JVM major version number - Java 8+

jmv.java

public class jmv {
    static String getJvmMajorVersion() {
        String version = System.getProperty("java.version");
        if (version.startsWith("1.")) {
            version = version.substring(2);
        }
        return version.replaceAll("(\\d+).+", "$1");
    }

    public static void main(String[] args) {
        System.out.println(getJvmMajorVersion());
    }
}