HashMap

An ArrayList finds items by position. But what if you want to find a person's phone number by their name ? A HashMap stores key→value pairs and looks any value up by its key, instantly — like a real dictionary or phone book.

Learn HashMap in our free Java course — a beginner-friendly interactive lesson with worked examples, a practice exercise and a quick reference.

Part of the free Java course at LearnCodingFast — hands-on lessons with examples you run in your browser, plus practice exercises and a quick quiz.

What You'll Learn in This Lesson

1️⃣ Keys and Values

A HashMap holds pairs. Each pair has a key (what you look up by) and a value (what you get back). Keys are unique; values can repeat.

💡 Analogy: A real dictionary maps a word (key) to its definition (value). You don't scan every page — you jump straight to the word. A HashMap does the same: get("Alice") goes directly to Alice's entry without checking everyone else.

You declare two types in the angle brackets — the key type and the value type. Both must be objects, so use wrapper classes for primitives:

2️⃣ The Core Methods

Method

What it does

put(k, v)

Add the pair, or overwrite if k exists

get(k)

Value for k, or null if absent

getOrDefault(k, d)

Value for k, or d if absent

containsKey(k)

Is there an entry for key k?

containsValue(v)

Does any entry hold value v?

remove(k)

Delete the entry for k

size()

Number of pairs

keySet()

All keys (for looping)

entrySet()

All key/value pairs (for looping)

3️⃣ Looping Over a HashMap

To visit every pair, loop over entrySet() and read each entry's key and value. To visit only keys, loop over keySet() .

4️⃣ The Tally Pattern (Counting Things)

The single most common HashMap pattern is counting occurrences : words in a sentence, votes per candidate, clicks per page. The trick is getOrDefault :

The first time a key is seen, getOrDefault returns the default 0 , so the count becomes 1. Every later time, it returns the existing count and adds one. Clean and bug-free, with no separate "is this the first time?" check.

🧩 Reorder Challenge

Reorder these lines to store one age and look it up.

Why: The map must exist before you put into it, and the pair must be stored before get can retrieve it. If you reversed put and get , the lookup would print null . Output: 28 .

🧠 Quick Recall

1 . Keys are unique, so the second put("a", 5) overwrites the value rather than adding a new entry.

-1 . The key "x" is absent, so getOrDefault returns the supplied default instead of null .

2 . First put: 0 + 1 = 1. Second put: 1 + 1 = 2. That's the tally pattern running twice.

Common Beginner Mistakes

• ❌ Calling a method on a null get(): map.get("missing").length() crashes. Use getOrDefault or containsKey .
• ❌ Primitive type arguments: HashMap<String, int> is illegal — use HashMap<String, Integer> .
• ❌ Expecting an order: HashMap iteration order is unpredictable. Use LinkedHashMap or TreeMap if order matters.
• ❌ Forgetting Map import: using Map.Entry needs import java.util.Map; .
• ❌ Duplicate keys "lost": putting the same key twice overwrites — it doesn't keep both.

📋 Quick Reference

Description

Example

new HashMap<>()

Create an empty map

HashMap<String, Integer> m = new HashMap<>();

Add pair, or overwrite if k exists

m.put("a", 1)

m.get("a")

m.getOrDefault("z", 0)

m.containsKey("a")

m.containsValue(1)

m.remove("a")

Number of key-value pairs

m.size()

for (String k : m.keySet())

values()

All values (for looping)

m.values()

All key-value pairs

for (Map.Entry<K,V> e : m.entrySet())

e.getKey() / e.getValue()

Read a pair while iterating

e.getKey()

getOrDefault(k,0)+1

Tally / count pattern

m.put(w, m.getOrDefault(w,0)+1)

merge(k, 1, Integer::sum)

Alternative tally one-liner

m.merge(w, 1, Integer::sum)

Frequently Asked Questions

🎉 Lesson Complete!

You now command Java's most useful collection for lookups: put and get pairs, dodge null with getOrDefault , check membership with containsKey , iterate with entrySet / keySet , and count things with the tally pattern.

Next up: a Checkpoint that combines data types, casting, Scanner, Math, StringBuilder, ArrayList and HashMap into one hands-on build.

Practice quiz

Continue this course

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