类的Java通用枚举器

Question

I want to create an enumerator with classes to get these classes in further code but I can't. I'll explain on example what I want to do:

public enum SomeEnumeratorForClasses {
  FIRST_CLASS( "Name 1", FirstClass.class ),
  SECOND_CLASS( "Name 2", SecondClass.class );

  private String name;
  private Class<T> response;

  SomeEnumeratorForClasses ( String name, Class<T> clazz ) {...}

  public Class<T> getResponse() {
    return response;
  }
}

For example each class first/second has some fields/method example:

getDescription()

And finally in other class I want something like that:

SomeEnumeratorForClasses.FIRST_CLASS.getResponse().getDescription();

But I can't do it. Each class inherit from other base class Zero.class example:

FirstClass extends Zero
SecondClass extends Zero

I try do this also instead

private Class<T> response;

use this:

private Class<? extends Zero> response;

But still doesn't work.

Method getDescription is not static, is a public String for example:

class FirstClass {
  public String getDescription() {
    return "Text";
  }
}

Some methods like getDesciptions are abstract in Zero class. How can I do it?

Answer 1

If you know that each class shares a constructor signature (preferably a public no-args version), you can write a function that instantiates them.

public class ObjectUtils {
    private ObjectUtils(){}

    public static <T> T init(Class<T> clazz, Object... args) {
        try {

            return (T) Arrays.stream(clazz.getConstructors())
                    .filter(construct -> Objects.equals(args.length, construct.getParameterCount()))
                    .filter(construct -> IntStream.range(0, construct.getParameterTypes().length)
                            .allMatch(val -> construct.getParameterTypes()[val].isAssignableFrom(args[val].getClass())))
                    .findFirst().orElseThrow(() -> new InstantiationException("Could not find compatible constructor."))
                    .newInstance(args);

        } catch (InstantiationException | InvocationTargetException | IllegalAccessException e) {
            throw new RuntimeException("Could not instantiate class for provided arguments.", e);
        }
    }
}

Now, your enum could look like:

public enum SomeEnumeratorForClasses {
  FIRST_CLASS( "Name 1", FirstClass.class ),
  SECOND_CLASS( "Name 2", SecondClass.class );

  private String name;
  private Class<? extends Zero> responseClass;

  SomeEnumeratorForClasses ( String name, Class<? extends Zero> responseClass ) {
       this.name = name;
       this.responseClass = responseClass;
  }

  public <T extends Zero> T getResponse() {
    return ObjectUtils.init(responseClass);
  }
}

And use it like so:

SomeEnumeratorForClasses.FIRST_CLASS.getResponse().getDescription();