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  • Yeah, so when an error occurs, and an exception is thrown,

  • it's our job as developers to handle it by providing a backup plan.

  • We catch the exception and

  • fall back to some reasonable default behavior, otherwise the app may crash.

  • In other words,

  • some application crashes are simply the result of the uncaught exceptions.

  • >> Yeah, so in order to do this, we're going to have to learn about something

  • called the try catch block in Java.

  • >> Try means you try to execute a group of statements,

  • that includes calling a method that could potentially thow an exception.

  • >> And when an exception is thrown, you catch it.

  • If no one catches the exception, well your app will crash.

  • You okay, Joe?

  • >> I'm fine.

  • And regardless of whether an exception was thrown or not,

  • we can finish executing code in the finally block.

  • >> So the full flow is try to do something that may throw an exception,

  • catch the exception if it's thrown.

  • And finally, execute some code regardless of whether or

  • not that exception occurred.

  • >> We should show them some code for this.

  • >> Yeah, for sure.

  • Let's do it. >> In the Tsunami App there are multiple

  • examples of try catch blocks.

  • Let's look at one in the create your own method.

  • The goal of this method is to return a URL object for

  • the provided input string URL.

  • Ideally, we would just call a URL constructor and pass in the string URL,

  • but via this red squiggly line Android Studio is notifying us of

  • an error situation that is detected.

  • That is, it says that we have an unhandled exception of

  • type malformed URL exception.

  • This means that the URL constructor throws a malformed URL exception, and

  • we need to catch and handle the error.

  • Otherwise, our app won't build and we can't run it on our device.

  • For any constructor or method call,

  • you can check if it will throw an exception by checking the documentation.

  • For example, if we visit the URL reference documentation,

  • we can see that the URL constructor that accepts a string as input through

  • MalformedURLException.

  • If the input parameter can't be converted into a URL,

  • that is it is MalformedURL.

  • To fix our code, we can use the shortcut Alt+Enter for a quick fix.

  • We can select the option, surround with try catch, and

  • Android Studio will automatically update the code.

  • Pretty neat, right?

  • Now we have the key word try followed by an opening curly brace.

  • Inside the try block we put all of our code in here

  • that could throw an exception.

  • Now in general, it's good practice to keep code inside this block lean.

  • You don't want full method by this here,

  • then we close the block with the curly brace.

  • Next, we have the keyword catch followed a set of parenthesis

  • containing the exact data type of the excephamore catching.

  • In this case, MalformedURLException and the variable name for

  • the exception object e.

  • Then we have an opening brace where we will handle the exception,

  • e.printStackTrace will print the error stack in detailed format.

  • Another option is to log areas using Androids log methods and

  • include our own log tag and our own custom message.

  • The log.e method can take an exception as it's third argument.

  • To recap, if all went well on the URL constructor,

  • then no exception is thrown and nothing on the catch block ever runs.

  • But if the code within the try block throws an exception, then we immediately

  • jump to the cash block with the exception and execute the code there.

  • Then we exit the try catch statement and

  • continue executing the code line by line after that.

  • Notice that the URL variable is initialized before the try catch block.

  • We have to consider variable scope here.

  • If we define the URL variable inside the try block,

  • we won't be able to access the URL variable after the statement is done.

  • So since we want to reference the URL variable at the end of the method

  • outside of the try catch statement.

  • We need to initialize the URL variable outside of it as well.

  • In this case, we can just set it to null, which means an empty value.

  • So now we've seen how Android Studio will notify you if you need to wrap your

  • code in a try catch statement.

  • If you want more information,

  • you can check out the official Java documentation.

  • This next example is a little bit more complex.

  • It shows that you can have as many lines of code in the try block as you want.

  • If any line of that code throws an exception, then we stop executing

  • the statements in the try block and jump directly to the catch block.

  • As you can see here you can and should have

  • multiple catch blocks to handle different types of errors if necessary.

  • Here, the code catches an index out of bound exception and

  • prints a system log message.

  • The code also catches an IO exception and prints a different log message.

  • Now depending on what exception gets thrown,

  • we will fall into one of these catch blocks, but not both.

  • So you can see how there's no guarantee of executing

  • all of the code in the try block.

  • Sometimes you have clean up that needs to be done even if there was

  • an exception thrown.

  • So the finally block will always be executed regardless of whether or

  • not an exception was thrown.

  • Before we wrap up this discussion,

  • I want to point out one final method of dealing with a check exception.

  • Now as you can see here by modifying the createURL methods, methods signature.

  • You do have the option to defer the exception handling up the call chain

  • by declaring that your method may throw the exception

  • which in turn requires that the calling methods handle it.

  • This is common if the exception is thrown from inside helper method.

  • And you want to handle it in a try catch somewhere else.

  • For example, whoever calls the createUrl method will now need a try catch block

  • around this method call, and have to catch the MalformedURLException.

  • We can see this here in the doInBackground method.

  • For the purposes of the Tsunami App, if you call some Android framework code

  • that throws an exception, you should catch it and handle it at the moment

  • it happens instead of deferring it to be handled later.

Yeah, so when an error occurs, and an exception is thrown,

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