Java equivalent of C header files - How to share definitions between classes

Question

I'm an experienced C developer, and I'm wanting to know the equivalent to a header file in Java. I have an enum that I would like to be shared between two classes, but without header files, I'm not sure how to do this.

Is the standard way to have a super class, with a definition inside of it, then have the other two classes inherit from that? Google mentioned interfaces, but I don't have any experience with that.

Answer 1

You should understand packages in Java.

Basics: https://en.wikipedia.org/wiki/Java_package

Access level modifiers: https://docs.oracle.com/javase/tutorial/java/javaOO/accesscontrol.html

If you have an enum which is not private, it can be accessed from that package.

Answer 2

It depends on what level of encapsulation you want to accomplish. If you want to have the enum available just in the two classes that you mentioned then you could define it in the following ways:

• in the same package with the two classes, having default(package protected) qualifier for the given enum
• if the two classes are in different packages than you can define it in a common abstract class and use protected qualifier for the given enum, like this it will be available for the classes extending the given abstract class

I would prefer the first approach, it my opinion it is cleanest and it also follows the OOP best practices.

As for interfaces, they used to be used before enums to provide a "container" for constants, but in my opinion currently they shouldn't be used for this case.

Answer 3

A concrete example:

mypackage/MyEnum.java (define the enum)

package mypackage;
public enum MyEnum {
  VALUE_1, VALUE_2;
}

mypackage/MyClass.java (use the enum)

package mypackage;
public class MyClass {
  public void doSomething(MyEnum myEnum) {
    switch(myEnum) {
      case VALUE_1:
        // do something
        break;
      case VALUE_2:
        // do something
        break;
    }
  }
}

otherpackage/OtherClass.java (use the enum in another package)

package otherpackage;
import mypackage.MyEnum; // explicitly import as MyEnum is not in the same package as this class.
public class OtherClass {
  public MyEnum provideEnum() {
    return MyEnum.VALUE_1;
  }
}

Answer 4

Jave does not use headers, but directly imports declarations from other classes. In C++ (or C) you commonly split the declarations that go into headers from the definitions that go in source files, and start the files with a bunch of #include to load the external declarations.

In Java, you start the files by an equivalent bunch of import statements to declare the external classes or static symbols you need to use in on source.

To answer the exact question, you can either make the enum a plain class, or make it an inner (static) class of one of the to classes and access it from the other - provided access rights allow it.

You must know that Java has no notion of friendyness. The closer that exist is the default access of package private that declares the symbol to be accessible by all classes from same package. You can use that if you do not want the enum to be publicly accessible.