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Collections beginner

Sequenced collections

Access first/last elements and reverse views with clean API methods.

Code Comparison
✕ Java 8 
// Get last element
var last = list.get(list.size() - 1);
// Get first
var first = list.get(0);
// Reverse iteration: manual
✓ Java 21+ 
var last = list.getLast();
var first = list.getFirst();
var reversed = list.reversed();
Why the modern way wins
📖

Self-documenting

getLast() is clearer than get(size()-1).

🔄

Reversed view

reversed() gives a view — no copying needed.

🧩

Uniform API

Works the same on List, Deque, SortedSet.

Old Approach
Index Arithmetic
Modern Approach
getFirst/getLast
Since JDK
21
Difficulty
beginner
JDK Support
Sequenced collections
Available

Widely available since JDK 21 LTS (Sept 2023)

How it works

SequencedCollection adds getFirst(), getLast(), reversed(), addFirst(), addLast() to List, Deque, SortedSet, and LinkedHashSet. No more size-1 arithmetic or manual reverse iteration.

Related patterns
Collections intermediate

Collectors.teeing()

Java 8
long count = items.stream().count();
double sum = items.stream()
    .mapToDouble(Item::price)
    .sum();
var result = new Stats(count, sum);
Java 12+
var result = items.stream().collect(
    Collectors.teeing(
        Collectors.counting(),
        Collectors.summingDouble(Item::price),
        Stats::new
    )
);
Hover to see modern ➜
JDK 12+ →
Collections beginner

Immutable set creation

Java 8
Set<String> set =
    Collections.unmodifiableSet(
        new HashSet<>(
            Arrays.asList("a", "b", "c")
        )
    );
Java 9+
Set<String> set =
    Set.of("a", "b", "c");
Hover to see modern ➜
JDK 9+ →
Collections beginner

Immutable map creation

Java 8
Map<String, Integer> map = new HashMap<>();
map.put("a", 1);
map.put("b", 2);
map.put("c", 3);
map = Collections.unmodifiableMap(map);
Java 9+
Map<String, Integer> map =
    Map.of("a", 1, "b", 2, "c", 3);
Hover to see modern ➜
JDK 9+ →
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