# _ (unnamed variables and patterns) and compact source files with instance main()

$ javac UnnamedVariables.java      (JDK 21, no flag)
    <repo>/lanes/src/UnnamedVariables.java:12: error: unnamed variables are a preview feature and are disabled by default.
            if (o instanceof Point(int x, _)) {
                                           ^
      (use --enable-preview to enable unnamed variables)
    1 error
    exit=1

$ javac UnnamedVariables.java      (JDK 25, no flag)
    exit=0
$ java UnnamedVariables      (JDK 25)
    matched a Point, x = 3, y ignored with _
    caught NumberFormatException without naming it

$ java CompactHello.java      (JDK 21, no flag)
    <repo>/lanes/src/CompactHello.java:1: error: unnamed classes are a preview feature and are disabled by default.
    void main() {
    ^
      (use --enable-preview to enable unnamed classes)
    1 error
    error: compilation failed

$ java CompactHello.java      (JDK 25, no flag)
    compact source file: no class, no static, no String[] args