How to Put and Get in HashMap in Java - Simple Guide

In Java, you use put(key, value) to add a key-value pair to a HashMap and get(key) to retrieve the value for a given key. The put method stores the value, and the get method returns the value or null if the key is not found.

Syntax

The put method adds or updates a key-value pair in the HashMap. The get method retrieves the value for a given key.

• put(key, value): Adds the value with the specified key.
• get(key): Returns the value associated with the key or null if not found.

java

HashMap<KeyType, ValueType> map = new HashMap<>();
map.put(key, value); // Store value
ValueType value = map.get(key); // Retrieve value

Example

This example shows how to create a HashMap, add key-value pairs using put, and get values using get. It prints the stored values and shows what happens when a key is missing.

java

import java.util.HashMap;

public class HashMapExample {
    public static void main(String[] args) {
        HashMap<String, Integer> map = new HashMap<>();

        // Put key-value pairs
        map.put("apple", 10);
        map.put("banana", 20);
        map.put("orange", 30);

        // Get values
        System.out.println("apple: " + map.get("apple"));
        System.out.println("banana: " + map.get("banana"));

        // Try to get a key that does not exist
        System.out.println("grape: " + map.get("grape"));
    }
}

Output

apple: 10
banana: 20
grape: null

Common Pitfalls

Common mistakes include:

• Using get on a key that does not exist returns null, which can cause NullPointerException if not checked.
• Using mutable objects as keys without proper hashCode and equals methods can cause unexpected behavior.
• Forgetting to import java.util.HashMap.

java

import java.util.HashMap;

public class PitfallExample {
    public static void main(String[] args) {
        HashMap<String, String> map = new HashMap<>();
        map.put("key1", "value1");

        // Wrong: assuming key exists without check
        String val = map.get("key2");
        // This will cause NullPointerException if you call val.length()
        // System.out.println(val.length()); // Uncommenting causes error

        // Right: check for null
        if (val != null) {
            System.out.println(val.length());
        } else {
            System.out.println("Key not found");
        }
    }
}

Output

Key not found

Quick Reference

Method	Description	Example
put(key, value)	Adds or updates a key-value pair	map.put("name", "Alice");
get(key)	Retrieves value for the key or null	map.get("name");
containsKey(key)	Checks if key exists	map.containsKey("name");
remove(key)	Removes key-value pair	map.remove("name");

Key Takeaways

Use put(key, value) to add or update entries in a HashMap.

Use get(key) to retrieve values; it returns null if the key is missing.

Always check for null after get() to avoid errors.

HashMap keys should have proper hashCode and equals implementations.

Import java.util.HashMap before using it.