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Student: ex13

Read all lines of a file

Part of a series of tutorial articles about a Student class.

Code examples referred to on this page can be found here:

NOTE: NOT YET UPDATED FOR JAVA 17 and JUNIT 5 (TODO for W22, 01/05/22)

In this exercise we show how to read a list of students from a text file in .csv format (comma separated values) into a list of Student object in memory. This is a preliminary step to showing how to do sorting of lists in Java.

More specifically:

  • Show how to read all lines from a text input file into a List<String>.
  • Show how to convert that list of strings into an list of Student objects, more specifically an ArrayList<Student>.
  • Show how to define custom exceptions
  • Show how to set up the code so that most of the work is done in the Student class with full test coverage.

Some background that would be helpful

You will better understand the code in this lesson if you have first read about

  • class vs. interface in Java
  • Java Generics (the <> stuff in expressions such as ArrayList<Student>
  • Java Collections (e.g. List<E>, ArrayList<E>, etc.)

You can find out more about these topics in:

  • Chapters 6,7,8 and 16 in HFJ2
  • Chapter 7 in JN7, pp. 143-158 (from start of chapter, stopping when you get to Introducing Covariance)
  • The section titled The List Interface in Chapter 8 of JN7 (pp. 270-274). (Note that ArrayList is one concrete implementation of the List interface.)

A few notes on List<String>

Note that List is an interface, rather than a class; that means that all we know about the object that List<String> refers to is that it implements all the methods of List<E>, namely, the ones listed on this page of javadoc:

The List<String> might be an instance of ArrayList<String>, but not necessarily. It could also be a LinkedList<String> or a Vector<String> for example.

A few notes on ArrayList<Student>

The ArrayList<E> (where E is the generic element type) is probably the most basic and most often used class for a collection in Java. A Java ArrayList is similar to a List in Python:

  • It has some of the qualities of an array, i.e. you can directly index elements
  • But it also has some of the qualities of a linked list, in that you can dynamically grow and shrink the size of the list.

One significant difference between a Java ArrayList and a Python list is that while a Python list can store any type of object, and indeed a mixture of different types in the same list, in Java, when we declare an ArrayList, we typically give it a specific type, e.g. ArrayList<Student> or ArrayList<Dog>, and then it can only hold objects of that type.

(While it is possible to declare an ArrayList<Object> that can hold objects of any type (including primitives, through the wrapper classes, such as Integer, Float, etc.), the cases where that’s a good design are rare.)

For a more complete discussion of ArrayList, see:

  • Chapters 6 and 16 in HFJ2
  • The section titled The List Interface in Chapter 8 of JN7 (pp. 270-274). (Note that ArrayList is one concrete implementation of the List interface.)

Reading All Lines from a file

The file is a new class introduced here in ex13. It has a main method that will

  • take a filename as a command line argument
  • open that file
  • read all lines from that file
  • process each line, converting each line into a Student object
  • store each of those objects into an ArrayList<Student>

It is divided into two parts. First the main:

   public static void main(String[] args) {

        String usageMessage = "Usage: java Main filename\n"
                + "  filename should be a text file containing students in CSV format";

        if (args.length != 1) {

        String filename = args[0];

        List<String> allLines = null;

        try {
            allLines = Files.readAllLines(Paths.get(filename));
        } catch (IOException e) {
            System.err.println("ERROR: could not open file " + filename);

        ArrayList<Student> students = linesToStudents(allLines);


    } // main

The job of main() is to do four things:

  • Make sure there is a command line argument for a filename
  • Open that filename and read its contents into a List<String>
  • Pass those contents to a separate static method linesToStudents that converts a List<String> into an ArrayList<Student>
  • Print that ArrayList<Student> on System.out

It does all of these things with appropriate error checking and messaging.

The separate function linesToStudents does the job of

  • setting up an empty ArrayList<Student>
  • converting each separate line of input to a Student, checking for exceptions (and printing appropriate error messages)
  • returning the ArrayList<Student>

The fromCSV method

To support this code, we needed to add one additional method to the Student class, one called fromCSV. This is a static method because it is not associated with any instance of the class.

We could have written this as a constructor, but instead, we wrote it as a factory method.

Factory methods are an alternative to constructors for producing new instances of objects. In this case, there is no particular reason to make this a factory method rather than a constructor; we did so to familiarize you with the concept, since you’ll see it in other people’s code.

(There are situations where factory methods have advantages over constructors but that’s a topic for another time.)

The fromCSV method looks like this. Note that we declare that this method throws two different exceptions. These are custom exceptions; we’ll get to that in a moment.

    public static Student fromCSV(String csv) throws InvalidCSVLineException, InvalidPermException {
            String [] parts = csv.split(",");

            if (parts.length != 2) {
                throw new InvalidCSVLineException("Invalid: "+csv);

            String name=parts[0];
            int perm = 0;

            try {
            } catch (NumberFormatException nfe) {
                throw new InvalidPermException("Invalid: " + parts[1]);

            if (!validPerm(perm)) {
                throw new InvalidPermException("Invalid: " + perm);

            return new Student(name, perm);

The custom exceptions

The exceptions InvalidCSVLineException and InvalidPermException are custom exception types that we created with these new lines of code in

    public static class InvalidCSVLineException extends Exception {
        private static final long serialVersionUID = 1L;
        public InvalidCSVLineException(String msg) {

    public static class InvalidPermException extends Exception {
        private static final long serialVersionUID = 1L;
        public InvalidPermException(String msg) { 
            super (msg);

These are declared as static inner classes; that is they are a class nested inside the class Student. We could have made these standalone classes, but because they are so very small and used only in connection with the fromCSV method of Student, it is convenient to just tuck them inside the same source file,

The custom exceptions are declared as static inner classes. If an inner class is not declared as static, then instances of that class have to be tied to instances of the outer class (in this case Student). That’s not appropriate in this case, because the instances of these exceptions are created specifically in the case where we cannot make an instance of the Student class.

(There are many more things to say about inner classes that are not static, but that’s really a topic for another time; for now, it’s sufficient to know that if we want an inner class where the objects are independent of any instance of the main class, we need to declare the class as static.)

The constructors for each of these classes have one job, and that’s to pass the parameter msg to the constructor of the parent class, Exception. This allows us to customize the exception with a message that is revealed by the toString() method.

Test coverage

We have excluded the ReadStudents class from both Jacoco and PIT (mutation) test coverage checks. But we’ve pushed a lot of the details of error checking of the CSV input into the Student class in the fromCSV method which is subject to test coverage. Here are the additional tests we wrote to check the various cases for fromCSV. Some of these test cases were written by iterating on the test coverage reports until each case was covered.

    public void test_fromCSV_1() {
        String csv = "Chris Gaucho,1111111";
        Student s = null;
        try {
            s = Student.fromCSV(csv);
        } catch (Exception e) {
        assertEquals(s.getName(), "Chris Gaucho");
        assertEquals(s.getPerm(), 1111111);

    public void test_fromCSV_3() throws Student.InvalidCSVLineException, Student.InvalidPermException {
        String csv = "";
        Student s = null;
        s = Student.fromCSV(csv);

    public void test_fromCSV_4() throws Student.InvalidCSVLineException, Student.InvalidPermException {
        String csv = "Chris Gaucho,not-an-integer";
        Student s = null;
        s = Student.fromCSV(csv);

    public void test_fromCSV_5() throws Student.InvalidCSVLineException, Student.InvalidPermException {
        String csv = "Chris Gaucho,1111119";
        Student s = null;
        s = Student.fromCSV(csv);

Note that because our custom exceptions are checked exceptions (see Chapter 11 in HFJ2), they have to be “caught or declared to be thrown”. In our test cases, since we want the exceptions to be thrown (that’s what the test case is checking for), we don’t want to wrap the call to fromCSV in a try/catch. Accordingly, we declare the exceptions on the first line of the method with the throws Student.InvalidCSVLineException, Student.InvalidPermException between the closing ) of the parameter list and the opening { of the method.

public void test_fromCSV_5() throws Student.InvalidCSVLineException, Student.InvalidPermException {

Example of Running ReadStudents

To test our program, we set up two files in the directory data:

  • The file data/goodData.csv contains these lines, each of which has valid data:

    Chris Gaucho,1111111
    Laurel del Playa,1234566
    Alex Sabado,999996
  • The file `data/mixedData.csv has good data mixed in with bad data:

    Chris Gaucho,1111111
    This is a bad line of data
    Laurel del Playa,1234566
    Bad Perm,not-an-integer
    Alex Sabado,999996
    Luhn McBadperm,1234567

Next we compile the code to a .jar file with mvn package, then run our program using the command:

java -cp jar-file-name class-name cmd-line-args


  • jar-file-name is target/student-1.0.0.jar, the .jar (Java Archive) produced by the mvn package command
  • class-name is the name of the class containing the main method that we want to run, with the full package name.

    In this case, that’s edu.ucsb.cs156.student.ReadStudents

  • cmd-line-args are the value we want to pass in to the command line arguments, in this case the filename of our data

    That can be data/goodData.csv for example.

Here’s what that looks like.

% mvn package
... [output omitted] ...
% java -cp target/student-1.0.0.jar edu.ucsb.cs156.student.ReadStudents data/goodData.csv
[[name: Chris Gaucho, perm: 1111111], [name: Laurel del Playa, perm: 1234566], [name: Alex Sabado, perm: 999996]]

Notice that the type ArrayList<Student> has a toString() method defined that outputs the entire array; it invokes the toString() method of each Student object to output each student.

If we use a .csv file that has a few bad lines in it mixed in with the good lines, we get this as output:

% java -cp target/student-1.0.0.jar edu.ucsb.cs156.student.ReadStudents data/mixedData.csv
Invalid line ignored: This is a bad line of data
Line with invalid perm ignored: Bad Perm,not-an-integer
Line with invalid perm ignored: Luhn McBadperm,1234567
[[name: Chris Gaucho, perm: 1111111], [name: Laurel del Playa, perm: 1234566], [name: Alex Sabado, perm: 999996]]


In this exercise, ex13, we:

  • Added a new main class that reads from a file into an ArrayList<Student>
  • Added a static method to Student to create a new Student object from a String instance that represents one line of a .csv file
  • Added two custom exception types that allow us to customize the error output in our main program.
  • Unit tests for the conversion of the line from CSV
  • Added lines in our pom.xml so that ReadStudent would not appear in test coverage reports.

In ex14, we’ll discuss how to sort ArrayList<Student> instances by either name or perm.