HashMap

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layout: default title: Map Implementation Class - HashMap

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To create a Map, we need an implementation class.

HashMap is the general-purpose implementation of the Map interface.

 Map<Integer, String> map = new HashMap<>();

Using the Map

We use put(Integer, String) to add data to the HashMap.

map.put(42, "Robinson");

Use get(Integer) to get Strings from the Map.

String value = map.get(42);

If there is no mapping for a key, get(Integer) will return null.

Key and Value Methods

boolean containsKey(Object) - returns true if the key is in the Map. Takes any Object.

boolean containsValue(Object) - returns true if the value is in the Map. Takes any Object.

Iterating Through a Map's Keys or Values

Map does not define an iterator() method. * Set<K> keySet() retrieves a Set of keys that you can then iterate on.

```java
Set<Integer> s = map.keySet();      // Set of Integer keys
Iterator<Integer> it = s.iterator();

while (it.hasNext()) {
  Integer key = it.next();
  System.out.print("Key: " + key);
  System.out.println(" Value: " + map.get(key));
}
```

• Collection<V> values() retrieves a Collection of values you could iterate on.

Remember: * We put into a Map; we add to a Set or List. * keySet() returns a Set; values() returns a Collection.

Practice Exercise

Why the name hash map?

HashMap calls the hashCode() method of each key to evenly distribute key-value pairs in the underlying data structure (which is an array).

Drill

MapInterface/com.example.mapinterface.drills.UsingHashMap

This drill has you declaring a Map, putting some data into it, and retrieving the data. * Follow instructions in the class. * Note the order of the key-value pairs when you iterate through the Map.

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