Preview Features and How to Enable Them

void main() throws Exception {
    Path dir = Files.createTempDirectory("preview-run");
    Files.writeString(dir.resolve("Check.java"), """
        void main() {
            Object retries = 3;
            if (retries instanceof int n && n > 0) IO.println("retrying " + n + " times");
        }
        """);
    String java = Path.of(System.getProperty("java.home"), "bin", "java").toString();

    for (List<String> args : List.of(List.of("Check.java"), List.of("--enable-preview", "Check.java"))) {
        List<String> cmd = new ArrayList<>(List.of(java));
        cmd.addAll(args);
        Process p = new ProcessBuilder(cmd).directory(dir.toFile()).redirectErrorStream(true).start();
        String firstLine = new String(p.getInputStream().readAllBytes()).lines().findFirst().orElse("");
        IO.println("java " + String.join(" ", args) + " -> exit " + p.waitFor());
        IO.println("  " + firstLine);
    }
}
// Output:
// java Check.java -> exit 1
//   Check.java:3: error: primitive patterns are a preview feature and are disabled by default.
// java --enable-preview Check.java -> exit 0
//   retrying 3 times

void main() throws Exception {
    Path dir = Files.createTempDirectory("preview-class");
    Files.writeString(dir.resolve("Job.java"), """
        public class Job {
            public static void main(String[] args) {
                Object code = 7;
                if (code instanceof int c) System.out.println("exit code " + c);
            }
        }
        """);
    String bin = Path.of(System.getProperty("java.home"), "bin").toString();
    List<List<String>> steps = List.of(
        List.of("javac", "--release", "25", "--enable-preview", "-d", "out", "Job.java"),
        List.of("java", "-cp", "out", "Job"),
        List.of("java", "--enable-preview", "-cp", "out", "Job"));

    for (List<String> step : steps) {
        List<String> cmd = new ArrayList<>(step);
        cmd.set(0, Path.of(bin, step.get(0)).toString());
        Process p = new ProcessBuilder(cmd).directory(dir.toFile()).redirectErrorStream(true).start();
        String out = new String(p.getInputStream().readAllBytes());
        String last = out.lines().reduce((a, b) -> b).orElse("").strip();
        IO.println(step.get(0) + " " + String.join(" ", step.subList(1, step.size())) + " -> exit " + p.waitFor());
        if (!last.isEmpty()) IO.println("  " + last);
    }
}
// Output:
// javac --release 25 --enable-preview -d out Job.java -> exit 0
//   Note: Recompile with -Xlint:preview for details.
// java -cp out Job -> exit 1
//   java.lang.UnsupportedClassVersionError: Preview features are not enabled for Job (class file version 69.65535). Try running with '--enable-preview'
// java --enable-preview -cp out Job -> exit 0
//   exit code 7

import javax.tools.*;
import java.net.URI;

int[] compileAndReadVersion(String code, List<String> options) throws IOException {
    Path out = Files.createTempDirectory("ver");
    var file = new SimpleJavaFileObject(URI.create("string:///Probe.java"), JavaFileObject.Kind.SOURCE) {
        @Override public CharSequence getCharContent(boolean ignore) { return code; }
    };
    var opts = new ArrayList<>(options);
    opts.addAll(List.of("-d", out.toString()));
    ToolProvider.getSystemJavaCompiler().getTask(null, null, d -> {}, opts, null, List.of(file)).call();
    byte[] b = Files.readAllBytes(out.resolve("Probe.class"));
    int minor = ((b[4] & 0xff) << 8) | (b[5] & 0xff);
    int major = ((b[6] & 0xff) << 8) | (b[7] & 0xff);
    return new int[] {major, minor};
}

void main() throws IOException {
    int[] plain = compileAndReadVersion("class Probe { int x = 1; }", List.of());
    int[] preview = compileAndReadVersion(
        "class Probe { boolean b(Object o) { return o instanceof int; } }",
        List.of("--release", "25", "--enable-preview"));
    IO.println("plain   " + plain[0] + "." + plain[1]);
    IO.println("preview " + preview[0] + "." + preview[1] + (preview[1] == 0xFFFF ? " (needs --enable-preview)" : ""));
}
// Output:
// plain   69.0
// preview 69.65535 (needs --enable-preview)