How to Work with Hash Map in Java
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Working With HashMap HashMap is HashTable based implementation of Map. This is
the reason why interviewer always ask for difference
between HashMap and HashTable. HashMap is mostly
equals to HashTable except below two differences.
HashMap is unsynchronized while HashTable is
synchronised.
HashMap permits null while HashTable doesn’t.
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Working With HashMap DEFAULT_INITIAL_CAPACITY : Default Initial Capacity(2
power n). Number of element HashMap can contain.
MAXIMUM_CAPACITY : Maximum Capacity of HashMap (2
power n).
LOADFACTOR: Defines threshold of HashMap. When re-
sizing will occur of HashMap.
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Working With HashMap DEFAULT_LOAD_FACTOR : Will be used when no load factor
is defined in constructor of HashMap.
SIZE : Number of key-value pair mapping, HashMap
contains.
When there is no parameter defined while creating
HashMap default Initial Capacity(16) and Default load
factor(0.75) will be used.
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Working With HashMap HashMap can contain up to 16 element and resizing of
HashMap will occur when 13th element will be inserted.
This is because load factor is 75%(.75) and this threshold
will be crossed when you add 13th element(12+1).
We can also provide initial capacity and load Factor. But
initial capacity can not be more than maximum capacity (2
power 30) and load factor can not be zero or negative
number.
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Working With HashMap In order to add any element we need to provide 2 thing, key
and value.
Key : key with which specified value will be
associated. null is allowed.
Value : value to be associated with specified key.
First HashMap will generate hashcode for given key and
then check if there is any value already associated with
given key or not.
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Working With HashMap If there is any value already associated with given key then it
will return already associated value. Else it will add value in
HashMap with provided key.
HashMap doesn’t give any Guarantee in order of elements in
Map(Means Order can change over time).
HashMap provide Constant time performance
for get & set operation(If proper Hashing algorithm is used).
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Working With HashMap Time requires to Iterate collection is proportional to
“Capacity“(Elements it can hold) & Size(Elements it is
holding currently) of HashMap.
In case iteration performance is more important then it is
advisable to not set initial capacity too high and load factor
too low. As performance is directly proportional to Initial
Capacity and load Factor.
capacity is the number of buckets in the hash table.
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Working With HashMap initial capacity(Default Value is 16) is simply the capacity at the time
the hash table is created.
The load factor(Default value .75) is a measure of how full the hash
table is allowed to get before its capacity is automatically increased.
When the number of entries in the hash table exceeds the product of
the load factor and the current capacity, the hash table
is rehashed (that is, internal data structures are rebuilt) .
“Collections.synchronizedMap()” method is used to make
Map synchronised.
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Working With HashMap Iterators returned by HashMap class is “fail-fast“.
HashMap is backed by an Array(Key) and LinkedList(Value).
HashMap uses hashcode(Using Key) to identify exact
location where object should be placed or retrieved in
HashMap.
In the end HashCode return the exact location(Index) in
backing array.
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Working With HashMap Backed Array have a fixed size. So whenever Array is
full(Number of keys in this map reaches its threshold). A
new Array with new capacity will be created and all element
will be added to this new Array.
HashCode will be used in both cases(Adding and Retrieving
Object) while equals() method may or may not be used in
any case.
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Working With HashMap Best candidate for Key in HashMap would be an Immutable
Class with properly implement Equals and Hashcode
method(Example: String Class and Wrappers Class).
The better hashcode and equals method implementation is
better performance of HashMap would be.
In such way String and Wrapper classes of all Primitives will
be great candidate for keys in HashMap.
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