy.

While using the debugger, you can click on any line number to set a breakpoint at that line. When the debugger Runs or Continues, it will pause once it hits that line of code.

Your Turn

Try setting some breakpoints in the following program!

Click on a line number to set a breakpoint at that line.

Press Run to start the program and pause at the first breakpoint you set.

Press Continue to continue to the next breakpoint.

Note

Breakpoints must be put in place before you press Run.

If you modify your breakpoints while your program is running, you'll need to re-run your program in order to pause at your new breakpoints.

User Input

You may have noticed that providing input to the program while debugging can be tricky. The debugger tries to interpret your input as debugger commands, but numbers like "10" or "20" are not valid debugger commands.

The fix is to simulate input by creating your Scanner from a String of input, rather than from System.in

// Simulates inputting 10 and then 20 while the program is running

Scanner = new Scanner("10\n20\n");

Curious to Learn More?

CodeHS is using the industry standard built-in Java debugger, JDB, to pause and step through your Java programs.

In this tutorial, you learned about using the following JDB commands:

• step - steps to the next instruction
• next - skips to the next instruction in the current method
• cont - continues execution without pausing
• print - prints out the value of a variable
• dump - dumps out the value of every field within an Object
• stop - set a breakpoint at a particular line of code

These are the main commands you'll be using to debug Java programs, but you can check out all the JDB commands at your disposal by reading the JDB Documentation