ConfigLoader.java

// file: lib/config/ConfigLoader.java

public interface ConfigLoader {
    void openConfig();

    /**
     * Allow user to access a specified table.
     * @param String name - name of table in config file.
     * @return Optional in case config is missing a table with the specified name.
     */
    Optional<ConfigTable> getTable(String name);
}

ConfigTable.java

// file: lib/config/ConfigTable.java

public interface ConfigTable {
    /**
     * Get double value from specified field.
     * @param String name - name of field in table (ex: "speed.min")
     * @return Optional in case config is missing a field with the specified name, or unable to convert string in config into a double.  Might need a Result type to indicate missing vs bad value.
     */
    Optional<double> getDouble(String name);
    Optional<boolean> getBoolean(String name);
}

Drivetrain.java

// file: subsystems/drivetrain/Drivetrain.java

import lib.config.ConfigTable;

public class Drivetrain {
    public Drivetrain(ConfigTable table) {
        double turnP;

        {
            Optional<double> kP = table.getDouble("swerve.turn.kP");

            if (!kP.isPresent()) {
                System.err.println("Error: drivetrain cannot find toml property swerve.turn.kP");
                throw new Exception("Error: drivetrain cannot find toml property swerve.turn.kP");
            }
        }
    }
}

Robot.java

public class Robot : TimedRobot {
    public void RobotInit() {
        ConfigLoader config = null;   // Will eventually replace with TomlConfigLoader one we find a library.

        mDrivetrain = new Drivetrain(config.getTable("drivetrain").orElseThrow(() -> throw new Exception("Config file missing table \"drivetrain\"")));
    }
}

TomlConfigLoader.java

// file: lib/config/TomlConfigLoader.java

public class TomlConfigLoader implements ConfigLoader {
    public TomlConfigLoader(String config_filename) {
      // Do nothing until we find a library.
    }

    public void openConfig() {
      // Do nothing until we find a library.
    }

    public Optional<ConfigTable> getTable(String name) {
      // Do minimum until we find a library, then rewrite to create ConfigTable for the specified name.
      return Optional.of(new TomlConfigTable());
    }
}

TomlConfigTable.java

// file: lib/config/TomlConfigTable.java

public class TomlConfigTable implements ConfigTable {
    public Optional<double> getDouble(String name) {
      return Optional.empty();
    }

    public Optional<boolean> getBoolean(String name) {
      // Do the same as getDouble() until we find a java toml library.
    }
}