HashMap

Syntax

HashMap<KeyType, ValueType> map = new HashMap<>();

Examples

import java.util.HashMap;

HashMap<String, Integer> ages = new HashMap<>();
ages.put("Fola", 21);
ages.put("Zain", 22);
ages.put("Jamal", 20);

System.out.println(ages);              // {Fola=21, Zain=22, Jamal=20} (order not guaranteed)
System.out.println(ages.get("Zain"));   // 22

import java.util.HashMap;

HashMap<String, Integer> stock = new HashMap<>();
stock.put("apples", 50);
stock.put("bananas", 30);

System.out.println(stock.containsKey("apples"));  // true
System.out.println(stock.getOrDefault("mango", 0)); // 0 - safe default

stock.put("apples", 45);  // overwrites the existing value
stock.remove("bananas");

System.out.println(stock);  // {apples=45}

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

HashMap<String, Integer> scores = new HashMap<>();
scores.put("Alice", 85);
scores.put("Bob", 92);

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

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

import java.util.HashMap;

HashMap<String, Integer> wordCount = new HashMap<>();
String[] words = {"apple", "banana", "apple", "cherry", "apple"};

for (String word : words) {
    wordCount.merge(word, 1, Integer::sum);  // increments count, or sets to 1 if new
}

System.out.println(wordCount);  // {banana=1, cherry=1, apple=3}

Best practices

• Use getOrDefault() or putIfAbsent() instead of manual containsKey() + get() checks, for cleaner and more efficient code
• Use merge() or compute() for update-in-place patterns like counting occurrences, rather than a manual get-then-put sequence
• Choose LinkedHashMap if you need predictable insertion-order iteration, or TreeMap if you need keys kept in sorted order
• Always override equals() and hashCode() properly on any custom class you use as a HashMap key, or lookups will not behave correctly

At a glance

Purpose

General-purpose application development

File extension

.java

Runs in

Java Virtual Machine

Usually used with

JDK and Java libraries

Specifications & further reading

• Java Language Specification

Related Java documentation