CS 46A - Lecture 18

Pre-class reading

• None :-)

The First Bug

Using a Debugger

• Debugger: a program to execute your program and analyze its run-time behavior
• A debugger lets you stop and restart your program, see contents of variables, and step through it
• The larger your programs, the harder to debug them simply by inserting print commands
• Debuggers can be part of your IDE (e.g. Eclipse, BlueJ) or separate programs (e.g. JSwat)
• Three key concepts:
  • Breakpoints
  • Single-stepping
  • Inspecting variables

The Debugger Stopping at a Breakpoint

Inspecting Variables

Debugging

• Execution is suspended whenever a breakpoint is reached
• In a debugger, the program runs at full speed until it reaches a breakpoint
• When execution stops you can:
  • Inspect variables
  • Step through the program a line at a time
  • Or, continue running the program at full speed until it reaches the next breakpoint
• When the program terminates, the debugger stops as well
• Breakpoints stay active until you remove them
• Two variations of single-step command:
  • Step Over: skips method calls
  • Step Into: steps inside method calls

Single-step Example

• Current line:

  String input = in.next();
  Word w = new Word(input);
  int syllables = w.countSyllables();
  System.out.println("Syllables in " + input + ": " + syllables);

• When you step over method calls, you get to the next line:

  String input = in.next();
  Word w = new Word(input);
  int syllables = w.countSyllables();
  System.out.println("Syllables in " + input + ": " + syllables);

• However, if you step into method calls, you enter the first line of the countSyllables method

  public int countSyllables()
  {
     int count = 0;
     int end = text.length() - 1;
     . . .
  }

Lecture 18 Clicker Question 1

1. Step over it
2. Step into it
3. Inspect n
4. Set a breakpoint

Lecture 18 Clicker Question 2

1. Step over the method
2. Step into the method
3. Inspect the method parameters
4. Set a breakpoint