Posts July 2026
38 posts
Use Collection.stream, parallelStream, spliterator characteristics, and Collectors.toCollection to leave a collection and land in the right one — without re-teaching Stream.map.
Read MoreVisit vertices in hop order with a FIFO queue — level order and unweighted shortest path — and see why a stack is the wrong frontier.
Read MoreHands-on batch editing on the command line — find the right files, feed them safely through xargs, and rewrite them with sed.
Read MoreUse ConcurrentLinkedQueue and ConcurrentLinkedDeque for non-blocking ends, and ConcurrentSkipListMap/Set when you need a concurrent sorted map — not a BlockingQueue and not a HashMap.
Read MoreSort RAM-sized chunks, write runs to disk, then k-way merge with a heap of heads — the in-memory merge-sort cousin when the file cannot be loaded.
Read MoreUse BlockingQueue to pass work between threads — put/take that block, offer/poll with timeouts, and when to pick Array, Linked, Delay, Synchronous, or Transfer queues.
Read MoreNatural runs, insertion on short runs, then merge — what object Arrays.sort and Collections.sort actually run, and why you should not hand-roll it.
Read MoreUse CopyOnWriteArrayList and CopyOnWriteArraySet when iteration is the hot path — every mutation copies the array, and iterators walk a snapshot.
Read MoreLSD digit passes as counting sort, when integer or string keys have a digit layout, and why Arrays.sort is not this.
Read MoreUse ConcurrentHashMap for a shared map — per-bin concurrency, no nulls, weakly consistent iterators, and atomic compute/merge — not Hashtable and not synchronizedMap.
Read MoreHistogram, prefix of counts, and a stable place-from-the-right pass when keys come from a small universe.
Read MoreHands-on rsync recipes for everyday work — incremental sync, remote backups over SSH, progress output, and safe dry-runs.
Read MoreRecognize Vector, Stack, Hashtable, Dictionary, Enumeration, and Properties as legacy — synchronized whole-table types you should not choose for new code.
Read MoreBuild a max-heap and extract the max into the tail — guaranteed O(n log n), little extra memory, and why PriorityQueue poll-all is not this procedure.
Read MoreUse Collections and Arrays for sort, binarySearch, shuffle, nCopies, emptyXxx, synchronized wrappers, and asList — the static methods around the types, not a fourth List implementation.
Read MorePartition around a pivot and recurse — and why last-element vs random vs median-of-three vs dual-pivot is the algorithm, not textbook Lomuto as Arrays.sort.
Read MoreUse List.of, Set.of, Map.of, and copyOf for unmodifiable collections that reject nulls — Java 9 factories, not Collections.unmodifiableList wrapping a live ArrayList.
Read MoreHands-on sudo and sudoers recipes — run commands as root or another user, edit policy safely with visudo, and delegate fine-grained privileges without handing out full root.
Read MoreSplit, sort the halves, merge two sorted runs — why the extra array buys a guaranteed O(n log n) bound and stability, and what object Arrays.sort actually runs.
Read MoreInsertion still earns a seat on small or nearly-sorted ranges; selection swaps the min into place; bubble is a whiteboard name — none of them are the production default.
Read MoreUse IdentityHashMap when keys are compared with ==, and WeakHashMap when the GC should drop entries whose keys are otherwise unreachable — two unusual maps, two different jobs.
Read MoreUse EnumSet and EnumMap when the key universe is an enum — a bit vector and an ordinal array, not a hash table.
Read MoreSort by start, sweep, and merge overlapping calendar or busy ranges — and when this is not interval scheduling.
Read MoreHands-on df and du recipes — check filesystem free space, hunt directory hogs, and resolve “no space left on device” with confidence.
Read MoreA decision guide for the data-structures series: given the hot operation, which layout and which JDK type — with links to every post in the series.
Read MoreRunning best ending here, empty-array and all-negative pitfalls, and why this is one pass of DP you do not need to name as a table.
Read MoreSpatial trees split the plane so range and nearest-neighbor queries skip whole regions. Quadtrees cut into four boxes; k-d trees alternate axis splits.
Read MoreInclusive vs exclusive prefixes, a 2D sketch, difference arrays for range updates, and when a Fenwick or segment tree is the next bill.
Read MoreFixed and variable windows, the add-right / drop-left invariant, and why this is not the same thing as a rate-limit window.
Read MoreA suffix array is the sorted list of a string’s suffixes as start indices. Binary search finds substrings; a short section contrasts the heavier suffix tree.
Read MoreA treap is a BST on keys and a heap on random priorities. Rotations restore heap order so the shape stays balanced in expectation without color bits.
Read MoreHands-on ss and netstat recipes — list listening sockets, inspect open connections, check routing tables, and read interface statistics.
Read MoreOpposite-end, pair-sum on sorted data, and fast/slow — when two indexes replace O(n²) and when they are just two indexes.
Read MoreA splay tree is a BST that rotates the accessed node to the root. Recently used keys get cheap; amortized O(log n) without storing balance factors.
Read MoreThe sorted invariant, lower and upper bound, off-by-one, and what Arrays.binarySearch encodes when the key is missing.
Read MoreLeast-recently-used eviction is O(1) only when a hash map points at nodes of a doubly linked list. How the two layouts share the work, and LinkedHashMap as the JDK shortcut.
Read MoreA map of search, sort, graphs, strings, DP, and systems algorithms — what job each one solves, when to skip it, and a living index of every post in this series.
Read MoreHands-on kill, pkill, and killall recipes — choose SIGTERM, SIGKILL, or SIGHUP, target the right PIDs, and stop processes without wrecking managed services.
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