Question

I want to refactor some of my methods into one method for converting string values into int/float based on method argument. for this matter I have create a method like following :

private <T>Number setNumericValue(String value , T type)
{

    value = value.trim();
    if(isNumeric(value))
    {
        if (type instanceof Integer)
            return Integer.parseInt(value);
        if (type instanceof Float)
            return Float.parseFloat(value);

    }
    return 0;
}

first off I want to know is my method correct?
and for calling this method how should I use it? I’ve created something like this but java give me runtime error

this.temperatureC = setNumericValue(temperatureC, Float);

the error saying : “unable to cast Float to T”

Answer 1

You need to return the default value of the appropriate type too:

private <T extends Number> T setNumericValue(String value, Class<T> type) {
    value = value.trim();
    if (isNumeric(value)) {
        if (type.isAssignableFrom(Float.class))
            return type.cast(Float.parseFloat(value));
        if (type.isAssignableFrom(Integer.class))
            return type.cast(Integer.parseInt(value));
    } else {
        if (type.isAssignableFrom(Float.class))
            return type.cast(Float.valueOf(0));
        if (type.isAssignableFrom(Integer.class))
            return type.cast(Integer.valueOf(0));
    }

    throw new IllegalArgumentException(String.format(
            "Cannot convert '%s' to %s", value, type.getSimpleName()
    ));
}

Your original construct wasn’t typesafe, you can’t cast a primitive value to T extends Number. Java’s primitive type wrappers also don’t support converting between different number types using Class.cast, so you have to do this explicitly.