HashMap

The HashMap Class in Java

HashMap is one of the most widely used implementations of the Map interface in the Java Collection Framework.

It stores data in key--value pairs, where:

• Keys must be unique
• Values can be duplicated

Internally, HashMap uses a hash table to store entries, which enables very fast operations.

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Key Characteristics

• Unique Keys -- Duplicate keys overwrite existing values
• No Order Guarantee -- Iteration order is unpredictable
• Null Support -- Allows one null key and multiple null values
• High Performance -- Average O(1) time complexity

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Common Use Cases

• Caching systems
• Lookup tables
• Mapping IDs to objects
• Counting frequencies
• Configuration storage

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Important Methods

Method	Description
put(K key, V value)	Inserts or updates a key-value pair
get(Object key)	Retrieves value by key
remove(Object key)	Removes entry
containsKey(Object key)	Checks key existence
containsValue(Object value)	Checks value existence
keySet()	Returns set of keys
values()	Returns collection of values
entrySet()	Returns key-value entries

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Example 1: Basic Operations

import java.util.HashMap;
import java.util.Map;

public class HashMapExample {

    public static void main(String[] args) {

        Map<String,Integer> fruitCounts = new HashMap<>();

        fruitCounts.put("Apple",3);
        fruitCounts.put("Banana",5);
        fruitCounts.put("Cherry",7);

        System.out.println(fruitCounts);

        System.out.println(fruitCounts.get("Apple"));

        System.out.println(fruitCounts.containsKey("Banana"));

        fruitCounts.remove("Cherry");

        System.out.println(fruitCounts);

        for(Map.Entry<String,Integer> entry : fruitCounts.entrySet()){

            System.out.println(entry.getKey()+" : "+entry.getValue());

        }
    }
}

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Example 2: Null Keys and Values

import java.util.HashMap;
import java.util.Map;

public class HashMapNullExample {

    public static void main(String[] args) {

        Map<String,String> colors = new HashMap<>();

        colors.put("Red","Primary");
        colors.put(null,"Unknown");
        colors.put("Blue",null);

        System.out.println(colors);
    }
}

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Example 3: Iterating HashMap

import java.util.HashMap;
import java.util.Map;

public class HashMapIterationExample {

    public static void main(String[] args) {

        Map<String,Integer> scores = new HashMap<>();

        scores.put("Alice",95);
        scores.put("Bob",87);
        scores.put("Charlie",92);

        for(String key : scores.keySet()){
            System.out.println(key);
        }

        for(Integer value : scores.values()){
            System.out.println(value);
        }

        for(Map.Entry<String,Integer> entry : scores.entrySet()){
            System.out.println(entry.getKey()+" : "+entry.getValue());
        }

    }
}

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Performance Characteristics

Operation	Complexity
put()	O(1)
get()	O(1)
remove()	O(1)
iteration	O(n)

Performance depends heavily on good hashCode() implementation.

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When to Use HashMap

Use HashMap when:

• You need fast key-based lookup
• Ordering does not matter
• You need high performance

Avoid when:

• Order matters → use LinkedHashMap
• Sorted keys required → use TreeMap

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Comparison: HashMap vs LinkedHashMap vs TreeMap

Feature	HashMap	LinkedHashMap	TreeMap
Order	No order	Insertion order	Sorted
Complexity	O(1)	O(1)	O(log n)
Null Key	Allowed	Allowed	Not allowed
Structure	Hash table	Hash table + linked list	Red-Black Tree