Void value as return parameter

Question

I have this interface:

public interface Command {
    T execute(String... args);
}

it works fine for most uses. But when I try to mode

Answer 1

Here's a best-of-multiple-worlds implementation.

// Generic interface for when a client doesn't care
// about the return value of a command.
public interface Command {
    // The interfaces themselves take a String[] rather
    // than a String... argument, because otherwise the
    // implementation of AbstractCommand would be
    // more complicated.
    public void execute(String[] arguments);
}

// Interface for clients that do need to use the
// return value of a command.
public interface ValuedCommand extends Command {
    public T evaluate(String[] arguments);
}

// Optional, but useful if most of your commands are ValuedCommands.
public abstract class AbstractCommand implements ValuedCommand {
    public void execute(String[] arguments) {
        evaluate(arguments);
    }
}

// Singleton class with utility methods.
public class Commands {
    private Commands() {} // Singleton class.

    // These are useful if you like the vararg calling style.
    public static void execute(Command cmd, String... arguments) {
        cmd.execute(arguments);
    }

    public static  void execute(ValuedCommand cmd, String... arguments) {
        return cmd.evaluate(arguments);
    }

    // Useful if you have code that requires a ValuedCommand
    // but you only have a plain Command.
    public static ValuedCommand asValuedCommand(Command cmd) {
        return new VoidCommand(cmd);
    }

    private static class VoidCommand extends AbstractCommand {
        private final Command cmd;

        public VoidCommand(Command actual) {
            cmd = actual;
        }

        public Void evaluate(String[] arguments) {
            cmd.execute(arguments);
            return null;
        }
    }
}

With this implementation, clients can talk about a Command if they don't care about the return value, and a ValuedCommand if the need a command that returns a certain value.

About the only reason not to go with using Void straight up is all the unsightly return null; statements that you will be forced to insert.