2. JDK 26 Preview Notes

JDK 26 is the latest feature release of Java. This book uses JDK 25 LTS as the main baseline, so JDK 26-only features belong in a clearly marked preview section.

Use preview features deliberately. They require explicit compiler and launcher flags, and their syntax or API can still change before finalization.

javac --release 26 --enable-preview Example.java
java --enable-preview Example

2.1. Primitive patterns

JDK 26 previews primitive types in patterns, instanceof, and switch. This makes pattern matching more uniform across reference and primitive values. It can also check whether a value fits exactly into a narrower primitive type before binding it.

String statusMessage(int status) {
  return switch (status) {
    case 0 -> "okay";
    case 1 -> "warning";
    case 2 -> "error";
    case int code when code > 2 -> "unknown status: " + code;
  };
}

void printIfByte(int value) {
  if (value instanceof byte b) {
    IO.println("fits in a byte: " + b);
  } else {
    IO.println("too large for byte");
  }
}

Because this is preview syntax, do not use it in the core examples. The standard JDK 25 way to write the same beginner-friendly code is still a normal default branch.

2.2. HTTP/3

JDK 26 adds HTTP/3 support to the HTTP Client API. Most beginner code still starts with HttpClient, HttpRequest, and HttpResponse; the new protocol support mainly matters when an application talks to HTTP/3 servers.

2.3. Structured concurrency

Structured concurrency is preview in JDK 26. It is important for the future of Java concurrency, but this book should teach stable virtual threads first. Preview structured-concurrency examples can be added later when the API is final.

2.4. Migration notes

When moving code from JDK 25 to JDK 26, watch for these areas:

Preview code must be rechecked against the exact JDK 26 syntax.
HTTP Client behavior changed in a few edge cases.
The Applet API was removed.
Deep reflection that mutates final fields now emits warnings to prepare for future restrictions.