Add more built-in converters

[remkop]

Gary has some good ideas. Picking Java 5-native types:

• Currency
• Locale
• TimeZone
• ByteOrder
• Class
• java.net.NetworkInterface
• java.sql.Connection
• java.sql.Driver
• java.sql.Timestamp

Special care needed:

• java.sql.Types - converts from String to int. Picocli may need to introduce a converter attribute on @Option and @Parameters to support the ability to have different converters on a per-field basis instead of just on a per-type basis.

Java 6 or higher (reflection?)

• java.time.Duration
• java.time.* (Duration, Instant, LocalDate, ... etc)
• -PosixFilePermission- (enum, no special converter needed, will work out of the box)
• java.nio.file.* (Path, FileSystem, LinkPermission)

Unsure

Unsure about some of the remaining converters; these types may not be used often...

Picocli makes registering custom converters much easier than JCommander. In JCommander one must specify a converter class name in the annotation, so an actual named class needs to be defined. In picocli, a single line lambda expression is sufficient. For example,

CommandLine cmd = new CommandLine(new SomeCommand());
cmd.registerConverter(MessageDigest.class, s -> MessageDigest.getInstance(s));

Given the above ease of use I'm not sure we need built-in converters for the below:

• javax.xml.datatype.Duration
• javax.xml.namespace.QName
• javax.crypto.Cipher, javax.crypto.KeyGenerator, javax.crypto.Mac, javax.crypto.SecretKeyFactory
• java.security.AlgorithmParameterGenerator, java.security.AlgorithmParameters, java.security.KeyFactory, java.security.KeyPairGenerator, java.security.KeyStore, java.security.MessageDigest, java.security.Signature

[garydgregory]

Note that Duration is a Java 8 class.

[garydgregory]

java.time.Duration that is.

[garydgregory]

Single (long) line or not, if I do it more than once, I will want to refactor it. My vote is for ease of use the first go around.

[garydgregory]

I would make Java 8 the min req and avoid reflection hacks.