Lecture 4 (Accelerated 10/15/16) I. Advanced Input/Output Methods II. Exceptions III. Command Line Arguments

Transcription

1 Lecture 4 (Accelerated 10/15/16) I. Advanced Input/Output Methods II. Exceptions III. Command Line Arguments I. Advanced Input/Output (IO) A. How to read from a file First, import the classes we need import java.io.*; What * means: import ALL classes in the IO package We can only import specific classes, and in our case, we actually need the following classes from the IO package: File, FileNotFoundException, and PrintWriter class You can write separate commands for each of these: (e.g.) import java.io.file; Or you can just import all of them: import java.io.* import java.util.scanner; Create a File object File inputfile = new File("Example.txt"); Make a Scanner from this File object Scanner input = new Scanner(inputFile); Recall how the Scanner was made when it was created from user input: Scanner input = new Scanner(System.in); Method depends on what you want to read in Desired input Method to check if it's there Method to store it One line of text input.hasnextline() input.nextline() One character input.hasnext() input.next() Integer input.hasnextint() input.nextint() Double input.hasnextdouble() input.nextdouble() Note that each line in a text file is set apart by a newline character "/n," but we just don't see it. This is what the Scanner will use to delimit lines. nextline() stores up to the next "/n" character and gets rid of it for you!

2 (e.g.) How to read the entire file line by line and print it out: while (input.hasnextline()) { String line = input.nextline(); System.out.println(String); B. How to write to a file Create a Printwriter object for a new file named "out.txt" PrintWriter outfile = new PrintWriter( out.txt ) Print the string stuff on a new line in "out.txt": It is not actually printed yet; it is just sent to the computer memory. outfile.println( stuff ) Remember to always close the file after making all the commands. This is when the string "stuff" is actually printed to the file and the file appears in your directory! outfile.close() II. Exceptions A. What are they? A mechanism for handling problems that may occur in the methods (the actions of the program) Think of a Java program of calling methods in an arranged order B. Types of exceptions (2 main types we should consider) 1) Checked Exceptions: IOExceptions Typically, not your fault but CAN happen so Java anticipates these exceptions (e.g.) File you want to read is not in the directory You MUST deal with these (Java refuses to compile the program if you don't) Or use the command throws if you want to ignore them. 2) Unchecked Exceptions: run-time exceptions This is your fault! (e.g.) ArrayIndexOutOfBounds, Arithmetic error dividing by 0

3 The programmer should have checked the length of the array or whether the denominator is 0 before putting this in the method Java doesn t anticipate these because they re not supposed to exist C. How do I deal with exceptions? (**See and run "UseThrows.java" and "UseTryCatch.java" to compare the two ways below) try/catch/finally statements Put the commands that could cause an exception in a try block Put what you would like to do with that exception (if it happens) in a catch block Any command that the program should go through REGARDLESS of exception occurrence should go in a finally: (e.g.) output file closing (e.g.) try{ File newfile = new File("Example.txt"); Scanner input = new Scanner(newFile); PrintWriter outfile = new PrintWriter("output.txt"); // more code catch(filenotfoundexception e){ System.out.println("No file!"); finally( outfile.close() Ignore them with a throws statement in the method declaration This is telling the compiler that this method might throw this exception so that the compiler can sufficiently deal with it Useful if you just don't want to deal with an IOException yourself! (e.g.) public static void openfile() throws FileNotFoundException { File newfile = new File("Example.txt"); Scanner input = new Scanner(newFile); // more code you don't need to do anything more

4 D. How to throw a runtime exception when you want Use the throw command in your method Notice it's throw, not throws III. Command Line Arguments public static void main(string[] args){ // the String array args stores all the command line arguments String first = args[0]; // print out all the command line arguments for (i = 0; i < args.length; i++){ System.out.println(args[i]) If you don't know how to use Terminal, here are some basic commands (all you need to know for now) ü "cd DirectoryName" : go to that directory ü "cd -" : back up one directory ü "cd ~" : go to home directory ü "ls" : print me a list of every directory I can access ü "javac FileName.java": compile this java file ü "java FileName": run this java application" ü "java FileName arg1 arg2 ": run this java application with these command line arguments cf1) Remember how we use arrays! datatype[] arrayname = new datatype[arraysize]; datatype var = arrayname[0]; //get first index of array int size = arrayname.length; // number of elements in an array

5 cf2) Useful for today's lab: String splitting! String[] arr = "a b c d e".split("\\s); // this splits the string using one whitespace as a delimiter à // stores a, b, c, d, e, etc. as each element in the array String[] arr= "a b c d e".split("\\s+"); // this gets rid of all the unneeded whitespace for you (hence, same // result as above String[] arr = "a b c d e".split("\\s"); // whereas this sets the whitespace between the letters as an array element as well String[] arr = "abcde".split(""); // this uses to separate the elements and takes them out