Collections, Maps and Iterators - Absolute Java - page 923

The HashMap<K,V> , of course, implements all the methods in the Map<K,V>
interface and adds no other methods beyond constructors and an implementation of
the clone() method. A summary of the HashMap<K,V> constructors and clone() is
given in Display 16.10 .
As with the HashSet<T> class, if you intend to use your own class as the parameterized
type K in a HashMap<K,V> then your class must override the following methods:

public int hashCode();
public boolean equals(Object obj);

These methods are required for indexing and checking for uniqueness of the key.
See the discussion in Section 16.1 about overriding these methods and Section 15.5
about hash functions.

Display 16.10 Methods in the HashMap<K,V> Class

The HashMap<K,V> class is in the java.util package.
The HashMap<K,V> class extends the AbstractMap<K,V> class and implements the
Map<K,V> interface.
The HashMap<K,V> class implements all of the methods in the Map<K,V> interface (Display 16.9).
The only other methods in the HashMap<K,V> class are the constructors.
All the exception classes mentioned are the kind that are not required to be caught in a catch
block or declared in a throws clause.
All the exception classes mentioned are in the package java.lang and so do not require any
import statement.

public HashMap()
Creates a new, empty map with a default initial capacity of 16 and load factor of 0.75.

public HashMap( int initialCapacity)
Creates a new, empty map with a default capacity of initialCapacity and load factor of 0.75.
Throws an IllegalArgumentException if initialCapacity is negative.

public HashMap( int initialCapacity, float loadFactor)
Creates a new, empty map with the specified capacity and load factor.
Throws an IllegalArgumentException if initialCapacity is negative or loadFactor is
nonpositive.

public HashMap(Map<? extends K,? extends V> m)
Creates a new map with the same mappings as m . The initialCapacity is set to the same
size as m and the loadFactor to 0.75.
Throws a NullPointerException if m is null .

public Object clone()
Creates a shallow copy of this instance and returns it. The keys and values are not cloned.

The remainder of the methods are the same as those described for the Map<K,V> interface
(Display 16.9).