Modern / Recent Platform Additions

They are a debugging aid, not a prevention tool. You still use Optional, null checks, requireNonNull, and annotations to avoid NPEs; the helpful message just tells you fast where one slipped through.

HttpURLConnection is an old, awkward, blocking-only API with no HTTP/2 support. java.net.http replaces it with a modern, immutable builder-based client that supports HTTP/2, WebSocket, async calls via CompletableFuture, and pluggable request/response body handling.

A forward reference is when a method you define refers to a variable, method, or type that doesn't exist yet. JShell accepts the definition, marks it as not-yet-runnable, and makes it work automatically once you define the missing piece.

Use JShell for quick, throwaway experiments: trying an API, checking what a snippet returns, or teaching. For anything you need to keep, repeat reliably, or run with your project's dependencies and build, use a scratch class or a unit test instead.

indent(n) (Java 12) adds n spaces of indentation to each line (or removes up to n with a negative value) and normalizes line endings to \n, ensuring each line ends with a newline. transform(fn) applies a function to the whole string and returns its result, letting you chain custom operations fluently.

Add a shebang line #!/path/to/java --source 21 as the first line of an extension-less file, make it executable with chmod +x, and run ./myscript. The --source flag is required because, without a .java extension, the launcher needs to be told to use source mode and which language level.

Incubating means it is an unfinished, opt-in module whose API can change between Java releases. You must explicitly enable it. It is waiting on Project Valhalla so vectors can become lightweight value types with no heap overhead before it is finalized.

Use SequencedCollection in a signature when callers genuinely need a defined order plus end access, but you don't want to lock them into List. Accept it as a parameter to be permissive; return it (or a narrower type) when order is part of your contract. Don't use it if order is irrelevant.

Single-file execution (Java 11) was an early step to make Java easier for small programs. Later, JEP 458 let you run multi-file programs from source, and implicit classes plus instance main methods removed most boilerplate, so a beginner can run a tiny program from source with almost no ceremony.

Use it only for hot, data-parallel numeric kernels where you have measured a real bottleneck and the JIT is not vectorizing reliably. For small or branchy code, let the JIT handle it; for heavy, well-tuned math, a native BLAS library may beat it. Weigh the cost of an unstable, incubating API.