Intro to Debuggers

(Written largely because newbies in ##java never seem -to have this knowledge.)

A “debugger” is a mechanism for monitoring and controlling the execution of -your program, usually interactively. Using a debugger, you can stop your -program at known locations and examine the actual values of its variables -(to compare against what you expected), monitor variables for changes (to see -where they got the values they have, and why), and step through code a line at -a time (to watch control flow and verify that it matches your expectations).

Pretty much every worthwhile language has debugging support of some kind, -whether it's via IDE integration or via a command-line debugger.

(Of course, none of this helps if you don't have a mental model of the -“expected” behaviour of the program. Debuggers can help you read, but can't -replace having an understanding of the code.)

Debugging Your First Program

Generally, you start running a debugger because you have a known problem -- an -exception, or code behaving strangely -- somewhere in your program that you -want to investigate more closely. Start by setting a breakpoint in your -program at a statement slightly before the problem area.

Breakpoints are instructions to the debugger, telling it to stop execution -when the program reaches the statement the breakpoint is set on.

Run the program in the debugger. When it reaches your breakpoint, execution -will stop (and your program will freeze, rather than exiting). You can now -inspect values and run expressions in the context of your program in its -current state. Depending on the debugger and the platform, you may be able to -modify those values, too, to quickly experiment with the problem and attempt -to solve it.

Once you've looked at the relevant variables, you can resume executing your -program - generally in one of five ways:

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Continue execution normally. The debugger steps aside until the program - reaches the next breakpoint, or exits, and your program executes normally.
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Execute the next statement. Execution proceeds for one statement in the - current function, then stops again. If the statement is, for example, a - function or method call, the call will be completely evaluated (unless it - contains breakpoints of its own). (In some debuggers, this is labelled “step - over,” since it will step “over” a function call.)
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Step forward one operation. Execution proceeds for one statement, then - stops again. This mode can single-step into function calls, rather than - letting them complete uninterrupted.
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Continue to end of function. The debugger steps aside until the program - reaches the end of the current function, then halts the program again.
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Continue to a specific statement. Some debuggers support this mode as a - way of stepping over or through “uninteresting” sections of code quickly and - easily. (You can implement this yourself with “Continue” and normal - breakpoints, too.)
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Whenever the debugger halts your program, you can do any of several things:

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Inspect the value of a variable or field, printing a useful representation - to the debugger. This is a more flexible version of the basic idea of - printing debug output as you go: because the program is stopped, you can - pick and choose which bits of information to look at on the fly, rather than - having to rerun your code with extra debug output.
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Inspect the result of an expression. The debugger will evaluate an - expression “as if” it occurred at the point in the program where the - debugger is halted, including any local variables. In languages with static - visibility controls like Java, visibility rules are often relaxed in the - name of ease of use, allowing you to look at the private fields of objects. - The result of the expression will be made available for inspection, just - like a variable.
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Modify a variable or field. You can use this to quickly test hypotheses: for - example, if you know what value a variable “should” have, you can set that - value directly and observe the behaviour of the program to check that it - does what you expected before fixing the code that sets the variable in a - non-debug run.
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In some debuggers, you can run arbitrary code in the context of the halted - program.
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Abort the program.
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