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Map Interface
The Map interface maps unique keys to values. A key is an object that you use to retrieve a value at a later date.
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Given a key and a value, you can store the value in a Map object. After the value is stored, you can retrieve it by using its key.
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Several methods throw a NoSuchElementException when no items exist in the invoking map.
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A ClassCastException is thrown when an object is incompatible with the elements in a map.
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A NullPointerException is thrown if an attempt is made to use a null object and null is not allowed in the map.
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An UnsupportedOperationException is thrown when an attempt is made to change an unmodifiable map.
Map Interface Methods
| Sr.No. | Method & Description |
|---|---|
| 1 |
void clear( ) Removes all key/value pairs from the invoking map. |
| 2 |
boolean containsKey(Object k) Returns true if the invoking map contains k as a key. Otherwise, returns false. |
| 3 |
boolean containsValue(Object v) Returns true if the map contains v as a value. Otherwise, returns false. |
| 4 |
Set entrySet( ) Returns a Set that contains the entries in the map. The set contains objects of type Map.Entry. This method provides a set-view |
| 5 |
boolean equals(Object obj) Returns true if obj is a Map and contains the same entries. Otherwise, returns false. |
| 6 |
Object get(Object k) Returns the value associated with the key k. |
| 7 |
int hashCode( ) Returns the hash code for the invoking map. |
| 8 |
boolean isEmpty( ) Returns true if the invoking map is empty. Otherwise, returns false. |
| 9 |
Set keySet( ) Returns a Set that contains the keys in the invoking map. This method provides a set-view of the keys in the invoking map. |
| 10 |
Object put(Object k, Object v) Puts an entry in the invoking map, overwriting any previous value associated with the key. The key and value are k and v, respectively. Returns null if the key did not already exist. Otherwise, the previous value linked to the key is returned. |
| 11 |
void putAll(Map m) Puts all the entries from m into this map. |
| 12 |
Object remove(Object k) Removes the entry whose key equals k. |
| 13 |
int size( ) Returns the number of key/value pairs in the map. |
| 14 |
Collection values( ) Returns a collection containing the values in the map. This method provides a collection-view of the values in the map. |
Classes that Implement Map
The following are the classes that implement a Map to use the functionalities of a Map –
- HashMap
- EnumMap
- LinkedHashMap
- WeakHashMap
- TreeMap
Interfaces that Extend Map
The following are the interfaces that extend the Map interface –
- SortedMap
- NavigableMap
- ConcurrentMap
Examples of Map Interface
Example 1
Map has its implementation in various classes like HashMap. Following is an example to explain map functionality −
import java.util.HashMap;
import java.util.Map;
public class CollectionsDemo {
public static void main(String[] args) {
Map<String, String> m1 = new HashMap<>();
m1.put("Zara", "8");
m1.put("Mahnaz", "31");
m1.put("Ayan", "12");
m1.put("Daisy", "14");
System.out.println();
System.out.println(" Map Elements");
System.out.print("t" + m1);
}
}
Output
Map Elements
{Daisy = 14, Ayan = 12, Zara = 8, Mahnaz = 31}
Example 2
Map has its implementation in various classes like TreeMap which sorts the entries based on keys. Following is an example to explain map functionality using TreeMap −
import java.util.Map;
import java.util.TreeMap;
public class CollectionsDemo {
public static void main(String[] args) {
Map<String, String> m1 = new TreeMap<>();
m1.put("Zara", "8");
m1.put("Mahnaz", "31");
m1.put("Ayan", "12");
m1.put("Daisy", "14");
System.out.println();
System.out.println(" Map Elements");
System.out.print("t" + m1);
}
}
Output
Map Elements
{Ayan=12, Daisy=14, Mahnaz=31, Zara=8}
Example 3
Map has its implementation in various classes like HashMap. Following is an example to explain map functions using HashMap to add and remove elements to the map−
import java.util.HashMap;
import java.util.Map;
public class CollectionsDemo {
public static void main(String[] args) {
Map<String, String> m1 = new HashMap<>();
m1.put("Zara", "8");
m1.put("Mahnaz", "31");
m1.put("Ayan", "12");
m1.put("Daisy", "14");
System.out.println();
System.out.println(" Map Elements");
System.out.print("t" + m1);
m1.remove("Daisy");
System.out.println(" Map Elements");
System.out.print("t" + m1);
}
}
Output
Map Elements
{Daisy=14, Ayan=12, Zara=8, Mahnaz=31} Map Elements
{Ayan=12, Zara=8, Mahnaz=31}
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