Usage of HashMap and Iterating through the contents of a Map

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Usage of HashMap

HashMap is an implementation of the Map interface that provides a Data Structure to store data in Key-Value pairs.

1. Declaring HashMap

Map<KeyType, ValueType> myMap = new HashMap<KeyType, ValueType>();

KeyType and ValueType must be valid types in Java, such as - String, Integer, Float or any custom class like Employee, Student etc..

Example

Map<String,Integer> myMap = new HashMap<String,Integer>();

2. Putting values in HashMap

To put a value in the HashMap, we have to call put method on the HashMap object by passing the Key and the Value as parameters.

myMap.put("key1", 1);
myMap.put("key2", 2);

If you call the put method with the Key that already exists in the Map, the method will override its value and return the old value.

3. Getting values from HashMap

For getting the value from a HashMap you have to call the get method, by passing the Key as a parameter.

myMap.get("key1"); //return 1 (class Integer)

If you pass a key that does not exists in the HashMap, this method will return null

4. Check whether the Key is in the Map or not

myMap.containsKey(varKey);

Check whether the Value is in the Map or not

myMap.containsValue(varValue)

The above methods will return a boolean value true or false if key, value exists in the Map or not.

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Iterating through the contents of a Map

Maps provide methods which let you access the keys, values, or key-value pairs of the map as collections. You can iterate through these collections. Given the following map for example:

Map<String, Integer> usersMap = new HashMap<>();
usersMap.put("Alice", 500_000);
usersMap.put("Bob", 300_000);
usersMap.put("Charlie", 750_000);

Iterating through map keys

// Iterating through map keys:
for (String user : usersMap.keySet()) {
    System.out.println(user);
}

Result:

Alice
Bob
Charlie

keySet() provides the keys of the map as a Set. Set is used as the keys cannot contain duplicate values. Iterating through the set yields each key in turn. HashMaps are not ordered, so in this example the keys may be returned in any order.

Iterating through map values:

// Iterating through map values:
for (Integer salary : usersMap.values()) {
    System.out.println(salary);
}

Result:

500000
300000
750000

values() returns the values of the map as a Collection. Iterating through the collection yields each value in turn. Again, the values may be returned in any order.

Iterating through keys and values together

// Iterating through keys and values together:
for (Map.Entry<String, Integer> entry : usersMap.entrySet()) {
    System.out.printf("%s has a salary of %d\n", entry.getKey(), entry.getValue());
}

Result:

Alice has a salary of 500000
Bob has a salary of 300000
Charlie has a salary of 750000

entrySet() returns a collection of Map.Entry objects. Map.Entry gives access to the key and value for each entry.

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Basic Programs

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• Creating Your First Java Program
• Type Conversion
• Getters and Setters
• Reference Data Types
• Java Compiler - javac
• Documenting Java Code
• Command line Argument Processing
• Java Command - java and javaw
• Literals
• Primitive Data Types

Strings in Java

• Comparing Strings
• Changing the case of characters within a String
• Finding a String Within Another String
• String pool and heap storage
• Splitting Strings
• Joining Strings with a delimiter and Reversing Strings
• String concatenation and StringBuilders
• Substrings and Platform Independent new line Separator
• Adding toString() method for custom objects
• Remove Whitespace from the Beginning and End of a String and Case insensitive switch
• Replacing parts of Strings and Getting the nth character in a String
• Counting occurrences of a substring or character in a string

StringBuffer and StringBuilder

• String Buffer
• String Builder
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Date Class

• Convert java.util.Date to java.sql.Date and Basic date
• LocalDate and LocalDateTime objects
• Creating a Specific Date and Converting Date to String format
• Convert formatted string representation of date to Date object
• Comparing Date objects
• Converting String into Date and Time Zones and java.util.Date

Dates and Time (java.time.*)

• Dates and Time
• Date Time Formatting and Date Manipulations
• LocalTime

BigDecimal

• Comparing and Using BigDecimals
• BigDecimal.valueOf()
• Mathematical operations with BigDecimal

BigInteger

• Initialization and Generating random BigIntegers
• BigInteger Mathematical Operations Examples
• NumberFormat

Bit Manipulation

• Checking, setting, clearing, and toggling individual bits. Using long as bit mask
• Checking if a number is a power of 2
• Signed vs unsigned shift and Expressing the power of 2

Arrays

• Creating and Initializing Arrays
• Creating a List from an Array
• Multidimensional and Jagged Arrays
• Array Covariance and Arrays to Stream
• Iterating over arrays
• Sorting arrays and Length of an Array
• Finding an element in an array nd change the size of an array
• Primitives and boxed types and Remove an element from an array
• Comparing arrays for equality and Copying arrays

Collections

• Removing items from a List within a loop
• Constructing collections from existing data
• ArrayList and adding objects and Iterating over Collections
• Immutable Empty , Sub and Unmodifiable Collection
• Removing matching items from Lists and Join lists
• Creating your own Iterable structure for use with Iterator or for-each loop

Lists

• Sorting a generic list
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• Replacement of a List element and Moving objects around in the list
• Creating, Adding and Removing element from an ArrayList
• Positional Access Operations and Iterating over elements in a list

Sets

• Initialization and Basics of Set
• Types and Usage of Sets

Maps

• Iterating Map Entries Effciently
• Using Default Methods of Map from Java 8
• Usage of HashMap and Iterating through the contents of a Map
• Merging, combine and composing Maps and Add multiple items
• Creating and Initializing Maps
• Check if key exists, Add an element and Clear the map
• LinkedHashMap, WeakHashMap and SortedMap

TreeMap and TreeSet

• TreeMap and TreeSet of a simple Java type
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• TreeMap and TreeSet Thread Safety

Queues and Deques

• Usage of the Queue and Deque
• Stacks and BlockingQueue
• LinkedList as a FIFO Queue and Queue Interface
• Dequeue Interface

Enums

• Declaring and using a basic enum
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• Enum Map , EnumSet class, Enum starting with number and Hashtable

Operators

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Constructors

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Object Class Methods and Constructor

• hashCode() method
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Immutable Class

• What is the advantage of immutability and Rules and Example

Immutable Objects

• Creating an immutable version of a type using defensive copying
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The java.util.Objects Class

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Reference Types

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Console I/O

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Collection Factory Methods

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Alternative Collections

• Multimap in Guava, Apache and Eclipse Collections
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Concurrent Collections

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super keyword

• Super keyword use with examples

Serialization

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Optional

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Object References

• Object References as method parameters

Exceptions and exception handling

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Calendar and its Subclasses

• Creating, Subtracting and Finding AM/PM Calendar

Using the static keyword

• Reference to non-static member from static context

Properties Class

• Reference to non-static member from static context

Lambda Expressions

• Introduction to Java lambdas
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• return only returns from the lambda and Execute-around Pattern

Basic Control Structures

• Switch, do while, For Each, Continue Statement in Java
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BufferedWriter

• Write a line of text to File

New File I/O

• Creating paths, Manipulating, Retrieving, Reading and Writing files

File I/O

• Migrating from java.io.File to Java 7 NIO
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Scanner

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Interfaces

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Regular Expressions

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Comparable and Comparator

• Sorting a List using Comparable<T> or a Comparator<T>
• compareTo, compare Methods and Natural vs explicit sorting

Java Floating Point Operations

• Comparing floating point values
• OverFlow and UnderFlow, Formatting the floating point values

Currency and Money

• Add custom currency

Object Cloning

• Cloning performing a deep copy and copy factory
• Cloning using a copy constructor and implementing Clonable interface

Recursion

• Basic idea of Recursion
• StackOverflowError & recursion to loop
• Computing the Nth Fibonacci Number and power of a number

Converting to and from Strings

• Converting String to other datatypes
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Random Number Generation

• Pseudo Random Numbers in Specific Range
• Generating Random Numbers with a Specified Seed

Singletons

• Enum Singleton and Singleton without use of Enum, Thread safe Singleton
• Extending singleton (singleton inheritance)

Autoboxing

• Using int and Integer interchangeably and Auto-unboxing may lead to NullPointerException
• Different Cases When Integer and int can be used interchangeably

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