In java I want to generate a compile-time error instead of a runtime error

Question

I'm currently doing something like this;

import java.util.*;

public class TestHashMap {

    public static void main(String[] args) {

        HashMap<Integer, String> httpStatus = new HashMap<Integer, String>();
        httpStatus.put(404, "Not found");
        httpStatus.put(500, "Internal Server Error");

        System.out.println(httpStatus.get(404));    // I want this line to compile,
        System.out.println(httpStatus.get(500));    // and this line to compile.
        System.out.println(httpStatus.get(123));    // But this line to generate a compile-time error.

    }

}

I want to ensure that everywhere in my code that there is a httpStatus.get(n), that n is valid at compile-time rather than finding out later at runtime. Can this be enforced somehow? (I'm using a plain text editor as my "development environment".)

I'm very new to Java (this week) so please be gentle!

Thanks.

Answer 1

In this specific example, it seems like an enum is what you may be looking for:

public enum HttpStatus {
  CODE_404("Not Found"),
  CODE_500("Internal Server Error");

  private final String description;

  HttpStatus(String description) {
    this.description = description;
  }

  public String getDescription() {
    return description;
  }
}

An enum is a handy way of creating constants in Java, which are enforced by the compiler:

// prints "Not Found"
System.out.println(HttpStatus.CODE_404.getDescription());

// prints "Internal Server Error"
System.out.println(HttpStatus.CODE_500.getDescription());

// compiler throws an error for the "123" being an invalid symbol.
System.out.println(HttpStatus.CODE_123.getDescription());

More information on how to use enums can be found in the Enum Types lesson from The Java Tutorials.

Answer 2

Define constants like static final int NOT_FOUND = 404, INTERNAL_SERVER_ERROR = 500; and so on or use enum types instead of using "magic constants" in your code.