CalliCoder

Software Engineering Principles: A Practical Guide

Sat, 11 Sep 2021 00:00:00 GMT

KISS, DRY, YAGNI and SOLID are quoted far more often than they are applied. What each one actually claims, the failure mode it prevents, and the cost of following it too literally.

Deploying a Go App with Redis on Kubernetes

Fri, 10 Sep 2021 00:00:00 GMT

Adding a stateful dependency: why a Deployment with a ReadWriteOnce volume deadlocks on rollout, StatefulSet against Deployment for Redis, the maxmemory that must sit below the container limit, and degrading rather than failing when the cache is gone.

Core Java Interview Questions with Answers

Sat, 04 Sep 2021 00:00:00 GMT

The topics Java interviews actually probe — equals and hashCode, the memory model, collections, exceptions, generics — with what each question is really testing and the follow-up that separates a memorised answer from an understood one.

Squares of a Sorted Array

Tue, 03 Aug 2021 00:00:00 GMT

Sorting the squares is the obvious O(n log n) answer and the wrong one. Two pointers from the outside in gives O(n), because the largest square is always at one end — and filling the result backwards is what makes it work.

Longest Substring with Same Letters After Replacement

Sun, 01 Aug 2021 00:00:00 GMT

A sliding window where the shrink condition is the count of the most frequent character, why that maximum never needs to be recomputed downward, and the off-by-one that makes the answer too large.

Sliding Window Maximum Explained

Tue, 27 Jul 2021 00:00:00 GMT

A deque holding indices in decreasing order of value gives O(n) where a heap gives O(n log k). The deque stores indices rather than values so the window boundary can be checked, and each index enters and leaves exactly once.

Maximum Sum Subarray of Size K

Sat, 24 Jul 2021 00:00:00 GMT

The sliding window that turns O(n·k) into O(n): add the entering element, subtract the leaving one, and never recompute. Plus the two off-by-ones in the window boundary and why negative numbers do not break it.

Find the Minimum in a Rotated Sorted Array

Tue, 20 Jul 2021 00:00:00 GMT

Comparing the midpoint to the right end rather than the left is what makes this work, why the loop uses < instead of <=, and the duplicate that forces the worst case back to O(n).

Go Command-Line Flags and Options with the flag Package

Tue, 06 Jul 2021 00:00:00 GMT

Why flags after a positional argument are silently ignored, single and double dashes being equivalent, custom flag types, and FlagSet for subcommands.

Golang Sorting and Custom Sorting by Functions

Sun, 04 Jul 2021 00:00:00 GMT

The slices package replaced most of sort, SortFunc wants a three-way comparison where sort.Slice wanted a boolean, and neither is stable unless you ask.

Golang URL Encoding and Decoding

Sun, 04 Jul 2021 00:00:00 GMT

QueryEscape turns a space into a plus and PathEscape turns it into %20, url.Values handles repeated keys, and building a URL by concatenating strings is how the encoding gets skipped.

Base64 Encoding and Decoding in Golang

Thu, 01 Jul 2021 00:00:00 GMT

Four encodings in one package and why picking the wrong one breaks a URL, the padding that some APIs reject, and the streaming encoder that avoids holding the whole payload twice.

Equal Subset Sum Partition Problem

Sun, 13 Jun 2021 00:00:00 GMT

An odd total is an instant no. Beyond that it is a subset-sum for half the total, and the one-dimensional table must be filled backwards or each item gets used more than once.

The 0/1 Knapsack Problem Explained

Thu, 27 May 2021 00:00:00 GMT

From the recursive definition to a one-dimensional table: why greedy fails here but works for the fractional version, why the 1D loop must run backwards, and what pseudo-polynomial actually means.

Spring Boot, MySQL and React with Docker Compose

Sun, 15 Nov 2020 00:00:00 GMT

A three-service stack in one file: healthcheck-gated startup so the API does not race the database, a React build served by nginx with the API proxied behind it, and a dev override that keeps hot reload.

Java CompletableFuture Tutorial with Runnable Examples

Wed, 30 Sep 2020 00:00:00 GMT

A CompletableFuture tutorial that runs: how to create one, chain transformations, combine several, handle exceptions properly, and pick the right executor so your pipeline never quietly stalls the common pool.

Java Concurrency Issues and Thread Synchronization

Sun, 20 Sep 2020 00:00:00 GMT

Thread interference and memory consistency are two different bugs with two different fixes. Why volatile solves one and not the other, what synchronized actually locks, and the lock objects that are shared with code you have never seen.

Java Callable and Future Tutorial

Tue, 15 Sep 2020 00:00:00 GMT

What Callable adds over Runnable, and the four things Future cannot do — no callbacks, no composition, no manual completion, no exception handling without blocking — with the CompletableFuture equivalent for each.

Java ExecutorService and Thread Pool Tutorial

Thu, 10 Sep 2020 00:00:00 GMT

Submitting work, sizing a pool, and shutting one down properly — plus the two Executors defaults that turn a slow consumer into an OutOfMemoryError, and why an uncaught exception in submit() vanishes.

How to Format Date and Time in Java

Mon, 01 Jun 2020 00:00:00 GMT

DateTimeFormatter patterns that mean something other than what they look like, the locale that changes the output of code you did not touch, and why formatting an Instant throws.

Add or Subtract Days and Hours from a Date in Java

Wed, 01 Apr 2020 00:00:00 GMT

plusDays against plusHours across a daylight-saving change, the month-end clamping that turns 31 January into 28 February, and why Calendar.add mutates the object you passed in.

Reading and Writing Environment Variables in Go

Tue, 10 Mar 2020 00:00:00 GMT

LookupEnv distinguishes unset from empty and Getenv does not, Setenv only changes this process, and an environment variable is readable by anything that can read /proc.

Reading Command Line Arguments in Go

Thu, 05 Mar 2020 00:00:00 GMT

os.Args includes the program name at index zero, why that name is not a reliable path, and the difference between raw arguments and what flag.Args returns after parsing.

How to Create a New File in Java

Sat, 15 Feb 2020 00:00:00 GMT

Files.createFile against File.createNewFile, the parent directory that has to exist first, POSIX permissions set at creation rather than after, and the temp-and-move pattern that makes a write atomic.

How to Get the Current Date and Time in Java

Sat, 15 Feb 2020 00:00:00 GMT

LocalDate, LocalTime, LocalDateTime, ZonedDateTime and Instant — which one has a time zone, which one is a moment, and why the answer to now() depends on a JVM default you did not set.

How to Get the Current Epoch Timestamp in Java

Sat, 15 Feb 2020 00:00:00 GMT

Seconds against milliseconds is the bug this causes, why an int epoch stops working in 2038, and the fact that a LocalDateTime cannot be converted without supplying a zone.

How to Read a File in Java

Thu, 06 Feb 2020 00:00:00 GMT

Files.readString for a small file, Files.lines for a large one, why the stream must be closed, and the charset default that changed in Java 18 and still bites on older runtimes.

How to Compare Date and Time in Java

Sat, 01 Feb 2020 00:00:00 GMT

isBefore and isAfter against compareTo and equals, why two ZonedDateTimes for the same moment are not equal, and the truncation that makes a timestamp comparison behave.

How to Parse a String to a Date in Java

Thu, 30 Jan 2020 00:00:00 GMT

LocalDate, LocalDateTime, ZonedDateTime and Instant from text — plus the two traps that pass every test and fail in production: yyyy under a strict resolver, and month names that depend on the default locale.

How to Check if a File or Directory Exists in Java

Tue, 07 Jan 2020 00:00:00 GMT

Files.exists, Files.notExists and why they are not opposites, the race between checking and acting that makes the check pointless, and what a symbolic link does to the answer.

How to Copy a File or Directory in Java

Tue, 07 Jan 2020 00:00:00 GMT

Files.copy will not overwrite unless you ask, copying a directory copies only the directory, and COPY_ATTRIBUTES is the difference between a copy and a duplicate.

How to Delete a File or Directory in Java

Tue, 07 Jan 2020 00:00:00 GMT

Files.delete throws where File.delete returns false, a directory must be empty first and the JDK has no recursive delete, and deleteOnExit accumulates filenames for the life of the JVM.

How to Move or Rename a File in Java

Tue, 07 Jan 2020 00:00:00 GMT

A move within one filesystem is atomic and nearly free; across filesystems it is a copy and a delete, and ATOMIC_MOVE throws rather than degrading silently. Renaming is the same call.

How to Create a Temp File or Directory in Java

Sun, 05 Jan 2020 00:00:00 GMT

Files.createTempFile creates with owner-only permissions and File.createTempFile does not, deleteOnExit leaks until the JVM stops, and the shared temp directory is a hostile place to write.

Valid Parentheses Problem Explained

Sat, 05 Oct 2019 00:00:00 GMT

A stack, and the three checks that make it correct: a closer with an empty stack, a closer that does not match the top, and anything left open at the end. Missing any one passes most tests.

Binary Search: Iterative and Recursive in Java

Thu, 03 Oct 2019 00:00:00 GMT

The midpoint calculation that overflows, why the loop condition is <= and not <, what the return value means when the target is absent, and the lower-bound variant that handles duplicates.

Longest Palindromic Substring

Thu, 03 Oct 2019 00:00:00 GMT

Expanding around each centre is O(n²) with O(1) space and beats the dynamic-programming table on both. The trick is that there are 2n−1 centres, not n — every gap between characters is one too.

Search an Element in a Rotated Sorted Array

Thu, 03 Oct 2019 00:00:00 GMT

One half of every split is always sorted, which is the single comparison the whole algorithm rests on — plus the boundary conditions that decide whether the target is in that half, and what duplicates cost.

Full-Stack Spring Boot on Kubernetes with Volumes

Sat, 27 Jul 2019 00:00:00 GMT

MySQL needs a PersistentVolumeClaim and a StatefulSet, a Secret is base64 rather than encrypted, and the React build has to be served by nginx that also proxies the API — with the routing decided at build time or not at all.

Deploying a Containerized Go App on Kubernetes

Sat, 20 Jul 2019 00:00:00 GMT

Deployment, Service and Ingress for a Go binary — plus the four settings that decide whether a rollout drops requests: probes that mean different things, resource requests, terminationGracePeriod and preStop.