Reading Files

We’ve already been reading data from files throughout most of this course. However, let’s take some time to review that code and improve it a bit to make it more flexible for use in the future.

Opening a File using java.nio.file

Here is the code we’ve used in the Exceptions module to open a file for reading:

reader = new Scanner(new File("input.txt""));

This code uses a file name "input.txt", and then creates a Scanner object to read data from that file, using an intermediate File object to represent the file itself. Scanners only open files for reading.

The File object used here is part of the older java.io package, which has been present in the Java API for quite a while. However, more recent versions of Java have included the new java.nio.file package, which includes many easier to use methods for handling files and directories.

So, instead of using a File object from java.io, we will use a Path object from java.nio.file, which is much more flexible. In fact, it is completely compatible with the older versions, so we can easily obtain a Path from a File and vice-versa.

To create a Path object, we use the Paths.get() static method. So, an updated version of our starter code might look like this:

import java.util.Scanner;
import java.nio.file.Paths;

public class Read{
  
  public static void main(String[] args) throws Exception{
    
    Scanner reader;
    reader = new Scanner(Paths.get("input.txt"));
    
    /* -=-=-=-=- MORE CODE GOES HERE -=-=-=-=- */
    
  }
}

There are a couple of important changes:

1. Instead of import java.io.File; at the top of the code, we must now use import java.nio.file.Paths; to get access to the new Paths class.
2. When creating the Scanner, we can now use reader = new Scanner(Paths.get("input.txt"));. This will use the Paths.get() static method to create a Path object, and then use that object to construct a Scanner which can read data from the file found at that path.

With those changes in place, we are now using the java.nio.file library, which we’ll use throughout this chapter.

Exceptions

Of course, as we learned in an earlier chapter, we should also add some Try-Catch and Try with Resources statements to this code to prevent any exceptions. So, let’s do that now:

import java.util.Scanner;
import java.nio.file.Paths;
import java.lang.ArrayIndexOutOfBoundsException;
import java.nio.file.InvalidPathException;
import java.nio.file.NoSuchFileException;

public class Read{
  
  public static void main(String[] args) throws Exception{
    
    try(
      //Try with Resources will automatically close the file
      Scanner scanner = new Scanner(Paths.get("input.txt"));
    ){
    
      /* -=-=-=-=- MORE CODE GOES HERE -=-=-=-=- */
     
    //add additional catch statements here to handle expected exceptions
    }catch(InvalidPathException e){
      //path is invalid
      System.out.println("Error: invalid file path!");
      return;
    }catch(NoSuchFileException e){
      //file is not found
      System.out.println("Error: file not found!");
      return;
    }catch(Exception e){
      //generic catch statement
      System.out.println("Error: unknown error while reading input!");
    }
    
  }
}

This code is very similar to the code we saw in a previous chapter, with a couple of major changes:

1. At the bottom, we are using a Try with Resources statement to actually handle reading our input. This will automatically close the file once we are done with it. Otherwise, we’d need to add scanner.close() at the end of our program, possibly in a finally block.
2. Since we are using the java.nio.file package, we now must use import java.nio.file.NoSuchFileException; to get the correct exception when a file cannot be found.
3. In addition, the Paths.get() method can throw an InvalidPathException if the path provided cannot be converted to a proper path.

Reading Data

Once we have our file open, we can use the same methods we’ve been using to read data from the file. For example, we can use a simple While loop to read each line of the file:

while(scanner.hasNext()){
  String line = scanner.nextLine().trim();
  if(line.length() == 0){
    break;
  }
}

In this code, we use the hasNext() method to check and see if the file has any additional lines to read. If so, it will read the line using the nextLine() method. We are also using the trim() method to remove any extra spaces from the beginning and end of the line. Finally, we have a short If-Then statement to check and see if the line is empty. If so, we’ll assume that we’ve reached the end of the input file, or that the user typing input via the terminal is done, and we’ll break out of the loop.

Writing Files

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Beyond just reading data from files, we can also create our own files and write data directly to them.

Opening a File for Writing

In Java, we’ll use a class named BufferedWriter to actually handle writing to a file. So, to create a BufferedWriter object, we could do something like the following example:

import java.io.BufferedWriter;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.io.IOException;
import java.lang.UnsupportedOperationException;
import java.nio.file.InvalidPathException;
import java.lang.ArrayIndexOutOfBoundsException;

public class Write{

  public static void main(String[] args){
  
    try(
      BufferedWriter writer = Files.newBufferedWriter(Paths.get("output.txt"));
    ){
   
      writer.write("Hello World");
      writer.newLine();
   
    }catch(ArrayIndexOutOfBoundsException e){
      //no arguments provided
      System.out.println("Error: no arguments provided!");
      return;
    }catch(InvalidPathException e){
      //path is invalid
      System.out.println("Error: invalid file path!");
      return;
    }catch(IOException e){
      //cannot open file or error while writing
      System.out.println("Error: I/O error!");
      return;
    }catch(UnsupportedOperationException e){
      //unable to open the file for writing
      System.out.println("Error: unable to open file for writing!");
      return;
    }catch(Exception e){
      //something went wrong
      System.out.println("Error: unknown error!");
      return;
    }
  }
}

Let’s break this code down into smaller parts so we can understand how it works.

First, we are using a Try with Resources statement to handle creating the BufferedWriter to write to the output file. This will automatically handle closing the file when we are done with it. Otherwise, we’d need to add a finally block that includes writer.close() to make sure the file is closed properly. If we don’t do that step, there is a chance that our data may not get written to the file correctly.

Inside of the Try with Resources block, we have this line:

BufferedWriter writer = Files.newBufferedWriter(Paths.get("output.txt"));

This line uses the Files class in java.nio.files to handle opening a file and creating a new BufferedWriter to write to that file. The newBufferedWriter() method needs a single input, which is a Path object representing the file to be opened. So, we can use the same Paths.get() method we use when opening a file for reading. Here we are using "output.txt" as the path to the file, but we can use any valid String as well.

It is important to note that, by default, if the file we are writing to already exists, it will be overwritten with the new output. If it doesn’t exist, it will be created. There are ways to open a file and append new data to it without overwriting the file, which we’ll discuss below.

Inside of the Try with Resources statement, we see two lines that write data to the file using the BufferedWriter object. The first method, writer.write() can be used to write any String to the file. So, we can use this just like we would System.out.print() when writing output to the terminal. We can even use formatted strings as well!

The second line, writer.newLine() prints a newline character to the output file. This is because the writer.write() method does not output a newline character by default each time it is used. This is different than System.out.println(), which always outputs a newline after each output. So, we need to use writer.newLine() each time we want to start on a new line.

Finally, there are several exception handlers at the end of the Try with Resources statement. They handle the most common exceptions that can occur when opening and writing to a file. The only one we haven’t covered so far is the UnsupportedOperationException, which is used when the operating system doesn’t allow us to write to a file, usually because the file permissions do not allow us to change or modify the file. It can also happen when we aren’t allowed to create a new file in the location we’ve specified. There are several other exceptions that could be thrown when we are unable to write to a file based on the type of operating system we are using. So, we’ll also need to catch a generic Exception here, just to be safe.

Standard Open Options

When opening a file, we can also give a set of options, known as StandardOpenOptions in Java, to specify how we’d like to handle the file when it is opened. By default, when we use the Files.newBufferedWriter() method to open a file, it uses the following options:

• StandardOpenOption.CREATE - create a file if it does not already exist
• StandardOpenOption.WRITE - open for write access
• StandardOpenOption.TRUNCATE_EXISTING - if the file exists, it will be truncated (all data will be removed)

If we’d like to change those options, we can specify them when opening the file. For example, if we’d like to append to an existing file, we can use the following code to open the file:

BufferedWriter writer = Files.newBufferedWriter(Paths.get("input.txt"), StandardOpenOption.CREATE, StandardOpenOption.WRITE, StandardOpenOption.APPEND);

We’ll also need to add import java.nio.file.StandardOpenOption; to the top of the file to give us access to those options.

There are many other options available in the Java StandardOpenOption class. Feel free to read the documentation linked below in the resources section to learn more.

Flushing Output Buffers

When writing data to a file using a program, it is important to understand how the underlying operating system handles that process. In many cases, the operating system will store, or buffer the output in memory, then write the output directly to the file a bit later. This allows the operating system to tell our program that the write was successful while it waits for the storage device the file is actually stored on to respond. So, our programs appear to run very quickly.

However, at times we want to tell the operating system to write the data it has stored in memory directly to the file. To do that, we can use the flush() method of our BufferedWriter class to flush the buffer, or make sure the data is written to the file. Here’s an example:

writer.write("Hello World");
writer.newLine();
writer.flush();
writer.write("More data");
writer.close();

Thankfully, the close() method will automatically write any buffered data to the file before closing it. So, we can either use the close() method ourselves, or use a Try with Resources statement to make sure that the file is closed automatically for us.

We can even use System.out.flush() to perform the same operation when printing output to the terminal. In most cases all of our output is printed directly to the terminal, but we can make sure that the output buffer is empty by using the flush() method anytime.

File Operations

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Beyond just reading and writing files, we can also perform several operations on files and directories from within our programs. In fact, pretty much any operation that can be done in the terminal can also be done in our programs, though some are more difficult than others. Let’s review a few of the common file operations and how we can use them in Java.

Paths

First, we’ll need to know how to access a path using Java. A path is a string that references a particular file or directory in a file system, identified by the path needed to move from the root node to that item. So, for example, we may use the path /home/codio/workspace/file.txt to reference a particular file in our Codio workspace.

In Java, we also use the term Path to refer to an object that points to an item on the file system.

To create a Path object in Java, we use code similar to this

import java.nio.file.Paths;
import java.nio.file.Path;
import java.nio.file.Files;
import java.nio.file.InvalidPathException;
import java.io.IOException;

public class Manipulate{
  public static void main(String[] args){
    
    try{
      Path pathObject = Paths.get("/home/codio/workspace/file.txt");
    
      /* -=-=-=-=- MORE CODE GOES HERE -=-=-=-=- */
      
    }catch(InvalidPathException e){
      //cannot convert string to path
      System.out.println("Error: Invalid Path");
      return;
    }catch(IOException e){
      //file system error
      System.out.println("Error: IOException");
      return;
    }
  }
}

In the code above, we can simply replace the string "/home/codio/workspace/file.txt" with any valid file path stored in a string to create the indicated Path object. It will even accept absolute paths, relative paths, and paths to directories instead of just individual files. It’s a very versatile tool to use. For the rest of the examples below, we’ll be using the pathObject variable created in the example above, with the code placed where the MORE CODE GOES HERE comment is in the skeleton above.

What Is It?

Once we have a Path object, we can use a few methods to determine what type of an object it is:

//Determine if a file or directory exists at that path
Files.exists(pathObject);

//Is that object a directory?
Files.isDirectory(pathObject);

//Is that object a regular file?
Files.isRegularFile(pathObject);

Each of those methods returns a boolean value, either true or false. So, they can easily be used with If-Then statements to take different actions based on the type of object found. So, in our code, we can use some of these tests before trying to open a file, avoiding some of the more common exceptions. As we discussed in the chapter on exceptions, it is really up to us whether we prefer to use If-Then statements to avoid these exceptions, or Try-Catch statements to deal with them when they do happen.

How Big Is It?

We can also get the size of the object:

Files.size(pathObject);

If the item is a regular file, this method will return the size in bytes of the file. However, if we use this method on a file that doesn’t exist, or a directory, it will throw an IOException, so we’ll probably need to pair it with one of the other methods above to avoid that problem.

Copy and Move

There are also methods we can use to copy or move an item from one path to another path:

Path source = Paths.get("/home/codio/workspace/dir1/file1.txt");
Path dest = Paths.get("/home/codio/workspace/dir2/file2.txt");

Files.copy(source, dest);
Files.move(source, dest);

These work very similarly to the cp and mv commands we’ve already seen on the Linux terminal. In addition, if the destination path already exists, these methods will throw a FileAlreadyExistsException unless we specify that it should overwrite existing files. We can refer to the documentation linked below to see examples for how to accomplish that.

Delete

We can also delete an existing file or path:

Files.delete(pathObject);

This method will delete a single file if the pathObject variable references a single file. If it references a directory, that directory must be empty, or else it will throw a DirectoryNotEmptyException .

Create

Of course, we can also create either a file or directory based on a Path:

Files.createFile(pathObject);
Files.createDirectory(pathObject);

These methods will also throw a FileAlreadyExistsException if something already exists at that path.

References

• Files in the Java 8 API Documentation
• Paths in the Java 8 API Documentation
• Java File I/O Tutorial from Oracle’s Java Tutorials

A Worked Example

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Now that we’ve seen how to handle working with files in Java, let’s go through an example program to see how we can apply that knowledge to a real program.

Problem Statement

Here’s a problem statement we can use:

Write a program that accepts three files as command line arguments. The first two represent input files, and the third one represents the desired output file. If there aren’t three arguments provided, either input file is not an existing file, or the output file is an existing directory, print “Invalid Arguments” and exit the program. The output file may be an existing file, since it will be overwritten.

The program should open each input file and read the contents. Each input file will consist of a list of whole numbers, one per line. If there are any errors parsing the contents of either file, the program should print “Invalid Input” and exit. As the input is read, the program should keep track of both the count and sum of all even inputs and odd inputs.

Once all input is read, the program should create the output file and print the following four items, in this order, one per line: number of even inputs, sum of even inputs, number of odd inputs, sum of odd inputs.

Finally, when the program is done, it should simply print “Complete” and exit. Don’t forget to close any open files!

So, let’s break down this problem statement and see if we can build a program to perform this action.

Parsing Arguments

First, let’s handle parsing the command line arguments. So, we can start with a simple program skeleton, containing both a class declaration and a main method declaration:

public class Example{
  public static void main(String[] args){
    
  }
}

Next, we’ll want to make sure there are exactly three arguments. This is probably best done using an If-Then statement, since it makes the code a bit simpler to read than if we would use a Try-Catch statement. However, either approach will work.

public class Example{
  public static void main(String[] args){
    if(args.length != 3){
      System.out.println("Invalid Arguments");
      return;
    }
  }
}

Next, we’ll need to check and make sure that each of the first two arguments is a valid file that we can open. Since we intend to open them anyway, let’s just use a Try with Resources statement:

import java.util.Scanner;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.nio.file.InvalidPathException;
import java.nio.file.NoSuchFileException;
import java.io.BufferedWriter;
import java.io.IOException;
import java.lang.NumberFormatException;

public class Example{
  public static void main(String[] args){
    if(args.length != 3){
      System.out.println("Invalid Arguments");
      return;
    }
    
    try(
      Scanner scanner1 = new Scanner(Paths.get(args[0]));
      Scanner scanner2 = new Scanner(Paths.get(args[1]));
      BufferedWriter writer = Files.newBufferedWriter(Paths.get(args[2]))
    ){
      
      /* -=-=-=-=- MORE CODE GOES HERE -=-=-=-=- */
      
    }catch(InvalidPathException e){
      System.out.println("Invalid Arguments");
      return;
    }catch(NoSuchFileException e){
      System.out.println("Invalid Arguments");
      return;
    }catch(IOException e){
      System.out.println("Invalid Arguments");
      return;
    }
    
    /* -=-=-=-=- MORE EXCEPTIONS GO HERE -=-=-=-=- */
  }
}

There are a few new things in this code that we haven’t seen before:

• We can include multiple variables in a Try with Resources statement by simply separating them with a semicolon ;.
• Since we aren’t worrying about reading input from System.in, we can just create our Scanner and BufferedWriter objects directly in the Try with Resources statement. Actually, this makes the code very straightforward.
• Recall that the args variable is actually an array, so we can access additional command line arguments by simply using different array indices. We haven’t done that yet, but it should make sense.

Now that we’ve confirmed that we can open each file, we can start coding the program’s logic. For the rest of this example, we’ll look at a smaller portion of the code. That code can be placed where the MORE CODE GOES HERE comment is in the skeleton above. We’ll also need to handle a few more exceptions, which can be added where the MORE EXCEPTIONS GO HERE comment is above.

Logic

The program’s logic should be pretty straightforward. First, we’ll need to create loops to read input from each input file:

while(scanner1.hasNext()){
  String line = scanner1.nextLine().trim();
  
}

while(scanner2.hasNext()){
  String line = scanner2.nextLine().trim();
  
}

Notice that we are using two separate While loops here. Since we are dealing with two different input files that are unrelated, this is the simplest way to go.

Next, we can parse the input to an integer, and then determine if it is even or odd:

while(scanner1.hasNext()){
  String line = scanner1.nextLine().trim();
  int input = Integer.parseInt(line);
  if(input % 2 == 0){
    //even
  }else{
    //odd
  }
}

while(scanner2.hasNext()){
  String line = scanner2.nextLine().trim();
  int input = Integer.parseInt(line);
  if(input % 2 == 0){
    //even
  }else{
    //odd
  }
}

Finally, we can add a few state variables to keep track of how many of each type we’ve had, and their sum as well:

int countEven = 0;
int countOdd = 0;
int sumEven = 0;
int sumOdd = 0;

while(scanner1.hasNext()){
  String line = scanner1.nextLine().trim();
  int input = Integer.parseInt(line);
  if(input % 2 == 0){
    countEven += 1;
    sumEven += input;
  }else{
    countOdd += 1;
    sumOdd += input;
  }
}

while(scanner2.hasNext()){
  String line = scanner2.nextLine().trim();
  int input = Integer.parseInt(line);
  if(input % 2 == 0){
    countEven += 1;
    sumEven += input;
  }else{
    countOdd += 1;
    sumOdd += input;
  }
}

Exceptions

In the new code above, we are converting strings to integers, which could result in a NumberFormatException. So, we’ll need to add one more catch block to the Try with Resources statement in the skeleton at the top of this page:

catch(NumberFormatException e){
  System.out.println("Invalid Input");
  return;
}

That will handle any problems with the input files themselves.

Printing Output

Finally, we can simply print our four variables to the output file:

int countEven = 0;
int countOdd = 0;
int sumEven = 0;
int sumOdd = 0;

while(scanner1.hasNext()){
  String line = scanner1.nextLine().trim();
  int input = Integer.parseInt(line);
  if(input % 2 == 0){
    countEven += 1;
    sumEven += input;
  }else{
    countOdd += 1;
    sumOdd += input;
  }
}

while(scanner2.hasNext()){
  String line = scanner2.nextLine().trim();
  int input = Integer.parseInt(line);
  if(input % 2 == 0){
    countEven += 1;
    sumEven += input;
  }else{
    countOdd += 1;
    sumOdd += input;
  }
}

writer.write("" + countEven);
writer.newLine();
writer.write("" + sumEven);
writer.newLine();
writer.write("" + countOdd);
writer.newLine();
writer.write("" + sumOdd);
writer.newLine();

System.out.println("Complete");

The eight lines at the end should be pretty self-explanatory. We can simply print each number, but we’ll need to convert them to a string first. The simplest way to do that is simply to use the concatenate operator + and concatenate each number with a string, which will automatically convert everything to a string. We’ll also need to remember to print a newline between each of them using the newLine() method. Of course, we also need to print Complete once we are finished!

Testing

Once we’ve completed the code, we can use the button below to test it and see if it works. Don’t forget to open the example.pregrade.html file that it creates to see detailed feedback from your program.