What is HashMap in Java ?

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Previously we have discussed about
What is Map Interface in Java ?

Here, we learn What is HashMap In Java.But before starting we must know what is Map Interface In Java.?
Class HashMap<K,V>

Type Parameters:

K - the type of keys maintained by this map

V - the type of mapped values

All Implemented Interfaces:

Serializable, Cloneable, Map<K,V>

Declaration of HashMap:

public class HashMap<K,V>

extends AbstractMap<K,V>

implements Map<K,V>, Cloneable, Serializable

HashMap is a implementation class of Map Interface.It belongs to java.util.*; package .The HashMap class is roughly equivalent to Hashtable, except that HashMap is unsynchronized and permits nulls. 

This class does not guarantee that the order will remain constant over time. 

An instance of HashMap has two parameters that affect its performance: initial capacity and load factor.  

The capacity is the number of buckets in the hash table, and the initial capacity is simply the capacity at the time the hash table is created.  

The load factor is a measure of how full the hash table is allowed to get before its capacity is automatically increased,the default load factor is(.75). 

HashMap implementation is not synchronized. If multiple threads access a hash map concurrently, and at least one of the threads modifies the map means that thread can adds or deletes one or more mappings.To prevent this we can synchronized the Map using 

Map m = Collections.synchronizedMap(new HashMap(...));

Hashmap is an example of fail-fast.And This class is a member of the Java Collections Framework.

Constructor and Description :-

HashMap()

Constructs an empty HashMap with the default initial capacity (16) and the default load factor (0.75).

HashMap(int initialCapacity)

Constructs an empty HashMap with the specified initial capacity and the default load factor (0.75).

HashMap(int initialCapacity, float loadFactor)

Constructs an empty HashMap with the specified initial capacity and load factor.

HashMap(Map<? extends K,? extends V> m)

Constructs a new HashMap with the same mappings as the specified Map.

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What is Arrays Class In Java ?

Previously we have discussed about

What is Comparator Interface In Java ?

Class Arrays

Declaration of Arrays:

public class Arrays

extends Object

Arrays class contains various methods for manipulating arrays (such as sorting and searching) and they are useful when working with arrays. This class also contains a static factory that allows arrays to be viewed as lists.

The asList( ) method returns a List that is backed by a specified array. In other words, both the list and the array refer to the same location. It has the following signature:

                           static <T> List asList(T... array)

The binarySearch( ) method uses a binary search to find a specified value.This method must be applied to sorted arrays. Here is one of its forms.

                           static int binarySearch(int array[ ], int value)

The copyOf( ) method returns a copy of an array and it has one of its form:

                           static int[ ] copyOf(int[ ] source, int len)

This class is a member of the Java Collections Framework.

Method Summary of Arrays:

Modifier and Method Name	Description
static <T> List<T> asList(T... a)	Returns a fixed-size list backed by the specified array.
static int binarySearch(byte[] a, byte key)	Searches the specified array of bytes for the specified value using the binary search algorithm.
static int binarySearch(byte[] a, int fromIndex, int toIndex, byte key)	Searches a range of the specified array of bytes for the specified value using the binary search algorithm.
static int binarySearch(char[] a, char key)	Searches the specified array of chars for the specified value using the binary search algorithm.
static int binarySearch(char[] a, int fromIndex, int toIndex, char key)	Searches a range of the specified array of chars for the specified value using the binary search algorithm.
static int binarySearch(double[] a, double key)	Searches the specified array of doubles for the specified value using the binary search algorithm.
static int binarySearch(double[] a, int fromIndex, int toIndex, double key)	Searches a range of the specified array of doubles for the specified value using the binary search algorithm.
static int binarySearch(float[] a, float key)	Searches the specified array of floats for the specified value using the binary search algorithm.
static int binarySearch(float[] a, int fromIndex, int toIndex, float key)	Searches a range of the specified array of floats for the specified value using the binary search algorithm.
static int binarySearch(int[] a, int key)	Searches the specified array of ints for the specified value using the binary search algorithm.
static int binarySearch(int[] a, int fromIndex, int toIndex, int key)	Searches a range of the specified array of ints for the specified value using the binary search algorithm.
static int binarySearch(long[] a, int fromIndex, int toIndex, long key)	Searches a range of the specified array of longs for the specified value using the binary search algorithm.
static int binarySearch(long[] a, long key)	Searches the specified array of longs for the specified value using the binary search algorithm.
static int binarySearch(Object[] a, int fromIndex, int toIndex, Object key)	Searches a range of the specified array for the specified object using the binary search algorithm.
static int binarySearch(Object[] a, Object key)	Searches the specified array for the specified object using the binary search algorithm.
static int binarySearch(short[] a, int fromIndex, int toIndex, short key)	Searches a range of the specified array of shorts for the specified value using the binary search algorithm.
static int binarySearch(short[] a, short key)	Searches the specified array of shorts for the specified value using the binary search algorithm.
static <T> int binarySearch(T[] a, int fromIndex, int toIndex, T key, Comparator<? super T> c)	Searches a range of the specified array for the specified object using the binary search algorithm.
static <T> int binarySearch(T[] a, T key, Comparator<? super T> c)	Searches the specified array for the specified object using the binary search algorithm.
static boolean[]                copyOf(boolean[] original, int newLength)	Copies the specified array, truncating or padding with false (if necessary) so the copy has the specified length.
static byte[] copyOf(byte[] original, int newLength)	Copies the specified array, truncating or padding with zeros (if necessary) so the copy has the specified length.
static char[] copyOf(char[] original, int newLength)	Copies the specified array, truncating or padding with null characters (if necessary) so the copy has the specified length.
static double[] copyOf(double[] original, int newLength)	Copies the specified array, truncating or padding with zeros (if necessary) so the copy has the specified length.
static float[] copyOf(float[] original, int newLength)	Copies the specified array, truncating or padding with zeros (if necessary) so the copy has the specified length.
static int[] copyOf(int[] original, int newLength)	Copies the specified array, truncating or padding with zeros (if necessary) so the copy has the specified length.
static long[] copyOf(long[] original, int newLength)	Copies the specified array, truncating or padding with zeros (if necessary) so the copy has the specified length.
static short[] copyOf(short[] original, int newLength)	Copies the specified array, truncating or padding with zeros (if necessary) so the copy has the specified length.
static <T> T[] copyOf(T[] original, int newLength)	Copies the specified array, truncating or padding with nulls (if necessary) so the copy has the specified length.
static <T,U> T[] copyOf(U[] original, int newLength, Class<? extends T[]> newType)	Copies the specified array, truncating or padding with nulls (if necessary) so the copy has the specified length.
static boolean[]                copyOfRange(boolean[] original, int from, int to)	Copies the specified range of the specified array into a new array.
static byte[] copyOfRange(byte[] original, int from, int to)	Copies the specified range of the specified array into a new array.
static char[] copyOfRange(char[] original, int from, int to)	Copies the specified range of the specified array into a new array.
static double[] copyOfRange(double[] original, int from, int to)	Copies the specified range of the specified array into a new array.
static float[] copyOfRange(float[] original, int from, int to)	Copies the specified range of the specified array into a new array.
static int[] copyOfRange(int[] original, int from, int to)	Copies the specified range of the specified array into a new array.
static long[] copyOfRange(long[] original, int from, int to)	Copies the specified range of the specified array into a new array.
static short[] copyOfRange(short[] original, int from, int to)	Copies the specified range of the specified array into a new array.
static <T> T[] copyOfRange(T[] original, int from, int to)	Copies the specified range of the specified array into a new array.
static <T,U> T[] copyOfRange(U[] original, int from, int to, Class<? extends T[]> newType)	Copies the specified range of the specified array into a new array.
static boolean deepEquals(Object[] a1, Object[] a2)	Returns true if the two specified arrays are deeply equal to one another.
static int deepHashCode(Object[] a)	Returns a hash code based on the "deep contents" of the specified array.
static String deepToString(Object[] a)	Returns a string representation of the "deep contents" of the specified array.
static boolean equals(boolean[] a, boolean[] a2)	Returns true if the two specified arrays of booleans are equal to one another.
static boolean equals(byte[] a, byte[] a2)	Returns true if the two specified arrays of bytes are equal to one another.
static boolean equals(char[] a, char[] a2)	Returns true if the two specified arrays of chars are equal to one another.
static boolean equals(double[] a, double[] a2)	Returns true if the two specified arrays of doubles are equal to one another.
static boolean equals(float[] a, float[] a2)	Returns true if the two specified arrays of floats are equal to one another.
static boolean equals(int[] a, int[] a2)	Returns true if the two specified arrays of ints are equal to one another.
static boolean equals(long[] a, long[] a2)	Returns true if the two specified arrays of longs are equal to one another.
static boolean equals(Object[] a, Object[] a2)	Returns true if the two specified arrays of Objects are equal to one another.
static bolean equals(short[] a, short[] a2)	Returns true if the two specified arrays of shorts are equal to one another.
static void fill(boolean[] a, boolean val)	Assigns the specified boolean value to each element of the specified array of booleans.
static void fill(boolean[] a, int fromIndex, int toIndex, boolean val)	Assigns the specified boolean value to each element of the specified range of the specified array of booleans.
static void fill(byte[] a, byte val)	Assigns the specified byte value to each element of the specified array of bytes.
static void fill(byte[] a, int fromIndex, int toIndex, byte val)	Assigns the specified byte value to each element of the specified range of the specified array of bytes.
static void fill(char[] a, char val)	Assigns the specified char value to each element of the specified array of chars.
static void fill(char[] a, int fromIndex, int toIndex, char val)	Assigns the specified char value to each element of the specified range of the specified array of chars.
static void fill(double[] a, double val)	Assigns the specified double value to each element of the specified array of doubles.
static void fill(double[] a, int fromIndex, int toIndex, double val)	Assigns the specified double value to each element of the specified range of the specified array of doubles.
static void fill(float[] a, float val)	Assigns the specified float value to each element of the specified array of floats.
static void fill(float[] a, int fromIndex, int toIndex, float val)	Assigns the specified float value to each element of the specified range of the specified array of floats.
static void fill(int[] a, int val)	Assigns the specified int value to each element of the specified array of ints.
static void fill(int[] a, int fromIndex, int toIndex, int val)	Assigns the specified int value to each element of the specified range of the specified array of ints.
static void fill(long[] a, int fromIndex, int toIndex, long val)	Assigns the specified long value to each element of the specified range of the specified array of longs.
static void fill(long[] a, long val)	Assigns the specified long value to each element of the specified array of longs.
static void fill(Object[] a, int fromIndex, int toIndex, Object val)	Assigns the specified Object reference to each element of the specified range of the specified array of Objects.
static void fill(Object[] a, Object val)	Assigns the specified Object reference to each element of the specified array of Objects.
static void fill(short[] a, int fromIndex, int toIndex, short val)	Assigns the specified short value to each element of the specified range of the specified array of shorts.
static void fill(short[] a, short val)	Assigns the specified short value to each element of the specified array of shorts.
static int hashCode(boolean[] a)	Returns a hash code based on the contents of the specified array.
static int hashCode(byte[] a)	Returns a hash code based on the contents of the specified array.
static int hashCode(char[] a)	Returns a hash code based on the contents of the specified array.
static int hashCode(double[] a)	Returns a hash code based on the contents of the specified array.
static int hashCode(float[] a)	Returns a hash code based on the contents of the specified array.
static int hashCode(int[] a)	Returns a hash code based on the contents of the specified array.
static int hashCode(long[] a)	Returns a hash code based on the contents of the specified array.
static int hashCode(Object[] a)	Returns a hash code based on the contents of the specified array.
static int hashCode(short[] a)	Returns a hash code based on the contents of the specified array.
static void sort(byte[] a)	Sorts the specified array into ascending numerical order.
static void sort(byte[] a, int fromIndex, int toIndex)	Sorts the specified range of the array into ascending order.
static void sort(char[] a)	Sorts the specified array into ascending numerical order.
static void sort(char[] a, int fromIndex, int toIndex)	Sorts the specified range of the array into ascending order.
static void sort(double[] a)	Sorts the specified array into ascending numerical order.
static void sort(double[] a, int fromIndex, int toIndex)	Sorts the specified range of the array into ascending order.
static void sort(float[] a)	Sorts the specified array into ascending numerical order.
static void sort(float[] a, int fromIndex, int toIndex)	Sorts the specified range of the array into ascending order.
static void sort(int[] a)	Sorts the specified array into ascending numerical order.
static void sort(int[] a, int fromIndex, int toIndex)	Sorts the specified range of the array into ascending order.
static void sort(long[] a)	Sorts the specified array into ascending numerical order.
static void sort(long[] a, int fromIndex, int toIndex)	Sorts the specified range of the array into ascending order.
static void sort(Object[] a)	Sorts the specified array of objects into ascending order, according to the natural ordering of its elements.
static void sort(Object[] a, int fromIndex, int toIndex)	Sorts the specified range of the specified array of objects into ascending order, according to the natural ordering of its elements.
static void sort(short[] a)	Sorts the specified array into ascending numerical order.
static void sort(short[] a, int fromIndex, int toIndex)	Sorts the specified range of the array into ascending order.
static <T> void sort(T[] a, Comparator<? super T> c)	Sorts the specified array of objects according to the order induced by the specified comparator.
static <T> void   sort(T[] a, int fromIndex, int toIndex, Comparator<? super T> c)	Sorts the specified range of the specified array of objects according to the order induced by the specified comparator.
static String toString(boolean[] a)	Returns a string representation of the contents of the specified array.
static String toString(byte[] a)	Returns a string representation of the contents of the specified array.
static String toString(char[] a)	Returns a string representation of the contents of the specified array.
static String toString(double[] a)	Returns a string representation of the contents of the specified array.
static String toString(float[] a)	Returns a string representation of the contents of the specified array.
static String toString(int[] a)	Returns a string representation of the contents of the specified array.
static String toString(long[] a)	Returns a string representation of the contents of the specified array.
static String toString(Object[] a)	Returns a string representation of the contents of the specified array.
static String toString(short[] a)	Returns a string representation of the contents of the specified array.

Example of Arrays:

import java.util.Arrays;

public class ArraysExample {

static void display(int array[]) {

for(int i: array)

System.out.print(i + " ");

System.out.println();

}

public static void main(String[] args) {

// Allocate and initialize array.

int array[] = new int[10];

for(int i = 0; i < 10; i++)

array[i] = -3 * i;

// Display, sort, and display the array.

System.out.print("Original contents: ");

display(array);

Arrays.sort(array);

System.out.print("Sorted: ");

display(array);

// Fill and display the array.

Arrays.fill(array, 2, 6, -1);

System.out.print("After fill(): ");

display(array);

// Sort and display the array.

Arrays.sort(array);

System.out.print("After sorting again: ");

display(array);

// Binary search for -9.

System.out.print("The value -9 is at location ");

int index =

Arrays.binarySearch(array, -9);

System.out.println(index);

}

}

Program Output:

Original contents: 0 -3 -6 -9 -12 -15 -18 -21 -24 -27 

Sorted: -27 -24 -21 -18 -15 -12 -9 -6 -3 0 

After fill(): -27 -24 -1 -1 -1 -1 -9 -6 -3 0 

After sorting again: -27 -24 -9 -6 -3 -1 -1 -1 -1 0 

The value -9 is at location 2

      

Blog Author - Pushkar Khosla,
Software Developer by Profession with 3.0 Yrs of Experience , through this blog i'am sharing my industrial Java Knowledge to entire world. For any question or query any one can comment below or mail me at pushkar.itsitm52@gmail.com.

This blog is all about to learn Core Java ,Interview Programs and Coding tricks to polish your Java Knowledge. If you like the content of this blog please share this with your friends.

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