如何创建一个新的数据类型，称为标识符，它的每个实例都应该是一个有效的标识符名称？

Question

can I make this new class as I asked in the title, this is my attempt.

import javax.lang.model.*;

public class Ide{
    public Ide New(String ide){
        if (SourceVersion.isName(ide)) {
            return ide;
        } 
    }
}

But it won't compile because I can't convert the String ide to type Ide, how can I do it? I need to have type Ide, thanks in advance.

Answer 1

This code has several issues:

• String is final . This means, no class can inherit from String .
• New should be static , otherwise the method won't make any sense. Requiring an instance of a class to create other instances is like the chicken-and-egg-problem, only you haven't got either one.
• calling a method New is really bad style. new is a keyword in java and you're provoking coding errors, make your code less readable... . And method names shouldn't be uppercase.
• Even if it was possible to make a class extend from String , a String -parameter wouldn't automatically be an Ide -instance. Thus the return ide; won't compile.

A solution to your problem would be:
Use a private constructor combined with a static factory-method. The class can only be a wrapper-class though. Simply implement it in adapter-style to provide the methods provided by String that are required:

class Ide{
    private String str;

    private Ide(String str){
        this.str = str;
    }

    public static Ide createIde(String str)
        throws IllegalArgumentException
    {
        if(validIdentifier(str))
            return new Ide(str);
        else
            throw new IllegalArgumentException("Invalid identifier: " + str);
    }

    //examples for adapter-like method-implementation
    public String toString(){
        return str.toString();
    }

    public char charAt(int i){
        return str.charAt(i);
    }
}

Answer 2

Your code is wrong on many many levels. I suggest that you first learn about the fundamentals of Java; before you consider writing your own code.

Problems in your code:

1. Java String class is final , that means that no other class can inherit from String (so Ide can not extend String)
2. You are calling your method "New". That is a superbad name for a method. Call it for example: "generateIdeFromName()".
3. The compiler tells you what is wrong: a String object is not an Ide object. Therefore you can't assign String to Ide (and that is what happens your method that is supposed to return an Ide object returns a String)

Long story short: you are trying to build a skyscraper, but you are already lacking the skills to dig a whole in the ground. This is not meant to insult you, but the fact is: you have to do a lot of learning first. Don't expect us to explain everything to you that you would need to come up with a reasonable program.