JAVA HashMap 2D，无法获得制作 2D HashMap 的正确方法，我的意思是将 HashMap 转换为另一个 HashMap

Question

我想制作一个包含学生姓名和科目的板，每个学生在每个科目中都有一个成绩(或者没有..他可以离开考试而不写，然后他的案子将是空的)。我只想使用 HashMap。我的意思是，它会是这样的:

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

但我认为，我没有正确的想法，因为对于每个科目，会有很多成绩(值)，所以这是不可能的。我必须为每个学生制作一张 map 吗？与他的主题？但随后将不会安排输出表。你有提议吗？例如，我想要一张看起来像那样的 table 。

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

(我会使用所有的键/值作为字符串，这样会更简单。)

在我们的类实现之后(比如类名是String2D)，我们应该这样使用它。

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); 
} 

不会看到“HashMap”......并且不允许使用数组

Answer 1

你可以使用这个类:

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();
}

}

然后像这样创建使用它:

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

但是为了提高性能，您可能希望将不同的成绩存储在一个数组中(假设您有一组固定的类(class))