Java HashMap

When it is required to store data key, value pairs you can use Either java HashMap, LinkedHashMap or TreeMap. There are slight differences amongst the classes. But all of the classes implement Map interface.

# Syntax


HashSet map = new HashMap();

//Like: 
HashSet map = new HashMap();

//Integer will be use as key and String will be used as values.

Note: The key in HashMap must be unique, but the values can be duplicate. Null values are allowed in key and values but there is only one null value supported as key. Also, remember HashMap doesn't maintain insertion order. And if you want to iterate HashMap or Map interface, you have to use Map.Entry and entrySet() method. Map.Entry contains getKey and getValue() method. For getting the instance of Map.Entry you have to use the entrySet() method overall.

# There are many useful methods to work with java HashMap class or Map interface.

MethodsDescription
put(K key, V value)Insert new element with the specified value with the specified key in this map.
putAll(Map m)Copies all of the mappings from the specified map to this map.
remove(Object key)Removes the mapping for the specified key from this map if present.
size()
type int		Returns the number of key-value mappings in this map.
clear()
void		Removes all of the mappings from this map.
clone()Returns a shallow copy of this HashMap instance: the keys and values themselves are not cloned.
containsKey(Object key)
type boolean		Returns true if this map contains a mapping for the specified key.
containsValue(Object value)

boolean		Returns true if this map maps one or more keys to the specified value.
isEmpty()
boolean		Returns true if this map contains no key-value mappings.
size()
int		Returns the number of key-value mappings in this map.
values()Returns a Collection view of the values contained in this map.
compute()Attempts to compute a mapping for the specified key and its current mapped value
keySet()Returns a Set view of the keys contained in this map

# Creating HashMap and add new elements based on key, value pairs


import java.util.HashMap;

public class Main{
  public static void main (String[] args) {
    //creating HashMap
    HashMap map = new HashMap();
    
    //add new elements using put() method
    map.put(1, "Mark Smith");
    map.put(2, "Bob Smith");
    map.put(3, "Jekov Jenkov");
    map.put(4, "Shakil Ahmed");
    map.put(5, "Jhon Peterson");
    
    //print the HashMap
    System.out.println(map);
  }
}

Output you will get in the console:


{1=Mark Smith, 2=Bob Smith, 3=Jekov Jenkov, 4=Shakil Ahmed, 5=Jhon Peterson}

Note: in square bracket we use . Thus, Integer is the key and String are the values. Following this this figure, we have add new elements using the put() method.

# Retrieve specific value based on key using the get(Key) method


import java.util.HashMap;

public class Main{
  public static void main (String[] args) {
    //creating HashMap
    HashMap map = new HashMap();
    
    //add new elements using put() method
    map.put(1, "Mark Smith");
    map.put(2, "Bob Smith");
    map.put(3, "Jekov Jenkov");
    map.put(4, "Shakil Ahmed");
    map.put(5, "Jhon Peterson");
    
    //print the HashMap
    System.out.println(map);
    
    //get the first value usin get(key)
    System.out.println(map.get(1));
    
    //get the second value
    System.out.println(map.get(2));
    
    //get the last value
    System.out.println(map.get(5));
  }
}

After running above example you will get the following output in the console


{1=Mark Smith, 2=Bob Smith, 3=Jekov Jenkov, 4=Shakil Ahmed, 5=Jhon Peterson}

Mark Smith
Bob Smith
Jhon Peterson

Note: it is also possible though the key is String. (But case sensitive)

# Iterate HashMap


import java.util.HashMap;

public class Main{
  public static void main (String[] args) {
    //creating HashMap
    HashMap map = new
    HashMap();
    
    //add new elements using put() method
    map.put(1, "Mark Smith");
    map.put(2, "Bob Smith");
    map.put(3, "Jekov Jenkov");
    map.put(4, "Shakil Ahmed");
    map.put(5, "John Peterson");
   
    //iterate HashMap (keys and values)
    for(Map.Entry value: map.entrySet()){
      System.out.println(value.getKey +" "+value.getaValue);
    }
  }
}
/**
 * Output: (together keys and values)
 * 1 Mark Smith
 * 2 Bob Smith 
 * 3 Jekov Jenkov
 * 4 Shakil Ahmed 
 * 5 John Peterson 
 */

# Inserting new elements using put, putAll and putIfAbsent method and then iterate


import java.util.HashMap;

public class Main{
  public static void main (String[] args) {
    //creating HashMap
    HashMap map = new
    HashMap();
    
    //add new elements using put() method
    map.put(1, "Mark Smith");
    map.put(2, "Bob Smith");
    map.put(3, "Jekov Jenkov");
    map.put(4, "Shakil Ahmed");
    map.put(5, "John Peterson");
    
    //initial map
    System.out.println("Initial List");
    System.out.println(map);
    
    //using putIfAbsent() method
    map.putIfAbsent(6, "Abdul Hamid");
    map.putIfAbsent(3, "Eugune Uzbek"); // will not be updated
    
    //updated list 
    System.out.println("Updated List using putIfAbsent");
    System.out.println(map);
    
    //Creating new entry 
    HashMap newMap = new HashMap();
    
    //add new elements
    newMap.put(7, "Rebeka Ghom");
    newMap.putAll(map); //using putAll()
    
    //print newMap
    System.out.println("Updated List using putAll()");
    System.out.println(newMap);
    
    //iterate map and newMap now
    System.out.println("map iteration");
    for(Map.Entry value: map.entrySet()){
      System.out.println(value.getKey +" "+value.getaValue);
    }
    
    System.out.println("newMap iteration");
    for(Map.Entry value: newMap.entrySet()){
      System.out.println(value.getKey +" "+value.getaValue);
    }
  }
}

If you run the above example, you will get the following output


//output (together with keys and values)

Initial List
 {1=Mark Smith, 2=Bob Smith, 3=Jekov
 Jenkov, 4=Shakil Ahmed, 5=Jhon
 Peterson}
 
Updated List using putIfAbsent()
{1=Mark Smith, 2=Bob Smith, 3=Jekov Jenkov, 4=Shakil Ahmed, 5=Jhon Peterson, 6=Abdul Hamid}

Updated List using putAll
{1=Mark Smith, 2=Bob Smith, 3=Jekov Jenkov, 4=Shakil Ahmed, 5=Jhon Peterson, 6=Abdul Hamid, 7=Rebeka Ghom}

map iteration 
 1 Mark Smith
 2 Bob Smith 
 3 Jekov Jenkov
 4 Shakil Ahmed 
 5 John Peterson 
 6 Abdul Hamid
  
 newMap iteration 
 1 Mark Smith
 2 Bob Smith 
 3 Jekov Jenkov
 4 Shakil Ahmed 
 5 John Peterson 
 6 Abdul Hamid
 7 Rebeka Ghom

# Remove any entry based on key, value, and both (keys abd values) from HashMap


import java.util.HashMap;

public class Main{
  public static void main (String[] args) {
    //creating HashMap
    HashMap map = new
    HashMap();
    
    //add new elements using put() method
    map.put(1, "Mark Smith");
    map.put(2, "Bob Smith");
    map.put(3, "Jekov Jenkov");
    map.put(4, "Shakil Ahmed");
    map.put(5, "John Peterson");
    
    //print initial HashMap 
    System.out.println("Initial HashMap:  "+map);
    
    //remove based on key
    map.remove(5)
    
    //print updated list 
    System.out.println("Updated Entry "+map);
    
    //remove based on value 
    map.remove("Shakil Ahmed")
    
    //print updated list
    System.out.println("Updated Entry "+map);
    
    //remove based on keys and values
    map.remove(3, "Jekov Jenkov");
    
    //print update list
    System.out.println("Update d Entry " +map);
    
    //now iterate  HashMap (after removal)
    for(Map.Entry value: map.entrySet()){
      System.out.println(value.getKey +" "+value.getaValue);
    }
  }
}

You get the following output after running the above example


//Output

Initial HashMap:  {1=Mark Smith, 2=Bob Smith, 3=Jekov
 Jenkov, 4=Shakil Ahmed, 5=Jhon
 Peterson}
 
 Updated Entry:  {1=Mark Smith, 2=Bob Smith, 3=Jekov
  Jenkov, 4=Shakil Ahmed}
  
 Updated Entry:  {1=Mark Smith, 2=Bob Smith, 3=Jekov
  Jenkov,}
 
 Updated Entry:  {1=Mark Smith, 2=Bob Smith}
 
 //iteration entry after removal
1 Mark Smith
2 Bob Smith