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Now let's look at how we can do debugging within
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Eclipse. I created a new file called AddNumbers which I'm
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showing here. It takes two numbers, parses them into integers,
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adds them and prints the sum, supposedly, of the two numbers.
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Now we look at the run configuration for this program,
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and here you can see that we're passing two arguments,
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two and five, to the program. So now let's run
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our program and see what happens. And the result says that
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2 plus 5 is equal to 10, which is not
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exactly correct. So we need to debug our program. We
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need to figure out what's wrong with the program, why
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the wrong result was, produced. So we're going to add a break
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point here by double-clicking here on the side of the
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code. And the break point is basically a place where I'm
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telling my debugger to stop during the execution because I
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want to inspect the state of the program. So to start
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debugging, we select Debug as Java Application from the Context
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menu, similar to what we were doing for running the program.
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And as you can see, this asks us whether we want
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to pass to the debug perspective, which is a, a perspective
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specifically designed for debugging. We say yes. And as you
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see here, it shows us, it's like a different, set of
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views, so we can see the code down here with an
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indication of where the execution is. And of course the execution
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stopped at the break point, which is exactly where
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we told the debugger to stop. So let's look at
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some of the other views in this perspective. The view
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here on the right-hand side, for example, shows the variables
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in scope and the break points that are currently active
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for the debugging session. This is where the editor is
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at. The outline of the program and the console at
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the bottom. So now let's execute one line by clicking
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on the Step Over button here at the top, and this will
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execute the line that is currently highlighted and therefore it will move to
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the next line. And as you can see, one nice feature is that
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if I move the mouse over a variable, I can see the value
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of the variable. And the same thing I can do if I look
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at the variables windows here on the right. If I click it, it
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will tell me what is the value of the variable, and in case
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of more complex variables you can even expand it and get more details.
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So now let's step over another line by clicking again this button,
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and as you can see now we get to the line that
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is actually performing the sum, supposedly, so now let's do the same
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thing that we did before, and let's mouse over b, and we can
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see that the value of b is five, as expected. So now
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let's step over this line as well, and execute the actual sum. And
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doing the mouseover thing, we can see that the value of sum
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is ten, which is not right, of course. In fact, if we check
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a gain we can see that value of A is two. The
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value of B is five and therefore it's clear that there's something
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wrong going on here, and at this point we can notice that
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here we are doing multiplication instead
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of addition. And therefore that's what the
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error is. And this is clearly a very simple case. Right? A
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case in which probably you just needed to look at the code and
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you didn't need the debugger. But you probably got the idea right?
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So this can be extremely useful when you're debugging, when you're studying more
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complex programs. If you want to stop the debugger because you're
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done with your debugging session as in this case, you can
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either click here on the Terminate button or you can also
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just simply tell the debugger to continue the execution, to resume
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the execution until the program terminates naturally. So, in this case,
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we're going to click here just to show what happens. And what
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happens is that, you know, the execution will just continue until
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the program exits. So now let's say that we want to fix
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this problem that we just discovered. So we replace the multiplication
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with an addition, we save the program, and we execute the
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program again by clicking on this button. And at this point,
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unsurprisingly, we get the right result as shown in the console.
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