Question

I am having a specific problem implementing a parametrised class Parameter, but this is something I have come across before with generics, so a general solution would be good..

The class Parameter stores a value of one of a strict number of classes:

public class Parameter<T> {

/*
 * Specify what types of parameter are valid
 */
private static final Set<Class<?>> VALID_TYPES;
static {
    Set<Class<?>> set = new HashSet<Class<?>>();

    set.add( Integer.class );
    set.add( Float.class );
    set.add( Boolean.class );
    set.add( String.class );

    VALID_TYPES = Collections.unmodifiableSet(set);
}

private T value;

public Parameter(T initialValue) throws IllegalArgumentException {

    // Parameter validity check
    if (!VALID_TYPES.contains(initialValue.getClass())) {
        throw new IllegalArgumentException(
                initialValue.getClass() + " is not a valid parameter type");
    }

    value = initialValue;
}

    public T get() { return value; }

    public void set(T value) {
        this.value = value;
    }
}

This is all fine, until I try and store instances of Parameter in a collection. For example:

Parameter<Integer> p = new Parameter<Integer>(3); 
int value = (Integer)p.get();
p.set(2); // Fine

ArrayList<Parameter<?>> ps = new ArrayList<Parameter<?>>();
ps.add(p);
value = (Integer)(ps.get(0).get());

ps.get(0).set(4); // Does not compile due to type erasure

What would others do in this situation to get round this?

Thanks

Answer 1

Well, you can’t directly work around this.. But perhaps you could remember the class of the initial value?

class Parameter<T> {
    // ...
    private T value;
    private final Class<?> klass;

    public Parameter(T initialValue) throws IllegalArgumentException {
        if (!VALID_TYPES.contains(initialValue.getClass()))
            throw new IllegalArgumentException(...);
        value = initialValue;
        klass = initialValue.getClass();
    }

    @SuppressWarnings("unchecked")
    public void set(Object value) {
        if (value != null && value.getClass() != klass)
            throw new IllegalArgumentException(...);
        this.value = (T)value;
    }

However, you will lose compile-time type checks on set()..