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All good Java development environments including Eclipse provide good debuggers that allow you to inspect data at runtime, set breakpoints, and otherwise figure out what's going on in your code. I'm going to show you how to use the debugger and along the way, I'll show you how to work with parts of the Java class library that commonly throw exceptions, how you can find out what those exceptions might be and anticipate them. I'm working in an empty main method in the current project debugger. I'm going to declare a variable of a special type URI.
URI stands for Uniform Resource Identifier. This class is a part of the package java.net and is part of a collection of classes that are used for network communications in Java. After you have typed URI, press Ctrl+ Spacebar and choose the URI class from java.net. That will add an import statement at the top of the code. Then give it a variable name uri, all lower case, and set its initial value with the constructor method new URI and pass in a well formed domain string such as "http://somecompany.com." It doesn't matter what the name of the domain is or even whether it's real website, what matters is that it's well formed with http://at the beginning and then a well formed domain at the end.
When you save your changes and build the project, you'll see an error has occurred, Unhandled exception type URISyntaxException. The Java compiler seems to think that when I call this URI constructor method, it might throw an exception and that it's up to me as the programmer to make sure that I've handled that exception. Well, how would I know that as the programmer unless I have to wait for the compiler to tell me? And the answer as is so frequently the case in Java is that it's there in the documentation.
I'll go to the URI documentation, I'll double click URI, the data type and then I'll go to my Dynamic Help and click javadoc for java.net.uri and I'll maximize the Help screen. In the documentation, I'll click on Constructor, here is the method that I just called URI receiving a string argument. I'll click that method and it tells me and the documentation very explicitly, this method could throw an the instance of URI syntax exception.
So when you're working with a method that's part of the Java class library and that method has been defined as throwing an exception, it'll be there in the docs. So what do I have to do as a developer to deal with that? If a bit of code is going to throw an exception I have to handle it with a Try/catch method or by putting an appropriate throws declaration in the method signature where I'm actually calling the code. I'm going to wrap up this code with a Try /catch block, I'll select the code then right-click it and choose Surround With, Try/catch block.
Eclipse is smart enough to look at the syntax of the class or the method and say hey that method might throw URI syntax exception and so that's the class that puts into the catch block that it generates. Now, I'll delete the TODO comment and I will add a little bit more code down at the bottom, so I can see clearly that the application is still running System.out.println and I'll put the message "I'm alive!" I'll save the change and run the application.
And the first time everything works fine because the string that I passed into the URI method was a well formed domain. But now let's create an intentional error. I'll change those forward slashes to Backslashes. That will create a malformed URI. I'll save and run the application and now I get the stack dump. I still see I'm alive! at the end of the output. And by the way, you might not see this at the end of the output it might be mixed into the middle don't worry about that.
But if you see that message that means that the Trycatch block is working but in fact, this error message is being thrown as part of that complex exception object and this takes us to the debugger. Let's say, that I wanted to interrogate that exception object while I'm programming, I wanted to look at its structure. I can put a breakpoint on the line where the printStackTrace command is being executed. And I can freeze the execution of the application, suspend it, while it's suspended, I can use Eclipse to inspect that exception object and see what information is available.
To do this place a breakpoint on the line where the exception object is in scope, in this case it's on this line where I'm calling printStackTrace. To set a breakpoint, go to the left trough over on the left side of the editor and either double-click or you can right click and choose Toggle Breakpoint. Either way, you should see this little icon showing that there is now a breakpoint at this particular line. Now it's time to run the application in debug mode. Go to the toolbar and click debug, it's the little button that looks like it has a cockroach or a bug on it. Click the button.
If you see a message from your firewall application, let it know that everything is okay, and then you should see this message Confirm Perspective Switch. This is telling you that Eclipse wants to switch to something called the Debug perspective. The debug perspective is a collection of views that will allow you to more quickly inspect and fix your application. Click Yes. And this is the debug perspective. There is a Debug view in the upper left that shows your current call stack, a Variables view that shows all the currently in scope variables the Editor, the Outline and down at the bottom, the Console.
Go to the Variables view because you suspended the application at the point where the exception object became available, you should see the exception object named e listed there, double-click the tab for variables to expand it and then expands the e object. You'll see that there's all sorts of interesting information the cause, the detail message, the index, the input and the stack trace. Now some of these values occur on all exception objects like the detail message, but the values that they have may or may not be useful.
In this case, there's a pretty interesting error here, Illegal character in opaque part. But you'll also find that you can call methods of the exception object to get the information more elegantly. Once you've finished inspecting the exception object, double-click the Variables tab to return it to its original size, then go up to the toolbar and click the Terminate button, it's the red square button on the toolbar. And then return to your CustomForJava perspective.
You can choose the perspective from the upper right or you can go to the Menu and choose Window, Open Perspective, Other and choose the perspective you want, I've been using this one CustomForJava. Once you know what the structure of the exception object is, you can do some interesting stuff with it. So for example, instead of printing the stack trace, I'm going to do a little print line here System.out.println. And I'm going to output a value from e.getMessage. This is a method that's available on all exception objects.
I'll save my changes and run the application again. And I get that message Illegal character in opaque part at index 5, and it's talking about index 5 of the string. If you start counting at 0, the letter h is 0, ttp is 1, 2, 3, the : is 4 and the / is 5, that \\ was interpreted as a \ by Java and it's telling me exactly where the problem is in that string.
So now, I can come back to the code and go oh gosh, I put in \\ instead of // and I can fix it, save and run, and everything is okay. So whenever you are working with a method that might throw an exception, you should wrap it inside a Try/catch block and then within the catch block to can set a breakpoint and inspect that exception object. Once you know how to use the debugger for that, you know how to use it for everything. Whenever you have some information in your Java programming environment and you're not sure what the value of a variable is, set a breakpoint and use the debugger.
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