Java how to check if a string value is a type of given Class<?>

Question

Alright this is kind of a complicated question and I'm completely lost.

Assume you have a string, and a generic class. Like this.

String string;
Class<?> clazz;

How would you check to see if the String represented a value that the class could equal.

For example lets say that:

String string = "true";
Class<?> clazz = Boolean.class;

How would I check and see that the string "true" is in fact a boolean?

Here is another example. Lets say that:

String string = "true";
Class<?> clazz = Integer.class;

How would I check and see that the string "true" is not an Integer?

Answer 1

Given that you want this only for Wrapper Types, you can use some reflection hack here (Exception handling for irrelevant code is ignored here for brevity):

String string = "ABC";
Class<?> clazz = Integer.class;

Method method = clazz.getDeclaredMethod("valueOf", String.class);

if (method != null) {
    try {
        Object obj = method.invoke(null, string);       
        System.out.println("Success : " + obj);

    } catch (InvocationTargetException ex) {
        System.out.println("Failure : " + string + " is not of type " + 
                                          clazz.getName());
    }
}

I'm taking into account the fact that, every wrapper class has a static valueOf method that takes a parameter of type String, and returns the value of that wrapper type. And throws an exception, if the parameter is not convertible to the respective wrapper type.

So, in above case, if an exception is thrown, the string value is not of clazz type.

P.S.: Note that for Boolean.class, any string that is not "true" will be treated as false.

Answer 2

I'm assuming you are implementing some sort of Specification/Protocol or similar.

1. Look at the Spec.
2. Define a Grammar of valid input
3. Parse that grammar.

This is similar to what Programming Languages are doing with literals.