What happens to the values of duplicate keys in a tree map

Question

I have a text file which is read into a Tree Map. The Text File contains a list of students and their marks.

Harry 10
Sam    8
John   7
Harry  8
Sam    9

I have tried writing this code, which reads a file, stores it in the Tree Map and displays it on the console.

Scanner scanner = new Scanner(new FileReader("marks.txt"));
    TreeMap<String, Integer> students = new TreeMap<String, Integer>();
    while (scanner.hasNextLine()) {
        String[] columns = scanner.nextLine().split("\\s+");
        students.put(columns[0], Integer.parseInt(columns[1]));
    }
    System.out.println("Alpha Order");
    // Iterate over TreeMap
    for (String key : students.keySet()) {
        System.out.println(key + " :: " + students.get(key));
    }

The output I get on the console after executing the code is

Alpha Order
Harry :: 10
John :: 7
Sam :: 9 

I need the other values for further computations. is it possible to have an output like

Harry :: 10 8
John  ::  7
Sam   ::  9 8

Answer 1

As others have pointed out, the values will be replaced since a Map only allows one key -> value mapping.

The easiest way of mapping multiple values to a key is to use a multi(value)map. I prefer Guava for this:

Scanner scanner = new Scanner(new FileReader("marks.txt"));
TreeMultimap<String, Integer> students = TreeMultimap.create();
while (scanner.hasNextLine()) {
    String[] columns = scanner.nextLine().split("\\s+");
    students.put(columns[0], Integer.parseInt(columns[1]));
}
System.out.println("Alpha Order");
// Iterate over TreeMap
for (String key : students.keySet()) {
    System.out.println(key + " :: " + String.join(", ", students.get(key)));
}

But if you can't add dependencies you can use a TreeMap<String, TreeSet<Integer>>:

Scanner scanner = new Scanner(new FileReader("marks.txt"));
TreeMap<String, TreeSet<Integer>> students = new TreeMap<>();
while (scanner.hasNextLine()) {
    String[] columns = scanner.nextLine().split("\\s+");
    String student = columns[0];
    TreeSet<Integer> set;
    if(students.containsKey(student)) {
        // A set already exists in the map for this student,
        // append to it
        set = students.get(student);
    } else {
        // No set exists in the map for this student,
        // create a new one and put it in the map
        set = new TreeSet<>();
        students.put(student, set);
    }
    set.add(Integer.parseInt(columns[1]));
}
System.out.println("Alpha Order");
// Iterate over TreeMap
for (String key : students.keySet()) {
    System.out.println(key + " :: " + String.join(", ", students.get(key)));
}

Note that these two are using sets so won't allow duplicate key -> value mappings. I.e. you can't get this output:

Harry :: 10 8 8
John  ::  7 7
Sam   ::  9 9 8

That's easy to achieve with the latter solution though by just replacing the TreeSet with an ArrayList.