Iterate Map in Java

There are many ways to iterate a Map in Java. First let’s understand about Map in Java.

Map in Java stores elements in the form of key and value pairs. If the key is provided then map returns corresponding value.

There are following ways to iterate map in Java:

• Map.entrySet() with For-Each loop
• keySet() and values()
• forEach(action) method
• Searching of keys in map
• By using iterators

1. Using Map.entrySet() with For-Each loop

In this method of iteration, we use Map.Entry<K,V> in which Entry means key value pair. We get a collection-view of the map from Map.entrySet(), whose elements are in this class.

                  import java.util.Map; 
import java.util.HashMap; 
 
public class GeekCer { 
  public static void main(String[] arg) { 
    Map<String,String> map = new HashMap<String,String>(); 
      
    // Add data as key-key value pair 
    map.put("ADAMS", "CLERK"); 
    map.put("BLAKE", "MANAGER"); 
    map.put("FORD", "ANALYST"); 
    map.put("MILLER", "CLERK"); 
    map.put("TURNER", "SALESMAN");
          
    for (Map.Entry entry : map.entrySet()) {  
        System.out.println("Key = " + entry.getKey() + 
                             ", Value = " + entry.getValue());
    }
  } 
} 

Output:
Key = TURNER, Value = SALESMAN
Key = BLAKE, Value = MANAGER
Key = MILLER, Value = CLERK
Key = ADAMS, Value = CLERK
Key = FORD, Value = ANALYST

2. Using keySet() and values()

In this program, we have iterated over keys using keySet() method and iterated over values using values() method. The keySet() method returns a set view of the keys contained in this map and values() method returns of the values contained in this map.

                  import java.util.Map; 
import java.util.HashMap; 
 
public class GeekCer { 
  public static void main(String[] arg) { 
    Map<String,String> map = new HashMap<String,String>(); 
      
    // Add data as key-key value pair 
    map.put("ADAMS", "CLERK"); 
    map.put("BLAKE", "MANAGER"); 
    map.put("FORD", "ANALYST"); 
    map.put("MILLER", "CLERK"); 
    map.put("TURNER", "SALESMAN");
          
    // Iteration over keys using keySet()
    for (String name : map.keySet()) {
      System.out.println("Key: " + name);
    }

    // Iteration over values using  values()
    for (String job : map.values()) {
      System.out.println("Value: " + job);
    }
  } 
} 

Output:
Key: TURNER
Key: BLAKE
Key: MILLER
Key: ADAMS
Key: FORD
Value: SALESMAN
Value: MANAGER
Value: CLERK
Value: CLERK
Value: ANALYST

3. Iterate map in java Using forEach(action) method

This technique available in Java 8, here we iterate a map using lambda expression and forEach(action) method.

                  import java.util.Map; 
import java.util.HashMap; 
 
public class GeekCer { 
  public static void main(String[] arg) { 
    Map<String,String> map = new HashMap<String,String>(); 
      
    // Add data as key-key value pair 
    map.put("ADAMS", "CLERK"); 
    map.put("BLAKE", "MANAGER"); 
    map.put("FORD", "ANALYST"); 
    map.put("MILLER", "CLERK"); 
    map.put("TURNER", "SALESMAN");
          
     map.forEach((key ,value) -> System.out.println("Key = "
            + key + ", Value = " + value)); 
  } 
} 

Output:
Key = TURNER, Value = SALESMAN
Key = BLAKE, Value = MANAGER
Key = MILLER, Value = CLERK
Key = ADAMS, Value = CLERK
Key = FORD, Value = ANALYST

4. Searching of keys in map

It a simple way of iteration, firstly we get the key from the map and by using key we get the value from the map.

                  import java.util.Map; 
import java.util.HashMap; 
 
public class GeekCer { 
  public static void main(String[] arg) { 
    Map<String,String> map = new HashMap<String,String>(); 
      
    // Add data as key-key value pair 
    map.put("ADAMS", "CLERK"); 
    map.put("BLAKE", "MANAGER"); 
    map.put("FORD", "ANALYST"); 
    map.put("MILLER", "CLERK"); 
    map.put("TURNER", "SALESMAN");
          
    for (String name : map.keySet()) {
      String job = map.get(name);
      System.out.println("Key = " + name + ", Value = " + job);
    } 
  } 
} 

Output:
Key = TURNER, Value = SALESMAN
Key = BLAKE, Value = MANAGER
Key = MILLER, Value = CLERK
Key = ADAMS, Value = CLERK
Key = FORD, Value = ANALYST

5. Iterate map in java using iterators

First of all, we create a iterator for the collection-view returned by the entrySet() method and using while loop we iterate the map.

                  import java.util.HashMap;
import java.util.Iterator;
import java.util.Map; 
 
public class GeekCer { 
  public static void main(String[] arg) { 
    Map<String,String> map = new HashMap<String,String>(); 
      
    // Add data as key-key value pair 
    map.put("ADAMS", "CLERK"); 
    map.put("BLAKE", "MANAGER"); 
    map.put("FORD", "ANALYST"); 
    map.put("MILLER", "CLERK"); 
    map.put("TURNER", "SALESMAN");
          
    Iterator<Map.Entry<String, String>> itr = map.entrySet().iterator();    
    while(itr.hasNext()) 
    { 
      Map.Entry<String, String> entry = itr.next(); 
      System.out.println("Key = " + entry.getKey() +  
                             ", Value = " + entry.getValue()); 
    } 
  } 
} 

Output:
Key = TURNER, Value = SALESMAN
Key = BLAKE, Value = MANAGER
Key = MILLER, Value = CLERK
Key = ADAMS, Value = CLERK
Key = FORD, Value = ANALYST

More about collection

We can not store primitive dat types in the collection objects hence map doesn’t store the primitive type of data. The main purpose of collection is to handle object only, but not primitive data types.

Can you store a primitive data type into a collection?

No, Collections store the elements in the form of object only. But you can store a primitive data in the form of object by using wrapper class concept.