How do I split an arraylist dynamically?

Question

*Disclaimer: I am a super noob in java so bear with me.

I have a arraylist called hw_list that contains strings that were read in from a file like so:

    [Doe John 1 10 1 Introduction.java, Doe Jane 1 11 1 Introduction.java, Smith Sam 2 15 2 Introduction.java Test.java]

I was able to make each element of the array it's own sublist so it prints like so:

    [Doe John 1 10 1 Introduction.java] 
    [Doe Jane 1 11 1 Introduction.java]
    [Smith Sam 2 15 2 Introduction.java Test.java]

But to split each element into it's own sublist like above, I have to manually write each sublist out like:

    List<String> student1 = hw_list.subList(0, 1);
    List<String> student2 = hw_list.subList(1, 2);
    List<String> student3 = hw_list.subList(2, 3);

My problem is that the number of string that are read in can change so I don't know how many sublists to make ahead of time.

Is there a way to dynamically create new lists using a loop and then splitting each element based on hw_list.size()?

Is something sort of like this possible:

    for(int i=0; i<hw_list.size(); i++){
        List<String> student(i) = hw_list.sublist(i, i+1)
    }

TL;DR

How do I get a loop to create a new list for every element of an array?

Why are you splitting the element into list in the first place? I would avoid having to do this at all as it sounds overly complicated and inefficient. — Peter Lawrey
– Peter Lawrey, Commented Oct 13, 2013 at 15:14
@PeterLawrey Because I need to do certain things with each individual student. — Drieke
– Drieke, Commented Oct 13, 2013 at 15:17
Then it is better to create a Student class and do for (Student s : listOfStudents) — arynaq
– arynaq, Commented Oct 13, 2013 at 15:19
What do the numbers represent? The third one looks like a count of items to the right of it, but what are the first two? — Sergey Kalinichenko
– Sergey Kalinichenko, Commented Oct 13, 2013 at 15:22
Does the input file have a specific pattern, can a student have more than one name for instance? What you should do is to create a Student class and parse the lines in the file (according to a pattern) into these classes. After creating the objects you can easily work on them. Java is an object-oriented language so try to think in that paradigm while using it. — Birkan Cilingir
– Birkan Cilingir, Commented Oct 13, 2013 at 15:25

Birkan Cilingir · Accepted Answer · 2013-10-13 15:39:21Z

What you have coded runs fine, it does not make sense logically: single-item sublists that you have cannot be expanded by adding more elements, and they would also change with the underlying array list.

What you should do instead is building a class to represent the data stored in a single element as a group of related, meaningful items, such as first name, last name, section and date submitted, like this:

public class Student {
    private String firstName;
    private String lastName;
    private List<String> fileNames;
    private int section;
    private int date; // Consider changing this to a different type
    public Student(String firstName, String lastName, int section, int date) {
        this.firstName = firstName;
        this.lastName = lastName;
        this.section = section;
        this.date = date;
        fileNames = new ArrayList<String>();
    }
    public String getFirstName() { return firstName; }
    public String getLastName() { return lastName; }
    public int getSection() { return section; }
    public int getDateSubmitted() { return date; }
    public List<String> getFileNames() { return fileNames; }
}

Then you could make a method that takes a String, and produces a Student, like this:

private static Student studentFromString(String studentRep) {
    String[] tokens = studentRep.split(" ");
    Student res = new Student(tokens[0], tokens[1], Integer.parseInt(tokens[2]), Integer.parseInt(tokens[3]));
    // You can ignore tokens[4] because you know how many files are submitted
    // by counting the remaining tokens.
    for (int i = 5 ; i != tokens.length ; i++) {
        res.getFileNames().add(tokens[i]);
    }
    return res;
}

Flight Odyssey · Accepted Answer · 2013-10-13 15:38:33Z

1

Once you have followed dasblinkenlight's advice, convert each String into a student:

List<Student> students = new ArrayList<Student>();
for(String studentRep:hw_list){
    students.add(Student.studentFromString(studentRep));
}

Then you can do stuff with your list of students, like this:

for(Student student:students){
    System.out.println(student.getFirstName());
}

answered Oct 13, 2013 at 15:38

Flight Odyssey

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