A map is a collection in which data can exist in the form of key, and value pairs. retrieve of a value can be handled based upon the associated key. In the map, a key should be unique, whereas duplicate records are allowed for the value. a Map is broadly classified into three categories based on the iteration order they are

• Hashmap which is unordered.

• LinkedHashmap which iterates as per the key insertion order.

• Splay Treemap which iterated the keys in sorted order.

There are two approaches to defining a map

• via Map Literals

• via Map Constructor

Let's discuss the above declaration types in detail

Map Literals

A map literal consists of a pair of curly brackets, in which we specify the key/value pairs. These pairs are separated by a comma ( , ). The key is separated from the value by a colon ( : ).

Syntax:

var mapLiteral = { Key1: Value1, Key2: Value2, Key3: Value3, ........., Key_n: Value_n}

Eg:

void main(){

var studentInfo = {'Name': 'Aditya', 'Age': 23, 'Job': 'Student'};

print(studentInfo);

}

// Output: {Name: Aditya, Age: 23, Job: Student}

Map Constructor

It can be implemented in two ways. Initially, we need to declare a map using the map() constructor and then we initialize the map. Please go through the syntax given below

Syntax:

var map_initialization = Map();

var map_initialization['Key'] = value;

Eg:

void main(){

// Map initialization with constructor

var userInfo = Map();

userInfo['Name'] = 'Aditya';

userInfo['Age'] = '34';

userInfo['Gender'] = 'Male';

print(userInfo);

// Output : {Name: Aditya, Age: 34, Gender: Male}

}

Let's take a look at other items that are available in Maps.

Add All

when we want to add entries to an existing Map object we adopt this method

Eg:

void main(){

var countries = {1: 'India', 2: 'USA', 3: 'Canada', 4: 'Japan', 5: 'Israel'};

print(countries);

countries.addAll({6: 'Uk', 7: 'Australia'});

print(countries);

// Output: {1: India, 2: USA, 3: Canada, 4: Japan, 5: Israel, 6: Uk, 7: Australia}

}

Clear

When we want to clear data from Map, we use this method.

Eg:

void main(){

var countries = {1: 'India', 2: 'USA', 3: 'Canada', 4: 'Japan', 5: 'Israel'};

countries.clear();

// Data after clearing from Map

print(countries);

// Output: {}

}

Contains Key

It helps in checking whether given key is available or not and returns a boolean value.

Eg:

void main(){

var countries = {'India': 'New Delhi', 'USA': 'Washington DC', 'Canada': 'Ottawa', 'Japan' : 'Tokyo', 'Israel' :'Jerusalem' };

print('Is India Available : ${countries.containsKey('India')}');

// Output: Is India Available : true

print('Is Nepal Available : ${countries.containsKey('Nepal')}');

// Output: Is Nepal Available : false

}

Contains Value

For checking value we use this method

Eg:

void main(){

var countries = {'India': 'New Delhi', 'USA': 'Washington DC', 'Canada': 'Ottawa', 'Japan' : 'Tokyo', 'Israel' :'Jerusalem' };

print('Is New Delhi is the capital of India : ${countries.containsValue('New Delhi')}');

// Output: Is New Delhi is the capital of India : true

print('Is London is the capital of Germany : ${countries.containsValue('London')}');

// Output: Is London is the capital of Germany : false

}

For each

This method is useful for loading each and every key values that are available in the given map.

Eg:

void main(){

var countries = {'India': 'New Delhi', 'USA': 'Washington DC', 'Canada': 'Ottawa', 'Japan' : 'Tokyo', 'Israel' :'Jerusalem' };

countries.forEach((key, value){

print('Key : '+ key+' Value : '+value);

});

// Output

// Key : India Value : New Delhi

// Key : USA Value : Washington DC

// Key : Canada Value : Ottawa

// Key : Japan Value : Tokyo

// Key : Israel Value : Jerusalem

}

removeWhere

Removes the corresponding value when it satisfies the condition

Eg:

void main(){

var countries = {'India': 'New Delhi', 'USA': 'Washington DC', 'Canada': 'Ottawa', 'Japan' : 'Tokyo', 'Israel' :'Jerusalem' };

print(countries);

var removeCanada = countries.removeWhere((key, value) => value.startsWith('O') );

print(countries);

}

Note: other alternative methods for removing a value along with startsWith() are contains() & endsWith().

// Output : {India: New Delhi, USA: Washington DC, Japan: Tokyo, Israel: Jerusalem}

Update

When we want to update the value of a key instead of removing and re adding we can use this method.

Eg:

void main(){

var countries = {'India': 'New Delhi', 'USA': 'Washington DC', 'Australia': 'Sydney', 'Canada': 'Ottawa', 'Japan' : 'Tokyo', 'Israel' :'Jerusalem' };

var updateAustralia = countries.update('Australia', (value) => 'Canberra' );

print(countries);

// Output : {India: New Delhi, USA: Washington DC, Australia: Canberra, Canada: Ottawa, Japan: Tokyo, Israel: Jerusalem}

}