Java File Input/Output - It's Way Easier Than You Think

do you need to write data to a text file or read data from a text file in java that's what we're going to do from scratch in this video i'm going to show you the simplest way to read and write text files in java my name is john and i do a new java tutorial video every single week so please consider subscribing so you don't miss each week's video i also have a full java course available in a link down in the description if you're interested if not that's awesome too i'm thrilled to have you here and as always the full source of this program is available in a link down in the description so go and get it if you look around online you're going to find tons of different ways to do file input and output in java so what i'm going to show you definitely isn't the only way it can be done but it is the way that i think is the simplest and it's the way that i like to use myself let's start with writing data to a text file the class that i like to use most for file output in java is called buffered writer so let's make a new buffered writer object we're going to call it writer and we say equals new buffered writer using eclipse we can go ahead and import buffered writer but if you aren't using a fancy ide the import is java java.io.bufferedrider now notice it's still complaining to us here because this constructor is undefined and that's because we have to actually pass something into this constructor method the reason it needs to take something is because this kind of buffered writer can write to many different things and a file is just one of them so we need to tell it that we want to write to a file and we can do that by saying new file writer and we also need to tell this file writer which file we want it to write and for that we can just pass in the name of the file that we want to create for our output file let's just call it output.txt we also have to import filewriter which is going to be java.io.filewriter and you might notice that now java is complaining that there's this unhandled exception i o exception all that means is that this code can throw an i o exception so we either need to handle it here in a try catch or declare in our main method that we throw it so we should do the right thing and just surround this code with a try catch block and we don't really need to do anything special inside of our catch block if this exception happens we can just print the stack trace and that's fine for our purposes okay now that our writer object has been created honestly the hard part's over all you need to do after that is take your writer object and call writer.write and you just pass in to this write method the string that you want to write to the file so let's just pass in a writing to a file but you're not done quite yet this won't work completely the way it is if you're running this in an ide you might notice that there's a warning for this writer object that says resource leak writer is never closed so what you have to do after you're done writing everything to the file that you want to you need to call writer.close if you forget to do that your file will get created but nothing will be written to it it'll just be blank and you'll be pulling your hair out trying to figure out what you did wrong and it's just this simple you need to remember to close your writer all right now let's go ahead and run it and see what we get okay the program finished successfully we didn't write any console output we just have the file output so let's go and see if that got created successfully if you just specify the file name here in your file writer that's what's called a relative path that just means that output.txt file is going to go right next to wherever your program is on your computer so if you just have a java file it's probably going to be in that same directory if you're using an ide like eclipse it's going to be in your project directory and you can get there very easily from eclipse by just right-clicking anywhere in this file and going to show in system explorer that will open a window that shows your dot java file you just need to go one directory up to go to your project folder so for me that's this file io folder and we can see that it has created this output.txt file so let's just open it up and see what's in it and it just has writing to a file and that's exactly what we wanted it to have now that's just one line of course let's say you wanted to write another line in that file so what you might try is just calling writer.write again and passing in whatever line you want to write like here is another line let's run that again completed successfully now one thing to note the first time that you run this it will create this output.txt file but every time you run it after that it will overwrite that same file now most of the time that's just fine but if there's something in there that you want just remember that it's going to do that it's going to overwrite it so just be careful anyway let's look at our text file and so what happened it actually didn't put this on another line and put it on the same line so what you actually have to do is put a backslash n whenever you want to do a new line so now with our backslash n there let's run our program again completed successfully and now here is another line is on the next line and you can just keep calling this right method for however many lines you need to write to the file but most of the time if you're doing file io you're probably not writing hard-coded values you probably want to write to the file from some variable or some array that you have so let's show an example of doing that let's say we have this array of strings just called names that has three values john carl and jerry well what you can do is loop through that array and write each element to the file so you just have something like for string name in names so for each iteration in this loop we can just do writer dot right first a new line so we don't just smoosh all these names onto one line and then plus name let's run that and you can see we have the original text that we wrote and then every element of our names array on a new line so that's how you output to a relative path where it'll put the file right next to wherever your program is but if you want to put it in some specific spot on your computer you just have to pass it an absolute path here's just an example of an absolute path on my own computer so c users john m example output.txt notice that you do have to put a double backslash otherwise it just won't work right let's run that and now we can see we have this output.txt file in this exact absolute path that we specified and it has all the same content in it all right so now we've written to a file and we've even gotten fancy and wrote the contents of an array into that file now let's learn how to read from a file so to write to a file we used a buffered writer so it might not surprise you that you read from a file we're going to use a buffered reader so buffer reader reader equals new buffered reader import for buffered reader is java dot io dot buffered reader similarly to the buffered writer where we had to pass in a file writer for the buffered reader we have to pass in a file reader new file reader and we pass in either the full or absolute path here again to make it simple we're just going to read the file that we just wrote so just output.txt import file reader which is java.io.filereader and just like the filewriter this could throw an exception so we also want to surround this with another try catch and we'll just have it catch i o exception just like the writer did and still just print a stack trace if any exceptions happen okay now we have our reader object available how do we use it well to read a line of text from our file we just call reader dot this this read line returns a string so for your purposes you might want to save it to a variable or something but just to prove that we're actually reading from the file we're going to just print it out to the console so system.out.printline reader.readline so we're going to read the line from the file and print it out to the console but before we get ahead of ourselves once we're done using the reader in our program we do want to call reader.close as well to close our reader object okay now let's run it and see if we are reading from the file awesome we can see that it did read the first line of the file that we wrote we just wrote writing to a file so this is reading it successfully but if you're reading a file you probably don't want to just read one line you probably want to read each and every line to do that of course you're going to want to use some kind of a loop and here we're going to use a while loop this is going to look a little strange but just stay with me before that while loop let's declare a string to hold each line of text as we read it then in our while loops condition we're actually going to have line equals reader.readline so for each iteration through the loop this is going to read a line of text and put it in this line variable but of course this doesn't equate to true or false so it doesn't really work as a while condition yet so what we want to add is not equals null so what is this doing it's going to read a line of text from our file and put that line of text in our line variable when it has reached the end of the file this line will turn out to be null because there's no other lines to read so we can just say while this line does not equal null loop through the file once the line is null we know we've reached the end of the file and it'll exit the while loop we can get rid of this line now where we're just reading a single line of text inside this while loop all we need is system.out.printline line so this should loop through every single line of our file and print it out to the console and there we go writing to a file here's another line john carl jerry that matches our output text file perfectly if you enjoyed this video or learned something please let me know by liking and subscribing it's the only way these videos get out to help more people and so i really do appreciate it so if you'll do that i'll see you next video

Channel: Coding with John

Views: 12,670

Keywords: java file io, java file handling, java file input output tutorial, java file processing, java file, java file reader, java file writer, java file reading and writing, coding with john, codingwithjohn, java beginner lesson, java beginner tutorial, java beginner course, java beginner to advanced, file handling in java, java, java io, file io java, file io in java, file in java, file input output java, read file in java

Id: ScUJx4aWRi0

Length: 8min 17sec (497 seconds)

Published: Mon Apr 26 2021