$ cat src/Hello.java
// The smallest complete Java 25 program. No class, no public, no static, no String[] args.
void main() {
    IO.println("Hello, world");
}

$ java src/Hello.java        (JDK 25: OpenJDK Runtime Environment Temurin-25.0.4.1+1 (build 25.0.4.1+1-LTS))
Hello, world

$ java src/Hello.java        (JDK 27: OpenJDK Runtime Environment Temurin-27+35 (build 27+35))
Hello, world

$ ls src/*.class
  ls: cannot access 'src/*.class': No such file or directory

$ java src/Hello.java        (JDK 21: OpenJDK Runtime Environment (build 21.0.10+7-Ubuntu-124.04))
src/Hello.java:2: 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