Map

The Big Picture

The Map interface maps keys to values.

• Implementations: HashMap, TreeMap, LinkedHashMap
• Essential for data relationships and lookups
• JSON objects naturally map to Map<String, Object>
• HTTP headers, configs stored as maps

Core to Java programming - used everywhere.

How It Works

Maps keys to values using hashing or tree structures.

• put(key, value) adds or updates entry
• get(key) retrieves value or null
• Iterate: entrySet(), keySet(), values()
• Java 8+: computeIfAbsent(), merge()
• Immutable: Map.of() (Java 9+)

In the Java Landscape

Core to Java - HTTP params, configs, caches all use maps.

• Spring @RequestParam converts to Map
• Properties files are key-value maps
• EnumMap and IdentityHashMap for special cases
• Collectors: groupingBy() produces maps from streams
• Map.entry() (Java 9+) for creating entries

Pitfalls and Best Practices

• Use getOrDefault() for a fallback when a key is absent
• Hash-map keys: consistent hashCode()/equals(); TreeMap needs stable ordering
• Avoid mutable keys
• Use computeIfAbsent() for lazy initialization
• Return Map.of() for immutable results