Custom properties

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visionset - 03 May 2007 13:55 GMT

I'm using the Preferences class and have some objects to represent with
Strings for storage.
I have for example a rectangle class that may have a string representation
"0,0,50,100".

Now saving a property is not a problem I just have a CustomProperty
interface and call toPropertyString().
But loading a property is a case of documenting that CustomProperty classes
must provide a no arg public contstructor, and then I can call
CustomProperty.fromPropertyString(myPropStr) to populate the object.

Now I'd really like to have immutable properties, and I realise I can have a
toImmutableInstance() method but are there any better solutions to my mai
goal?

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Mike W

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Thomas Fritsch - 03 May 2007 15:06 GMT

> I'm using the Preferences class and have some objects to represent with
> Strings for storage.
[quoted text clipped - 3 lines]
> Now saving a property is not a problem I just have a CustomProperty
> interface and call toPropertyString().

So far, so good.

> But loading a property is a case of documenting that CustomProperty
> classes must provide a no arg public contstructor, and then I can call
> CustomProperty.fromPropertyString(myPropStr) to populate the object.

As you already noticed below the interface method
public void fromPropertyString(String)
breaks immutability.

> Now I'd really like to have immutable properties, and I realise I can have
> a toImmutableInstance() method but are there any better solutions to my
> goal?

I would modify your requirement (interface CustomProperty) as follows:
(*) The class has to provide a public constructor taking a String argument.
(*) The class does *not* have to provide a fromPropertyString(String)
method. (hence you don't have to give up immutability)

Obviously the loading from a string gets more clumsy then.
But at least one can tuck away that clumsiness into a utility method:
public static Object createFromPropertyString(String s) {
try {
Constructor ctor = c.getDeclaredConstructor(new Class[]{String.class};
return ctor.newInstance(new Object[]{s});
} catch(Exception e) {
throw new RuntimeException(e);
}
}

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Thomas

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Thomas Fritsch - 03 May 2007 15:29 GMT

[...]

> public static Object createFromPropertyString(String s) {

Oops, I meant
public static Object createFromPropertyString(Class c, String s) {

> try {
> Constructor ctor = c.getDeclaredConstructor(new Class[

{String.class});

> return ctor.newInstance(new Object[]{s});
> } catch(Exception e) {
> throw new RuntimeException(e);
> }
> }

Usage example:
MyRectangle rect =
(MyRectangle) createFromPropertyString(MyRectangle.class, "0,0,50,100");

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Thomas

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visionset - 03 May 2007 17:39 GMT

> I would modify your requirement (interface CustomProperty) as follows:
> (*) The class has to provide a public constructor taking a String
[quoted text clipped - 13 lines]
> }
> }

Thanks, that makes more sense.
I can also Generify it which I really wanted to do.

static <T> CompoundProperty

createFromPropertyString(String propStr, Class<T> clazz) {

try {

Constructor ctor = clazz.getDeclaredConstructor(

new Class[]{String.class});

Object instance = ctor.newInstance(new Object[]{propStr});

return (CompoundProperty)instance;

} catch(Exception ex) {

throw new RuntimeException(ex);

}

}

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Custom properties

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