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Introduction to Java HashMap
What is a HashMap?
A HashMap in Java is a part of the Java Collection Framework and is used to store data in key-value pairs. It allows null values and the null key. It is not synchronized and is not ordered, meaning the order of the map is not guaranteed.
import java.util.HashMap;
class Example {
public static void main(String args[]) {
HashMap map = new HashMap<>();
map.put("Apple", 1);
map.put("Banana", 2);
System.out.println(map);
}
}
Console Output:
{Apple=1, Banana=2}
Adding Elements to HashMap
Using put() Method
The put() method is used to add elements to the HashMap. If the key already exists, the new value overwrites the old value.
import java.util.HashMap;
class Example {
public static void main(String args[]) {
HashMap map = new HashMap<>();
map.put("Key1", "Value1");
map.put("Key2", "Value2");
map.put("Key1", "Value3"); // Overwrites Value1
System.out.println(map);
}
}
Console Output:
{Key1=Value3, Key2=Value2}
Removing Elements from HashMap
Using remove() Method
The remove() method removes the mapping for a key from this map if it is present.
import java.util.HashMap;
class Example {
public static void main(String args[]) {
HashMap map = new HashMap<>();
map.put("One", 1);
map.put("Two", 2);
map.remove("One");
System.out.println(map);
}
}
Console Output:
{Two=2}
Iterating Over HashMap
Using for-each Loop
HashMap can be iterated using the for-each loop along with the entrySet() method, which returns a set view of the mappings contained in this map.
import java.util.HashMap;
import java.util.Map;
class Example {
public static void main(String args[]) {
HashMap map = new HashMap<>();
map.put("A", "Apple");
map.put("B", "Banana");
for (Map.Entry entry : map.entrySet()) {
System.out.println(entry.getKey() + " = " + entry.getValue());
}
}
}
Console Output:
A = Apple
B = Banana
Checking if HashMap is Empty
Using isEmpty() Method
The isEmpty() method returns true if the map contains no key-value mappings.
import java.util.HashMap;
class Example {
public static void main(String args[]) {
HashMap map = new HashMap<>();
System.out.println("Is map empty? " + map.isEmpty());
map.put("X", 10);
System.out.println("Is map empty? " + map.isEmpty());
}
}
Console Output:
Is map empty? true
Is map empty? false
Getting Size of HashMap
Using size() Method
The size() method returns the number of key-value mappings in the map.
import java.util.HashMap;
class Example {
public static void main(String args[]) {
HashMap map = new HashMap<>();
map.put("C", "Cat");
map.put("D", "Dog");
System.out.println("Size of map: " + map.size());
}
}
Console Output:
Size of map: 2
Replacing Values in HashMap
Using replace() Method
The replace() method replaces the entry for a specified key only if it is currently mapped to some value.
import java.util.HashMap;
class Example {
public static void main(String args[]) {
HashMap map = new HashMap<>();
map.put("E", "Elephant");
map.replace("E", "Eagle");
System.out.println(map);
}
}
Console Output:
{E=Eagle}
Checking for Key in HashMap
Using containsKey() Method
The containsKey() method returns true if this map contains a mapping for the specified key.
import java.util.HashMap;
class Example {
public static void main(String args[]) {
HashMap map = new HashMap<>();
map.put("F", 6);
System.out.println("Contains key 'F'? " + map.containsKey("F"));
System.out.println("Contains key 'G'? " + map.containsKey("G"));
}
}
Console Output:
Contains key 'F'? true
Contains key 'G'? false
Cloning a HashMap
Using clone() Method
The clone() method is used to create a shallow copy of the HashMap instance.
import java.util.HashMap;
class Example {
public static void main(String args[]) {
HashMap map1 = new HashMap<>();
map1.put("G", "Giraffe");
HashMap map2 = (HashMap) map1.clone();
System.out.println("Original: " + map1);
System.out.println("Cloned: " + map2);
}
}
Console Output:
Original: {G=Giraffe}
Cloned: {G=Giraffe}
Merging Two HashMaps
Using putAll() Method
The putAll() method copies all of the mappings from the specified map to this map.
import java.util.HashMap;
class Example {
public static void main(String args[]) {
HashMap map1 = new HashMap<>();
map1.put("H", "Horse");
HashMap map2 = new HashMap<>();
map2.put("I", "Iguana");
map1.putAll(map2);
System.out.println(map1);
}
}
Console Output:
{H=Horse, I=Iguana}
