<aside> 💡 Key-value structures comes under the Map Interface.

</aside>

• Has Two type Parameters:
  • K is the type for keys.
  • V is the type for values.
• get(k) → Fetches the value for the key k.
• put(k,v) → Updates the value for key k.

public interface Map<K,V>{
	V get(Object key);
	V put(K key, V value);

	boolean containsKey(Object key);
	boolean containsValue(Object value);
	...
}

• Only one entry per key-value.
• Assigning a fresh value to existing key overwrites the old value.
• put(k,v) returns the previous value associated with k or null .

Updating a Map

• There is an initialization problem with Maps:
  • If key exists then the value related to the key should be updated.
  • If the key does not exist, then create a new key entry with the value.
  • For any value to be inserted , one has to do these checks.
  • What does Java provide to make this easy?

1. Map has the following default method → V getDefault(Object key,V defaultValue);

   • Example:

     Map<String,Integer> scores = ....;
     int score = scores.getDefault(bat,0);
     //sets score to 0 if key bat is not present
     

   • Now we can update the entry for key bat as follows:

     scores.put(bat, scores.getDefault(bat,0)+newscore); //Updates score of batsman with previus score + newscore
     

   ---

2. Can Use putIfAbsent() to initialize a missing key.

   scores.putIfAbsent(bat,0); //Sets the value of key bat to 0 if the key does not exist, else do nothing.
   scores.put(bat, scores.get(bat)+newscore);
   // now update value with newscore because the key bat is guaranteed to have a value(either 0 or
   // the previously stored value)
   

   ---

3. Or use merge()

   score.merge(bat,newscore, Integer::sum);
   

   • Initializes to newscore if key bat does not exist.
   • Otherwise, combines the current value with newscore using Integer::sum

Extracting Keys and Values

• Methods to extract keys and values:

  • Set<K> keySet(); →Is a Set because keys should not have duplicates.
  • Collection<V> values();
  • Set<Map.Entry<K,V>> entrySet() → A set of Key value mappings. The type returned is a special Set of type Map.Entry(Set of Map.Entry).
  • Java calls the above as views.

• Now use the above to iterate through a Map

  Set<String> keys = strmap.keySet();
  for(String key : keys){
  	// do something with key....
  }
  

  • To access the value associated with each key in the above loop, we have to look up the map again, i.e, use the get() function for the corresponding key.

• So use entrySet() to operate on key and associated value without looking up the map again:

  for(Map.Entry<String,Employee> entry : staff.entrySet()){
  	String k = entry.getKey();
  	Employee v = entry.getValue();
  	// now do something with k and v
  }
  

• We looked at the interface Map. Moving forward, we now look at the concrete implementations of Map