Creating console command list - how to do it right with Java

Question

For a homework I code in Java(with which I'm a newbie), I bump into the problem of creating a console command list. The user will be confronted with a set of commands, among which s/he will choose his/her choice with a number. Something like this:

Enter your choice:
0) Create a Table
1) List All Tables
2) Delete a Table
3) Insert a Record
4) List All Records
5) Delete a Record
6) Find a Record(by =)
7) Find a Record(by >)
8) Find a Record(by <)
9) Exit

The first way I have done it is as follows(unnecessary code parts are truncated):

...

outerLoop: while (true) {
    Scanner s = new Scanner(System.in);
    try {
        while (true) {
            System.out.println("Enter your choice:");
            displayChoiceList();
            int choice = s.nextInt();
            switch (choice) {
            case 1:
                processTableCreation();
                break;
            case 2:
                catalog.listAllTables();
                break;
            case 3:
                System.out.println("Enter the name of table:");
                String tableName = s.nextLine();
                catalog.deleteTable(tableName);
                break;
            case 4:
                processRecordInsertion();
                break;
            case 5:
                processListAllRecords();
                break;
            case 6:
                processDeleteRecord();
                break;
            case 7:
                processFindRecord(Constants.Operator.EQUAL);
                break;
            case 8:
                processFindRecord(Constants.Operator.SMALLER);
                break;
            case 9:
                processFindRecord(Constants.Operator.GREATER);
                break;
            case 10:
                break outerLoop;
            }
        }
    } catch (IllegalArgumentException e) {
        System.out.println("Error: " + e.getMessage());
    } catch (IOException e) {
        System.out.println(e.getMessage());
        return;
    }
}

...

private static void displayChoiceList() {
    String[] choices = new String[] { "Create Table", "List All Tables",
            "Delete a Table", "Insert a Record to a Table",
            "List all records", "Delete a record",
            "Find by Primary Key(=)", "Find by Primary Key(<)",
            "Find by Primary Key(>)", "Exit" };
    int id = 0;
    for (String choice : choices) {
        System.out.println((id + 1) + ") " + choice);
        ++id;
    }
}

Then, thinking that this is ugly, and for the sake of experimenting with Enums, I have tried the following:

private enum Command {
    CREATE_TABLE("Create a Table"),
    LIST_ALL_TABLES("List All Tables"),
    DELETE_TABLE("Delete a Table"),
    INSERT_RECORD("Insert a Record"),
    LIST_ALL_RECORDS("List All Records"),
    DELETE_RECORD("Delete a Record"),
    FIND_RECORD_EQ("Find a Record(by =)"),
    FIND_RECORD_GT("Find a Record(by >)"),
    FIND_RECORD_LT("Find a Record(by <)"),
    EXIT("Exit");

    private final String message;

    Command(String message) {
        this.message = message;
    }

    public String message() { return this.message; }
}

...

outerLoop: while (true) {
    Scanner s = new Scanner(System.in);
    try {
        while (true) {
            System.out.println("Enter your choice:");
            displayChoiceList();
            int choice = s.nextInt();

            if (choice == Command.CREATE_TABLE.ordinal())
                processTableCreation();
            else if (choice == Command.LIST_ALL_TABLES.ordinal())
                catalog.listAllTables();
            else if (choice == Command.DELETE_TABLE.ordinal()) {
                System.out.println("Enter the name of table:");
                String tableName = s.nextLine();
                catalog.deleteTable(tableName);                     
            }
            else if (choice == Command.INSERT_RECORD.ordinal())
                processRecordInsertion();
            else if (choice == Command.LIST_ALL_RECORDS.ordinal())
                processListAllRecords();
            else if (choice == Command.DELETE_RECORD.ordinal())
                processDeleteRecord();
            else if (choice == Command.FIND_RECORD_EQ.ordinal())
                processFindRecord(Constants.Operator.EQUAL);
            else if (choice == Command.FIND_RECORD_LT.ordinal())
                processFindRecord(Constants.Operator.SMALLER);
            else if (choice == Command.FIND_RECORD_GT.ordinal())
                processFindRecord(Constants.Operator.GREATER);
            else if (choice == Command.EXIT.ordinal())
                break outerLoop;
            else
                System.out.println("Invalid command number entered!");
        } 
    } catch (IllegalArgumentException e) {
        System.out.println("Error: " + e.getMessage());
    } catch (IOException e) {
        System.out.println(e.getMessage());
        return;
    }
}

...

private static void displayChoiceList() {
    for (Command c : Command.values())
        System.out.println(c.ordinal() + ") " + c.message());
}

Actually, what I have in mind is using Enum's in switch with their ordinal values, but Java does not allow non-constant values in switch cases. What is the right way to solve this problem; the most elegant/scalable/flexible? Any constructive comments are highly appreciated!

Answer 1

Using ordinals as identifiers is considered rather poor form, but perhaps you could use it like this?

private enum Command {
    CREATE_TABLE("Create a Table"),
    ...
    EXIT("Exit");

    private static final Map<Integer, Command> fromOrdinal;

    static {
        fromOrdinal = new HashMap<Integer, Command>();
        for (Command c : values()) {
            fromOrdinal.put(c.ordinal(), c);
        }
    }

    public static Command fromId(int commandId) {
        return fromOrdinal.get(c);
    }

    private final String message;

    Command(String message) {
        this.message = message;
    }

    public String message() { return this.message; }
}

and in your command handling class:

    ...
    Map<Command, Runnable> actions = new HashMap<Command, Runnable>(); // Fill this map with commands and implementations of runnable that does what the command should.

    ...
    void run(int command) {
        Runnable action = actions.get(Command.fromId(command));
        if (action == null)
            throw new IllegalArgumentException("No such command");
        action.run();
    }

But if you don't want to go the enum route there's nothing really stopping you from creating non-enum command objects. The upside of that is that they can implement, or can be configured to call the implementation of, the actual command:

interface Command {
    char getInputChar();
    String getMessage();
    void run();
}

Next step is to create a Map<Character, Command> constructed from map.put(c.getInputChar(), c) for all commands you want to use, much like the enum example. To execute you can just execute the run() method of the command.

Answer 2

You could use Command.values(), which contains the enums in ordinal order:

switch (Command.values[number]) {
case CREATE_A_TABLE:
...
}

A more elegant and maintainable way is to elimate the switch statement using polymorphism:

abstract class Command {
    private String name;

    protected Command(String name) {
        this.name = name;
    }

    @Override public String toString() {
        return name;
    }

    public abstract void execute();
}

and elsewhere:

Command[] commands = {
    new Command("Create a table") {
        @Override public void execute() {
            // code to create a table
        }
    },
    new Command("List all tables") {
        @Override public void execute() {
            // code to list all tables
        }
    }
};

for (int i = 0; i < commands.length; i++) {
    System.out.println(i + ":" + command);
}

int number = getInput();

commands[number].execute();

Advantages:

• Shorter, clearer code
• The compiler checks that each command is implemented (with a switch statement, you can forget to add a case statement, an error which will only occur at runtime. Granted, a good compiler will emit a warning if you forget a case when switching over enums, but a warning is more likely to be missed than a compilation error). ==> More robust during maintenance.

Answer 3

Well, it's a curious approach, but if you just want to look at the use of Enums, then "ordinal()" is not recommended.

Better to make an Enum with 2 members. Not ideal, but it might help ..

private enum Command {
        CREATE_TABLE(0,"Create a Table"),
        LIST_ALL_TABLES(1,"List All Tables"),
        DELETE_TABLE(2,"Delete a Table"),
        INSERT_RECORD(3,"Insert a Record"),
        LIST_ALL_RECORDS(4,"List All Records"),
        DELETE_RECORD(5,"Delete a Record"),
        FIND_RECORD_EQ(6,"Find a Record(by =)"),
        FIND_RECORD_GT(7,"Find a Record(by >)"),
        FIND_RECORD_LT(8,"Find a Record(by <)"),
        EXIT(9,"Exit");

        private final String message;
        private final int code;

        public static Command get(int code) {
            for(Command c : Command.values()) {
                if(code==c.code) {
                    return c;
                }
            }
            return null;
        }

        Command(int code, String message) {
            this.code= code;
            this.message = message;
        }
        public int getCode() { return this.code; }
        public String message() { return this.message; }
    }

Now you can get an enum using the static Command.get(int)

    private static void runCommand(int choice) {
        Command command = Command.get(choice);
        System.out.println("You Chose '"+command.message()+"'\n\n");
        switch(command) {
                ....
        }
    }

Answer 4

Would using Jakarta Commons CLI be cheating? ;-)