AtticIteration Patterns
Complete guide to all Map iteration techniques in Java, entrySet vs keySet performance, safe removal, and Streams
Iterating over a Map<K, V> in Java is fundamentally different from iterating over a List or Set because Map does not implement Iterable. Instead, iteration is performed through one of its three collection views (entrySet(), keySet(), values()), modern functional methods, or the Streams API.
The 6 Canonical Ways to Iterate a Map
1. entrySet() with Enhanced For Loop (Default Choice)
This is the most efficient and idiomatic way to iterate over a map when you need both the key and the value. It extracts both in a single pass without extra hash lookups.
Map<String, Integer> scores = new HashMap<>();
scores.put("Alice", 95);
scores.put("Bob", 88);
scores.put("Charlie", 92);
for (Map.Entry<String, Integer> entry : scores.entrySet()) {
String name = entry.getKey();
Integer score = entry.getValue();
System.out.println(name + " : " + score);
}Why entrySet() is Optimal: In a hash map, each bucket already stores the key and value
together in a single Node object. entrySet() visits each node directly, avoiding a second hash
computation and bucket lookup.
2. keySet() with Enhanced For Loop (Keys Only)
Reach for keySet() only when your loop body only concerns keys, or when you plan to pass keys to external methods:
for (String name : scores.keySet()) {
System.out.println("Student: " + name);
}The keySet() + get(k) Anti-Pattern:
// ❌ SUB-OPTIMAL: Double lookup per row!
for (String key : map.keySet()) {
Integer value = map.get(key); // Re-hashes key and traverses bucket again!
}
// ✅ OPTIMAL: Single pass!
for (Map.Entry<String, Integer> entry : map.entrySet()) {
Integer value = entry.getValue();
}3. values() with Enhanced For Loop (Values Only)
Use values() when you only need to process or aggregate values (e.g., calculating a sum or finding an average):
int totalScore = 0;
for (int score : scores.values()) {
totalScore += score;
}
System.out.println("Average: " + (double) totalScore / scores.size());4. forEach() with BiConsumer Lambda (Cleanest Syntax)
Introduced in Java 8, map.forEach() is the cleanest and most concise option for short loop bodies:
scores.forEach((name, score) -> {
System.out.println(name + " earned " + score + " points");
});5. Iterator via entrySet().iterator() (Safe In-Loop Removal)
Modifying a map directly inside an enhanced for loop throws a ConcurrentModificationException. To safely remove entries during traversal, use the iterator's own remove() method:
Iterator<Map.Entry<String, Integer>> it = scores.entrySet().iterator();
while (it.hasNext()) {
Map.Entry<String, Integer> entry = it.next();
if (entry.getValue() < 90) {
it.remove(); // Safely unlinks entry from the backing map
}
}6. Java Streams API (Filtering, Sorting, and Transforming)
The Streams API allows you to filter, transform, and sort map entries during iteration:
Filter and Collect to a New Map
Map<String, Integer> highScorers = scores.entrySet().stream()
.filter(e -> e.getValue() >= 90)
.collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));Iterate Sorted by Key
scores.entrySet().stream()
.sorted(Map.Entry.comparingByKey())
.forEach(entry -> System.out.println(entry.getKey() + ": " + entry.getValue()));Iterate Sorted by Value (Descending)
scores.entrySet().stream()
.sorted(Map.Entry.<String, Integer>comparingByValue().reversed())
.forEach(entry -> System.out.println(entry.getKey() + " -> " + entry.getValue()));Performance & Selection Summary
| Iteration Strategy | Time Overhead | Memory Overhead | Best Used For |
|---|---|---|---|
entrySet() loop | O(n) | None | Default choice when reading both keys and values |
keySet() loop | O(n) | None | When only keys are needed in the loop |
values() loop | O(n) | None | Aggregating or inspecting values only |
forEach() lambda | O(n) | None | Clean, concise read-only traversal |
iterator() with it.remove() | O(n) | Minimal | Removing entries conditionally during iteration |
entrySet().removeIf() | O(n) | None | Cleanest in-place conditional filtering |
entrySet().stream() | O(n) | Stream pipeline | Filtering, complex sorting, and transforming |