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javaclassobjectmethodsfield
Andrew
Andrew

Reputation: 1167

Having a Java class where one of the fields is a method?

I'm pretty new to Java, as the nature of my post will give away

I need to create a class which contains a set of methods which can easily be extended by a programmer, should it be needed. I thought about having two classes: Commands and Command. Commands contains an array of Command objects, and is where the programmer can add new commands. The Command class has two fields. The name of the class, and a method signature. I'm not sure how this can be done. In C, I think you can have a struct of functions, so can we have a class where the instances of the class are methods? Or am I completely on the wrong track?

I thought about trying to do something like this:

public class Commands
{
    private ArrayList<Command> commands;

    /**
     * Constructor for objects of class Command
     */
    public Commands()
    {
        createCommands();
    }

    /**
     * This is where a programmer can add new commands
     */
    public void createCommands()
    {
        commands.add(new Command("move", public void move()));
    }

    /**
     * This is where the programmer can define the move command
     */
    public void move()
    {
        ....
    }
}

public class Command
{
    private String command_name;
    private Method command;

    public Command(String command_name, Method command)
    {
        this.command_name = command_name;
        this.command = command;
    }
}

I know there are a lot of things wrong with this, but I'm stuck on finding the right way. Hints/help would be fantastic.

Upvotes: 1

Views: 165

Answers (4)

Ishtar
Ishtar

Reputation: 11662

Java does not have function pointers. This is a common trick:

abstract class (or interface) Command {
  void execute();
}

public void createCommands() {
  commands.add(new Command(){ 
      @Override
      void execute() {
        something.move();
      }
    });
}

Upvotes: 3

Adrian Mouat
Adrian Mouat

Reputation: 46480

Note that you can't directly pass methods about in Java. Instead, they will need to be objects that implement a common interface. For example, your methods could become objects that implement the Runnable interface. Then you just call "run" when you need to use the method.

Upvotes: 1

AngerClown
AngerClown

Reputation: 6229

Java doesn't have function pointers, so this doesn't work. What you probably want to do is have a Command interface with an execute() method that you implement in concrete subclasses.

It's a matter of style, but I usually wouldn't have a name field in the Command implementations. Rather, I would just create a Map<String, Command> that holds the name for each Command.

Upvotes: 1

MarcoS
MarcoS

Reputation: 13564

I think you want to use the Command pattern:

In object-oriented programming, the command pattern is a design pattern in which an object is used to represent and encapsulate all the information needed to call a method at a later time

The Wikipedia page has an example.

Your Command should be an interface with an execute method. A programmer can code a class that implements the Command interface, and thus she/he must implement the execute method to do what is needed. You can then have a Command[] array, and for each Command object you can simply call execute to perform the task.

Upvotes: 5

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