How can I instantiate a generic type in Java?

Question

I\'ve added a human-readable configuration file to my app using java.util.Properties and am trying to add a wrapper around it to make type conversions easier. Specifically,

Answer 1

The following uses functional interfaces.

You could change the method signature to use a provided "parser":

protected  T getProperty(String key, T fallback, Function parser) {
    String value = properties.getProperty(key);

    if (value == null) {
        return fallback;
    } else {
        return parser.apply(value);
    }
}

Also, for efficiency you might want to replace T fallback with Supplier fallbackSupplier to prevent having to create fallback values when they are not needed:

protected  T getProperty(String key, Supplier fallbackSupplier, Function parser) {
    String value = properties.getProperty(key);

    if (value == null) {
        return fallbackSupplier.get();
    } else {
        return parser.apply(value);
    }
}

Then you can use method references and lambda expressions as parser and fallback supplier, e.g.:

protected void func2() {
    foobar = getProperty("foobar", () -> 210, Integer::valueOf);
    // Better create own parsing method which properly handles 
    // invalid boolean strings instead of using Boolean#valueOf
    foobaz = getProperty("foobaz", () -> true, Boolean::valueOf);

    // Imagine creation of `ExpensiveObject` is time-wise or 
    // computational expensive
    bar = getProperty("bar", ExpensiveObject::new, ExpensiveObject::parse);
}

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The advantage of this approach is that it is not restricted to a (potentially non-existent) constructor anymore.