LECTURE NOTES
CHAPTER 10

USING A DEBUGGER

- programs rarely run perfectly the first time
- frustrating to find bugs
- logging
  - use to show key variables and program flow
  - time-consuming to insert and remove
  - as programs become longer, becomes less feasible
- debugger
  - special program
  - packaged with most modern development environments
  - locate bugs by following the execution of the program
  - can't avoid using, because most efficient debugging tool
  - still takes time to run, so important to write best program possible initially
  - vary widely among systems
  - book shows Eclipse, which we are using
  - I'll be Eclipse specific, too
- start by running in debug mode
- debugger runs program until you tell it to stop
  - in Eclipse, can tell debugger to stop automatically at the first line of main
  - breakpoint
    - tell the debugger to pause the program when it gets to a certain line of code
    - can have multiple at a time
    - pauses the program at every breakpoint
      - next stop is the first breakpoint reached
      - should periodically clear unused breakpoints
- when paused
  - look at current values of variables
    - purpose: verify all variables contain the expected values
    - variables window shows all variables in the current scope
      - useful for helping you understand scope!
    - inspect command
      - right click a variable in the code window
      - debugger shows you its current value
    - looking at objects:
      - in variable window, click + symbol
      - lets you inspect instance fields
  - how long to run before next pause
    - set another breakpoint (may depend on whether code working so far)
    - step
      - go through your code one line at a time
      - see how the code changes variables
      - see the program flow (how conditionals and loops depend on variables)
      - single step command
        - executes current line and stops at the next line
        - what happens when we get to a method call?
          - step into
            - jump to the method code
            - purpose: check if the method carries out its job correctly
          - step over
            - skip over this method
            - still executes the method
            - you continue on the next line after the method
            - purpose: streamline debugging when know method already works
- debugger stops when program terminates
- can rerun program in debugger
- find a bug
  - for now, exit debugger, fix bug, re-enter debugger
  - some debuggers let you edit code on the fly and continue debugging!