Java Map Collection Tutorial
Map stores the value in the form of key and value pair. A map cannot contain duplicate keys, Each key is associated with a single value.
Example:
- Mapping of Students name with their RollNo
- Mapping of Airport Nae with their Airport Code
- Mapping of Employee Name with their EmployeeId
Collection Framework:
- It is the set of Predefined classes & interfaces which is used to store multiple data.
- It contains 2 main parts(Packages):-
- java.util.Collection;
- java.util.Map;
Map Hierarchy in Java:
Collection Hierarchy in Java:
As a part of this blog we will keep our discussion only for Map Interface.
Below are the useful methods of MAP Interface:
| Method | Description |
| put(Object, Object) | Adds a new key: value pair to the map |
| putAll(Map) | Copies all the mappings of the specified map into the new map |
| size() | Returns the total number of key: value pairs in a map |
| clear() | Clears and removes all the mappings |
| equals(Object) | Checks the equality between two maps |
| containsKey(Object) | Returns a boolean value depending on whether the specified key is mapped or not |
| containsValue(Object) | Returns a boolean value depending on whether the specified value is mapped or not |
| get(Object) | Returns the value mapped with the specified key or null if the key is not mapped |
| isEmpty() | Checks whether the map is empty |
| keySet() | Returns a set view of the mapped keys |
| remove(Object) | Removes an element’s key mapping |
| value() | Creates a collection of the values of a map |
| putIfAbsent(Object key,Object value) | If the specified key is not already associated with a value (or is mapped to null) associates it with the given value and returns null, else returns the current value. |
| replace(Object Key,Object Oldvalue,Object NewValue) | Replaces the entry for the specified key only if currently mapped to the specified value |
| replace(Object key,Object value) | Replaces the entry for the specified key only if it is currently mapped to some value. |
1. Lets understand how to use put(Object,Object) in Map Interface.
package ab43;
import java.util.HashMap;
import java.util.Map;
public class Map_Interface
{
public static void main(String[] args)
{
Map m1=new HashMap();
//Upcasting HashMap to Map Interface
m1.put("student 1", "Manish");
//Adds a new key: value pair to the map
m1.put("student 2", "Abhishek");
m1.put("student 3", "Sunita");
m1.put("student 4", "Jeetu");
System.out.println(m1);
}
}
Output:
{student 3=Sunita, student 4=Jeetu, student 1=Manish, student 2=Abhishek}
Screenshot:
2. Lets understand how to use putAll(Collection) in Map Interface.
package ab43;
import java.util.HashMap;
import java.util.Map;
public class Map_Interface2
{
public static void main(String[] args)
{
Map m1=new HashMap();
m1.put("student 1", "Manish");
m1.put("student 2", "Abhishek");
m1.put("student 3", "Sunita");
m1.put("student 4", "Jeetu");
System.out.println(m1);
Map m2=new HashMap();
//Copies all the mappings of the m1map into the new map m2
m2.putAll(m1);
m2.put("Student 5", "Reeyansh");
System.out.println(m2);
}
}
Output:
{student 3=Sunita, student 4=Jeetu, student 1=Manish, student 2=Abhishek}
{student 3=Sunita, student 4=Jeetu, Student 5=Reeyansh, student 1=Manish, student 2=Abhishek}
Screenshot:
3. Lets understand how to use clear() in Map Interface.
package ab43;
import java.util.HashMap;
import java.util.Map;
public class Map_Interface3
{
public static void main(String[] args)
{
Map m1=new HashMap();
m1.put("student 1", "Manish");
m1.put("student 2", "Abhishek");
m1.put("student 3", "Sunita");
m1.put("student 4", "Jeetu");
System.out.println(m1);
m1.clear();
//Clears and removes all the mappings
System.out.println(m1);
}
}
Output:
{student 3=Sunita, student 4=Jeetu, student 1=Manish, student 2=Abhishek}
{}
Screenshot:
4. Lets understand how to use Equals in Map Interface.
package ab43;
import java.util.HashMap;
import java.util.Map;
public class Map_Interface4
{
public static void main(String[] args)
{
Map m1=new HashMap();
m1.put("student 1", "Manish");
m1.put("student 2", "Abhishek");
m1.put("student 3", "Sunita");
m1.put("student 4", "Jeetu");
System.out.println(m1);
Map m2=new HashMap();
m2.put("student 1", "Manish");
m2.put("student 2", "Abhishek");
m2.put("student 3", "Sunita");
m2.put("student 4", "Jeetu");
System.out.println(m2);
boolean a1= m1.equals(m2);
//Checks the equality between two maps
System.out.println(a1);
}
}
Output:
{student 3=Sunita, student 4=Jeetu, student 1=Manish, student 2=Abhishek}
{student 3=Sunita, student 4=Jeetu, student 1=Manish, student 2=Abhishek}
true
Screenshot:
5. Lets understand how to use get(Object) in Map Interface.
package ab43;
import java.util.HashMap;
import java.util.Map;
public class Map_Interface5
{
public static void main(String[] args)
{
Map m1=new HashMap();
m1.put("Rice",10);
m1.put("Sugar",2);
m1.put("Jaggery", 3);
m1.put("Cereals",25);
System.out.println(m1);
System.out.println(m1.get("Sugar"));
//Returns the value mapped with the specified key or null if the key is not mapped
}
}
Output:
{Cereals=25, Jaggery=3, Sugar=2, Rice=10}
2
Screenshot:
6. Lets understand how to use remove(Object) in Map Interface.
package ab43;
import java.util.HashMap;
import java.util.Map;
public class Map_Interface6
{
public static void main(String[] args)
{
Map m1=new HashMap();
m1.put("Rice",10);
m1.put("Sugar",2);
m1.put("Jaggery", 3);
m1.put("Cereals",25);
System.out.println(m1);
Map m2=new HashMap();
m2.putAll(m1);
System.out.println(m2);
m2.put("Oil", 5);
System.out.println("After Removing:");
m2.remove("Rice");
//Removes an element’s key mapping
System.out.println(m2);
}
}
Output:
{Cereals=25, Jaggery=3, Sugar=2, Rice=10}
{Jaggery=3, Cereals=25, Sugar=2, Rice=10}
After Removing:
{Jaggery=3, Cereals=25, Oil=5, Sugar=2}
Screenshot:
7. Lets understand how to use replace(Object,Value) in Map Interface.
package ab43;
import java.util.HashMap;
import java.util.Map;
public class Map_Interface7
{
public static void main(String[] args)
{
Map m1=new HashMap();
m1.put("Rice",10);
m1.put("Sugar",2);
m1.put("Jaggery", 3);
m1.put("Cereals",25);
System.out.println(m1);
m1.replace("Rice", 90);
/Replaces the entry for the specified key only if it is currently mapped to some value.
System.out.println(m1);
}
}
Output:
{Cereals=25, Jaggery=3, Sugar=2, Rice=10}
{Cereals=25, Jaggery=3, Sugar=2, Rice=90}
Screenshot:
8. Lets understand how to use size() in Map Interface.
package ab43;
import java.util.HashMap;
import java.util.Map;
public class Map_Interface8
{
public static void main(String[] args)
{
Map m1=new HashMap();
m1.put("Rice",10);
m1.put("Sugar",2);
m1.put("Jaggery", 3);
m1.put("Cereals",25);
System.out.println(m1);
System.out.println(m1.size());
//Returns the total number of key: value pairs in a map
}
}
Output:
{Cereals=25, Jaggery=3, Sugar=2, Rice=10}
4
Screenshot:
9. Lets understand how to use containsKey(Object key) in Map Interface.
package ab43;
import java.util.HashMap;
import java.util.Map;
public class Map_Interface89
{
public static void main(String[] args) {
Map m1=new HashMap();
m1.put("Rice",10);
m1.put("Sugar",2);
m1.put("Jaggery", 3);
m1.put("Cereals",25);
System.out.println(m1);
System.out.println(m1.containsKey("Rice"));
//Returns a boolean value depending on whether the specified key is mapped or not
}
}
Output:
{Cereals=25, Jaggery=3, Sugar=2, Rice=10}
true
Screenshot:
10. Lets understand how to use containsValue(Object value) in Map Interface.
package ab43;
import java.util.HashMap;
import java.util.Map;
public class Map_Interface10
{
public static void main(String[] args)
{
Map m1=new HashMap();
m1.put("Rice",10);
m1.put("Sugar",2);
m1.put("Jaggery", 3);
m1.put("Cereals",25);
System.out.println(m1);
//Returns a boolean value depending on whether the specified value is mapped or not
System.out.println(m1.containsValue(2));
}
}
Output:
{Cereals=25, Jaggery=3, Sugar=2, Rice=10}
true
Screenshot:
11. Lets understand how to use replace(Object key,Object oldvalue,Object newvalue) in Map Interface.
package ab43;
import java.util.HashMap;
import java.util.Map;
public class Map_Interface11
{
public static void main(String[] args)
{
Map m1=new HashMap();
m1.put("Rice",10);
m1.put("Sugar",2);
m1.put("Jaggery", 3);
m1.put("Cereals",25);
System.out.println(m1);
m1.replace("Sugar", 2, 20);
//Replaces the entry for the specified key only if currently mapped to the specified value
System.out.println(m1);
}
}
Output:
{Cereals=25, Jaggery=3, Sugar=2, Rice=10}
{Cereals=25, Jaggery=3, Sugar=20, Rice=10}
Screenshot:
12. Lets understand how to use values() in Map Interface.
package ab43;
import java.util.HashMap;
import java.util.Map;
public class Map_Interface112
{
public static void main(String[] args)
{
Map m1=new HashMap();
m1.put("Rice",10);
m1.put("Sugar",2);
m1.put("Jaggery", 3);
m1.put("Cereals",25);
System.out.println(m1);
System.out.println(m1.values());
//Creates a collection of the values of a map
}
}
Output:
{Cereals=25, Jaggery=3, Sugar=2, Rice=10}
[25, 3, 2, 10]
Screenshot:
13. Lets understand how to use keySet() in Map Interface.
package ab43;
import java.util.HashMap;
import java.util.Map;
public class Map_Interface13
{
public static void main(String[] args)
{
Map m1=new HashMap();
m1.put("Rice",10);
m1.put("Sugar",2);
m1.put("Jaggery", 3);
m1.put("Cereals",25);
System.out.println(m1);
System.out.println(m1.keySet());
//Returns a set view of the mapped keys
}
}
Output:
{Cereals=25, Jaggery=3, Sugar=2, Rice=10}
[25, 3, 2, 10]
Screenshot:
14. Lets understand how to use putIfAbsent(Object key,Object value) in Map Interface.
package ab43;
import java.util.HashMap;
import java.util.Map;
public class Map_Interface14
{
public static void main(String[] args)
{
Map m1=new HashMap();
m1.put("Rice",10);
m1.put("Sugar",2);
m1.put("Jaggery", 3);
m1.put("Cereals",25);
System.out.println(m1);
m1.putIfAbsent("Wheat", 100);
//If the specified key is not already associated with a value associates it with the given value
System.out.println(m1);
}
}
Output:
{Cereals=25, Jaggery=3, Sugar=2, Rice=10}
{Cereals=25, Jaggery=3, Wheat=100, Sugar=2, Rice=10}
Screenshot:
Example 1: Just because the “Wheat” key was not present it got added in m1.
Example 2: Just because the “Rice” key was already present it did not get added in m1.
15. How to fetch the value of MAP Interface:
Case 1: Using EntrySet to fetch both key and value
package ab43;
import java.util.HashMap;
import java.util.Map;
import java.util.Map.Entry;
public class Map_Interface17
{
public static void main(String[] args)
{
Map<String,String> m1=new HashMap<String,String>();
//Upcasting from HashMap to Map Interface
m1.put("INR","Indian Rupee");
m1.put("USD","US Dollars");
m1.put("CAD", "Canadian Dollars");
m1.put("GBP","United Kingdom Pound");
//let's use for each loop to fetch key and value
//fetching both key and value using entryset method
for(Map.Entry<String,String> value1: m1.entrySet())
{
System.out.println(value1);
}
}
}
Output:
GBP=United Kingdom Pound
USD=US Dollars
CAD=Canadian Dollars
INR=Indian Rupee
Screenshot:
Case 2: Using keySet() to fetch only keys and using values() to fetch only values
package ab43;
import java.util.HashMap;
import java.util.Map;
import java.util.Map.Entry;
public class Map_Interface16
{
public static void main(String[] args) {
Map<String,Integer> m1=new HashMap<String,Integer>();
//Upcasting from HashMap to Map Interface
m1.put("Rice",10);
m1.put("Sugar",2);
m1.put("Jaggery", 3);
m1.put("Cereals",25);
//lets use for each loop to fetch key separately and value separately
for(String key: m1.keySet())
{
System.out.println("Keys-> "+key);
}
for(Integer value: m1.values())
{
System.out.println("Values-> "+value);
}
//fetching both key and value using entryset method
for(Entry<String, Integer> value1: m1.entrySet())
{
System.out.println("Key and Value -> "+value1);
}
}
}
Output:
Keys-> Cereals
Keys-> Jaggery
Keys-> Sugar
Keys-> Rice
Values-> 25
Values-> 3
Values-> 2
Values-> 10
Key and Value -> Cereals=25
Key and Value -> Jaggery=3
Key and Value -> Sugar=2
Key and Value -> Rice=10
Screenshot:
Case 3: Using Iterator to fetch both key and value together.
package ab43;
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.Map.Entry;
public class Map_Interface18
{
public static void main(String[] args)
{
Map<String,String> m1=new HashMap<String,String>();
//Upcasting from HashMap to Map Interface
m1.put("INR","Indian Rupee");
m1.put("USD","US Dollars");
m1.put("CAD", "Canadian Dollars");
m1.put("GBP","United Kingdom Pound");
Iterator<Entry<String, String>> i1=m1.entrySet().iterator();
while(i1.hasNext())
{
System.out.println(i1.next());
}
}
}
Output:
GBP=United Kingdom Pound
USD=US Dollars
CAD=Canadian Dollars
INR=Indian Rupee
Screenshot:
Now let’s discuss how List Set and Map Interface are different from each other:
| List | Set | Map | |
| Package | java.util.List | java.util.Set | java.util.Map |
| Duplicate | List allows you to store duplicate elements in java. | Set does not allow storing duplicate elements in java. | Map stores data in the form of key-value pairs; it does not allow to store duplicate keys but allows duplicate values in java. |
| Insertion Order | Maintain Insertion order | Does not maintain Insertion Order | Most of the Map classes do not maintain insertion order. |
| Null | List allows you to store many null keys in java. | Most of the Set implementations allow to add only one null in java | HashMap allows one null key and many null values. TreeMap doesn’t allow null keys but allows many null values. LinkedHashMap allows one null key and many null values. |
| Classes | List implementation classes are Array List, LinkedList. | Set implementation classes are HashSet, LinkedHashSet, and TreeSet. | Map implementation classes are HashMap, HashTable, TreeMap, WeakHashMap, and LinkedHashMap. |
| When to use | If you need to access the elements frequently by using the index then we can use the list | If you want to create a collection of unique elements then we can use set | If you want to store the data in the form of a key/value pair then we can use the map. |
| Traversing | We can traverse using Iterator and ListIterator | We can traverse using Iterator | It can’t be traverse but you can fetch using keyset(),value() and entrySet() |
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