java.lang.OutOfMemoryError: Java Heap Space: Diagnosis and Fixes
Fix java.lang.OutOfMemoryError: Java heap space by checking -Xmx, capturing a heap dump and finding what retains memory in Eclipse MAT.
~8 min readread →
Developer interview notes & guides — Page 3
Focused explanations and longer guides, with worked scenarios, common mistakes and interview exercises across every chapter.
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Showing 49–72 of 200 articles
UnsupportedClassVersionError: Class File Version Mismatch in Java
Fix UnsupportedClassVersionError (class file version 65.0 vs 61.0) by matching the runtime JDK to your build or compiling with javac --release.
~9 min readread →
UnsupportedOperationException from Arrays.asList and List.of
Why Arrays.asList and List.of throw UnsupportedOperationException on add or remove, and how to choose a mutable copy or an immutable list.
~7 min readread →
Fixing 'Blocked by CORS Policy' Errors in JavaScript fetch
Fix 'blocked by CORS policy' fetch errors: which headers the server must send, preflights, credentials, dev proxies and why no-cors fails.
~8 min readread →
TypeError: Cannot Read Properties of Undefined in JavaScript
Fix TypeError: Cannot read properties of undefined by finding which value is missing, reading the stack trace and repairing the data flow.
~8 min readread →
Cannot Use Import Statement Outside a Module: ESM vs CommonJS
Fix SyntaxError: Cannot use import statement outside a module in Node and browsers: type module, .mjs, require of ESM and __dirname in ESM.
~8 min readread →
JavaScript Custom Error Classes
How to write a JavaScript Error subclass that keeps its name, stack and cause, when instanceof fails, and what to catch versus rethrow.
~6 min readread →
TypeError: x Is Not a Function in JavaScript: Causes and Fixes
Fix TypeError: x is not a function in JavaScript: wrong imports, shadowed names, lost this, a missing semicolon or .map on a non-array.
~7 min readread →
RangeError: Maximum Call Stack Size Exceeded in JavaScript
Fix RangeError: Maximum call stack size exceeded: missing base cases, self-calling setters, circular data, huge spreads and deep recursion.
~8 min readread →
ReferenceError: x Is Not Defined vs undefined in JavaScript
Fix ReferenceError: x is not defined: typos, block scope, the TDZ, window or require missing in Node, and scripts that load in the wrong order.
~7 min readread →
SyntaxError: Unexpected Token '<' in JSON: Debugging fetch
Fix SyntaxError: Unexpected token '<' in JSON from fetch: the server sent HTML. Check res.ok and content-type, then read the body as text.
~8 min readread →
Uncaught (in promise): Fixing Unhandled Promise Rejections
Fix Uncaught (in promise) errors and Node crashes from unhandled rejections: missing await, async forEach, fire-and-forget and rethrown errors.
~8 min readread →
Error: listen EADDRINUSE: address already in use in Node.js
Fix Node.js 'listen EADDRINUSE: address already in use :::3000': find the process holding the port, close servers on restart, use PORT and port 0.
~9 min readread →
npm error code EACCES: Permission Denied on npm install -g
Fix 'npm error code EACCES permission denied' on global installs without sudo: change the npm prefix, use nvm or fnm, prefer npx, fix Docker.
~8 min readread →
npm error code ERESOLVE: Unable to Resolve Dependency Tree
Fix npm 'code ERESOLVE unable to resolve dependency tree': read the peer conflict report, upgrade the right package, use overrides with care.
~9 min readread →
React: Cannot Update a Component While Rendering a Different One
Fix the React warning 'Cannot update a component while rendering a different component': a child sets parent state during render.
~8 min readread →
React Invalid Hook Call: Hooks Can Only Be Called Inside a Component
Fix 'Invalid hook call' in React: hooks in class components, callbacks or plain functions, duplicate React copies and mismatched react-dom.
~8 min readread →
Maximum Update Depth Exceeded in React: Causes and Fixes
Fix 'Maximum update depth exceeded' in React: effects with no or unstable dependencies, setState in componentDidUpdate and ref callbacks.
~8 min readread →
Objects Are Not Valid as a React Child: Causes and Fixes in React
Fix 'Objects are not valid as a React child' in React: rendering objects, Dates, Errors or props, plus async components on the client.
~7 min readread →
React State Not Updating Immediately After setState: How to Fix
Why React state isn't updated immediately after setState: render snapshots, batching, functional updaters, mutation and when to use useEffect.
~8 min readread →
Too Many Re-renders. React Limits the Number of Renders: How to Fix
Fix 'Too many re-renders' in React: a state setter runs during render, often onClick={setX(1)} or state synced in the component body.
~8 min readread →
Spring @Autowired Field Is Null: Why Injection Didn't Happen
Why is an @Autowired or @Value field null in Spring Boot? Objects built with new, static fields and constructor timing, and how to fix them.
~8 min readread →
Spring Boot Circular Dependency: 'Beans Form a Cycle' Fix
Fix 'the dependencies of some of the beans in the application context form a cycle' in Spring Boot 3 by redesigning beans, not flags.
~8 min readread →
Configuring CORS in Spring Boot: Fixing 'Blocked by CORS Policy'
Fix 'blocked by CORS policy' for a Spring Boot 3 API: global CORS config, Spring Security preflight handling, credentials and exposed headers.
~8 min readread →
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