What would be the best way to implement a constant object?

Question

First of all I should probably say that the term 'constant object' is probably not quite right and might already mean something completely different from what I am thinking of, but it is the best term I can think of to describe what I am talking about.

So basically I am designing an application and I have come across something that seems like there is probably an existing design pattern for but I don't know what it is or what to search for, so I am going to describe what it is I am trying to do and I am looking for suggestions as to the best way to implement it.

Lets say you have a class:

public class MyClass {
    private String name;
    private String description;
    private int value;

    public MyClass(String name, String description, int value) {
        this.name = name;
        this.description = description;
        this.value = value;
    }

    // And I guess some getters and setters here.
}

Now lets say that you know in advance that there will only ever be say 3 instances of this class, and the data is also known in advance (or at least will be read from a file at runtime, and the exact filename is known in advance). Basically what I am getting at is that the data is not going to be changed during runtime (once it has been set).

At first I thought that I should declare some static constants somewhere, e.g.

public static final String INSTANCE_1_DATA_FILE = "path/to/instance1/file";
public static final String INSTANCE_2_DATA_FILE = "path/to/instance2/file";
public static final String INSTANCE_3_DATA_FILE = "path/to/instance3/file";
public static final MyClass INSTANCE_1 = new MyClass(getNameFromFile(INSTANCE_1_DATA_FILE), getDescriptionFromFile(INSTANCE_1_DATA_FILE), getValueFromFile(INSTANCE_1_DATA_FILE));
public static final MyClass INSTANCE_2 = new MyClass(getNameFromFile(INSTANCE_2_DATA_FILE), getDescriptionFromFile(INSTANCE_2_DATA_FILE), getValueFromFile(INSTANCE_2_DATA_FILE));
public static final MyClass INSTANCE_3 = new MyClass(getNameFromFile(INSTANCE_3_DATA_FILE), getDescriptionFromFile(INSTANCE_3_DATA_FILE), getValueFromFile(INSTANCE_3_DATA_FILE));

Obvisouly now, whenever I want to use one of the 3 instances I can just refer directly to the constants.

But I started thinking that there might be a cleaner way to handle this and the next thing I thought about was doing something like:

public MyClassInstance1 extends MyClass {
    private static final String FILE_NAME = "path/to/instance1/file";

    public String getName() {
        if (name == null) {
            name = getNameFromFile(FILE_NAME);
        }
        return name;
    }

    // etc.

}

Now whenever I want to use the instances of MyClass I can just use the one I want e.g.

private MyClass myInstance = new MyClassInstance2();

Or probably even better would be to make them singletons and just do:

private MyClass myInstance = MyClassInstance3.getInstance();

But I can't help but think that this is also not the right way to handle this situation. Am I overthinking the problem? Should I just have a switch statement somewhere e.g.

public class MyClass {
    public enum Instance { ONE, TWO, THREE }

    public static String getName(Instance instance) {
        switch(instance) {
        case ONE:
            return getNameFromFile(INSTANCE_1_DATA_FILE);
            break;
        case TWO:
            etc.
        }
    }
}

Can anyone tell me the best way to implement this? Note that I have written the sample code in Java because that is my strongest language, but I will probably be implementing the application in C++, so at the moment I am more looking for language independent design patterns (or just for someone to tell me to go with one of the simple solutions I have already mentioned).

Answer 1

If you want the values to be constant, then you will not need setters, otherwise code can simply change the values in your constants, making them not very constant. In C++, you can just declare the instances const, although I'd still get rid of the setters, since someone could always cast away the const.

The pattern looks ok, although the fact that you are creating a new instance each time one is requested, is not usual for constants.

In java, you can create enums that are "smart" e.g.

public enum MyClass {
    ONE(INSTANCE_1_DATA_FILE),
    TWO(INSTANCE_2_DATA_FILE),
    //etc...

    private MyClass(String dataFile)
    {
       this(getNameFromDataFile(dataFile), other values...)
    }

    private MyClass(String name, String data, etc...)
    {
       this.name = name;
       // etc..
    }

    public String getName()
    {
        return name;
    }
}

In C++, you would create your MyClass, with a private constructor that takes the filename and whatever else it needs to initialize, and create static const members in MyClass for each instance, with the values assigned a new instance of MyClass created using the private constructor.

EDIT: But now I see the scenario I don't think this is a good idea having static values. If the types of ActivityLevel are fundamental to your application, then you can enumerate the different type of activity level as constants, e.g. a java or string enum, but they are just placeholders. The actual ActivityDescription instances should come from a data access layer or provider of some kind.

e.g.

enum ActivityLevel { LOW, MED, HIGH  }

class ActivityDescription
{
    String name;
    String otherDetails;
    String description; // etc..
    // perhaps also
    // ActivityLevel activityLevel;

    // constructor and getters
    // this is an immutable value object
}

interface ActivityDescriptionProvider
{
    ActivityDescription getDescription(ActivityLevel activityLevel);
}

You can implement the provider using statics if you want, or an enum of ActivityDescription instnaces, or better still a Map of ActivityLevel to ActivityDescription that you load from a file, fetch from spring config etc. The main point is that using an interface to fetch the actual description for a given ActivityLevel decouples your application code from the mechanics of how those descriptions are produced in the system. It also makes it possible to mock the implementation of the interface when testing the UI. You can stress the UI with a mock implementation in ways that is not possible with a fixed static data set.

Answer 2

Now lets say that you know in advance that there will only ever be say 3 instances of this class, and the data is also known in advance (or at least will be read from a file at runtime, and the exact filename is known in advance). Basically what I am getting at is that the data is not going to be changed during runtime (once it has been set).

I'd use an enum. And then rather in this flavor:

public enum MyEnum {

    ONE("path/to/instance1/file"),
    TWO("path/to/instance2/file"),
    THREE("path/to/instance3/file");

    private String name;

    private MyEnum(String name) {
        this.name = name;
    }

    public String getName() {
        return name;
    }

}

Which can be used as follows:

MyEnum one = MyEnum.ONE;
String name = one.getName();

Answer 3

Your first method seems to me like the best and the least prone to code rot. I'm not impressed by the idea of subclassing an object just to change the file name that contains the data that will be used to build it.

Of course, you could maybe improve on your original idea by wrapping these all in an outer class that provides some sort of enumeration access. A collection of MyClass's in other words. But I think you should discard this subclassing idea.

Answer 4

First, you really should be limiting where you use these instances in the code. Use them in as few places as possible. Given these are file names, I expect you want three class instances which accesses the files. How many classes are required depends on what your want to do with them? Look at the Singleton pattern for these classes.

Now you don't need the constants, but could have a helper class which will read the file containing the file names and supply them to the reader class. The code to find then name could also be a method called by the static initializer of the Singleton.

Answer 5

The common approach is to use a map:

private static final Map<String, YouClass> mapIt = 
     new HashMap<String, YouClass>(){{
         put("one", new YourClass("/name", "desc", 1 )),
         put("two", new YourClass("/name/two", "desc2", 2 )),
         put("three", new YourClass("/name/three", "desc", 3 ))
    }}


public static YourClass getInstance( String named ) {
   return mapIt.get( named );
}

Next time you need it:

 YouClass toUse = YourClass.getInstance("one");

Probably using strings as keys is not the best option but you get the idea.

Answer 6

(I'm too slow once again, you already accepted an answer, but here it is anyway...)

You want to (a) prevent changes to the data held in objects of MyClass, and (b) allow only a fixed set of MyClass objects to exist, implying that runtime code should not be able to create new instances of MyClass.

Your initial example has a public constructor, which violates (b)

I'd use a Factory approach so the Factory is the only thing that can create instances, and the class doesn't provide any setters so it's immutable.

Depending on how much flexibility you want for the future, you could put the factory and the class in the same package and limit scope that way, or you could make MyClass an inner class within the factory. You may also consider making MyClass an interface separate from its implementation.

A properties file could be used to configure the factory itself. The properties file (e.g. "foo.properties") could look something like

one=/path/to/datafile1
two=/another/path/to/datafile2
three=/path/to/datafile3

I use "Foo" instead of "MyClass" in the (Java) examples below.

public class FooFactory
{
    /** A place to hold the only existing instances of the class */
    private final Map<String, Foo> instances = new HashMap<String, Foo>();

    /** Creates a factory to manufacture Foo objects */
    // I'm using 'configFile' as the name of a properties file,
    // but this could use a Properties object, or a File object.
    public FooFactory(String configfile)
    {
        Properties p = new Properties();
        InputStream in = this.getClass().getResourceAsStream();
        p.load(in); // ignoring the fact that IOExceptions can be thrown

        // Create all the objects as specified in the factory properties
        for (String key : p.keys())
        {
            String datafile = p.getProperty(key);
            Foo obj = new Foo(datafile);
            instances.put(key, obj);
        }
    }

    public Foo getFoo(String which)
    {
        return instances.get(which);
    }

    /** The objects handed out by the factory - your "MyClass" */
    public class Foo
    {
        private String name;
        private String description;
        private int value;

        private Foo(String datafile)
        {
            // read the datafile to set name, description, and value
        }
    }
}

You're set to allow only your predefined instances, which can't be changed at runtime, but you can set it all up differently for another run at a later time.