JAVA HashMap 2D, cant get the right approach to make a 2D HashMap, i mean a HashMap into another HashMap

Question

I want to make a board of Students' names and Subjects and each student has a grade in each subject (or not.. he can leave the exam and doesnt write it, and then his case will be empty). I want to use just HashMaps. I mean, it will be something like that:

HashMap<String,HashMap<String,String>> bigBoard = 
    new HashMap<String,HashMap<String,String>>();

but I think, I dont have the right idea, because for each subject, there will be many grades (values) so that won't be possible. Do I have to make a map for each student? with his subject? but then the table on output won't be arranged. Do you have a proposition? I would like a table that look like something like that for example.

Column-Key →
  Rowkey↓      Mathematics         Physics       Finance

Daniel Dolter         1.3           3.7 

Micky Mouse                                      5

Minnie Mouse                        1.7          n/a

Dagobert Duck         4.0                        1.0

(I would use all the keys/values as Strings, it will be more simple like that.)

After the implementation of our class (for example class-name is String2D), we should use it like that.

public static void main(String[] args) {

    String2D map2D = new String2D(); 
    map2D.put("Daniel Doster", "Practical Mathematics", "1.3"); 
    map2D.put("Daniel Doster", "IT Systeme", "3.7"); 
    map2D.put("Micky Mouse", "Finance", "5");
    map2D.put("Minnie Mouse", "IT Systeme", "1.7");
    map2D.put("Minnie Mouse", "Finance", "n/a");
    map2D.put("Dagobert Duck", "Practical Mathematics", "4.0");
    map2D.put("Dagobert Duck", "Finance", "1.0");
    System.out.println(map2D); 
} 

No "HashMap" will be seen.. and Arrays aren't allowed

Answer 1

You can use Google Guava's Table<R, C, V> collection. It is similar to eabraham's answer. A value V is keyed by a row R and a column C. It is a better alternative to using HashMap<R, HashMap<C, V>> which becomes quickly unreadable and difficult to work with.

See their GitHub Wiki for more information.

Answer 2

With Java 8 it is possible to use computeIfAbsent to insert a default value if it is empty. So you can simply use this as the type of the 2d-map:

Map<RowType, Map<ColumnType, ValueType>> map = new WhateverMap<>();

let's say all types are int:

int get(int x, int y) 
  return map.computeIfAbsent(x, (key)->new WhateverMap<>()).computeIfAbsent(y,(key)->0);
}

void put(int x, int y, int value) 
  return map.computeIfAbsent(x, (key)->new WhateverMap<>()).put(y,value);
}

Note that is not atomic. therefore this is not thread-safe even if WhateverMap is.

Answer 3

I don't think a nested hashmap is the way to go. Create a Student class and Subject class.

public class Student{
    private ArrayList<Subject> SubjectList = new ArrayList<Subject>();
    private String name;

    public Student(String name){
        this.name=name;
    }
    public void addSubject(Subject s){
        SubjectList.add(s);
    }
    public String getName(){
        return this.name;
    }
    //...add methods for other operations
}
public class Subject{
    private ArrayList<double > GradeList = new ArrayList<double>();
    private String name;

    public Subject(String name){
        this.name=name;
    }
    public void addGrade(double s){
        GradeList.add(s);
    }
    //...add methods for other operations
}

Then you can store the Students instances in a hashmap.

public static void main(String[] args){
    HashMap<Students> hm = new HashMap<Students>();
    Student s = new Student("Daniel Dolter");
    Subject sub = new Subject("Mathematics");
    sub.addGrades(1.3);
    s.addSubject(sub);
    hm.put(s.getName(),s);
}

Answer 4

You can use this class:

public class BiHashMap<K1, K2, V> {

private final Map<K1, Map<K2, V>> mMap;

public BiHashMap() {
    mMap = new HashMap<K1, Map<K2, V>>();
}

/**
 * Associates the specified value with the specified keys in this map (optional operation). If the map previously
 * contained a mapping for the key, the old value is replaced by the specified value.
 * 
 * @param key1
 *            the first key
 * @param key2
 *            the second key
 * @param value
 *            the value to be set
 * @return the value previously associated with (key1,key2), or <code>null</code> if none
 * @see Map#put(Object, Object)
 */
public V put(K1 key1, K2 key2, V value) {
    Map<K2, V> map;
    if (mMap.containsKey(key1)) {
        map = mMap.get(key1);
    } else {
        map = new HashMap<K2, V>();
        mMap.put(key1, map);
    }

    return map.put(key2, value);
}

/**
 * Returns the value to which the specified key is mapped, or <code>null</code> if this map contains no mapping for
 * the key.
 * 
 * @param key1
 *            the first key whose associated value is to be returned
 * @param key2
 *            the second key whose associated value is to be returned
 * @return the value to which the specified key is mapped, or <code>null</code> if this map contains no mapping for
 *         the key
 * @see Map#get(Object)
 */
public V get(K1 key1, K2 key2) {
    if (mMap.containsKey(key1)) {
        return mMap.get(key1).get(key2);
    } else {
        return null;
    }
}

/**
 * Returns <code>true</code> if this map contains a mapping for the specified key
 * 
 * @param key1
 *            the first key whose presence in this map is to be tested
 * @param key2
 *            the second key whose presence in this map is to be tested
 * @return Returns true if this map contains a mapping for the specified key
 * @see Map#containsKey(Object)
 */
public boolean containsKeys(K1 key1, K2 key2) {
    return mMap.containsKey(key1) && mMap.get(key1).containsKey(key2);
}

public void clear() {
    mMap.clear();
}

}

And then create use it like this:

BiHashMap<String,String,String> bigBoard = new BiHashMap<String,String,String>();

However for performance you may want to store the different grades in an array (assuming that you have a fix set of courses)