How would I create an object that uses String type behaviour for creation?

Question

I'd like to be able to create an object that is created like a String object, and when created verifies that the String value is an appropriate option.

IE SpecificString can be "Bob" or "Jim".

SpecificString BadName = "Sam" //Throws exception SpecificString GoodName = "Bob" //Does not throw exception.

The most important functionality is that when it is accessed it must behave like a string when accessed.

printf(SpecificString); // outputs Bob.

Is this possible?

Answer 1

You probably want to use an enum for this. Enums were introduced to Java in version 1.5. You can read about them in the official documentation here .

If, for some reason, you are unable to use Java 1.5, the Apache commons-lang library has a reasonably good substitute.

Answer 2

If the question is can it be created like a String aka. String s = "foo" the answer is no. Only Strings can be constructed that way and String is final so you won't be able to define a child class for it...

If you look for the best way to create Strings from a given Set you may use a StringFactory or look at the other answers...

Answer 3

As has been said using the = operator is not possible.

For printf(SpecificString) you should override Object.toString() :

@Override
public String toString(){
    //return a String representation of the Object;
}

Answer 4

EDIT: Oops, thought this was a C# question to start with :)

Java doesn't allow user-defined conversions, so this isn't going to work in the way that you've shown. You'll need a static Parse method or a constructor, or something similar.

If there are only certain values which are permitted, could you use an enum instead?

Answer 5

If I understand your point correctly, you want to ensure that a given object is a String limited to a specific set of possible values.

I'd go for Enums, in this case. Just use the standard Enums if you are using a recent Java version, or roll up the "old" DIY version that may perhaps be more appropriate to your specific problem.

For an example of the latter one, see this

Answer 6

You can't. But if you could do this it would not be possible to distinguish a normal atribution from one that throws an exception (assuming your exception is unchecked) or do anything on atribution. Using a method call you would be making clear to someone using your class that some validation exists, and in Eclipse I could see what this validation is with a ctrl+click.