Group by Key - Java DSA

Build buckets keyed by a shared field, preserving the first-seen key order.

Algorithm

Canonical pairs (a,1), (b,2), (a,3), (c,4), (b,5) print {a: [1, 3], b: [2, 5], c: [4]}. The replay uses the same input in every language, so this Java DSA implementation can be compared directly with the rest of the DSA track.

Basic Implementation

Basic.java

import java.util.*;

public class Basic {
    public static void main(String[] args) {
        String[] keys = {"a", "b", "a", "c", "b"};
        int[] values = {1, 2, 3, 4, 5};
        Map<String, List<Integer>> groups = new LinkedHashMap<>();
        for (int i = 0; i < keys.length; i++) {
            groups.computeIfAbsent(keys[i], key -> new ArrayList<>()).add(values[i]);
        }
        List<String> parts = new ArrayList<>();
        for (Map.Entry<String, List<Integer>> entry : groups.entrySet()) {
            parts.add(entry.getKey() + ": " + entry.getValue());
        }
        System.out.println("{" + String.join(", ", parts) + "}");
    }
}

Complexity

Time: O(n) average
Space: O(k + n) for buckets and values

Implementation notes

Keep output formatting deterministic. Do not rely on unordered hash-map printing when the lesson needs cross-language comparison.
The trace highlights the hash table state after each write.

bucket map Each key owns a list. A new key creates a bucket; a repeated key appends to the existing bucket.