JAVA / JAVA COLLECTIONS FRAMEWORK /
HASHMAP

1. GENERALLY

All methods of Map is available.

Specific method are below.

Constructor signatures:

• (): default capacity is 16, default load factor is 0.75

• (capacity in int, load factor in float): how many entries it can store, if load factor is exceeded, the map is moved to an other map with double size

• (other Map class): converting from that map to HashMap; need identical pair types

EXAMPLE FOR CONVERSION:

EXAMPLE FOR USAGE OF HASHMAP:

EXAMPLE FOR ITERATION:

Differences between HashMap and WeakHashMap:

• HashMap keys are strong reference type, WeakHashMap keys are weak reference type

• by the GC, released (not used) keys will be left in HashMap while will be freed in WeakHashMap

Differences between ConcurrentHashMap and HashMap:

• ConcurrentHashMap is thread-safe, HashMap is not

• ConcurrentHashMap provides methods for bulk operations like forEach(), search() and reduce(), HashMap does not

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2. CONVERT TO ARRAYLIST AND BACK

EXAMPLE FOR CONVERTING TO ARRAYLIST AND BACK:

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