Java – How to use java.lang.Enum

java

This is not a question of the new features in enum.
This is not a question that waits for an answer that says
enum is equivalent to class xyz extends Enum. If you wish to preach on how enums are used, as found in numerous tutorials, please don't answer.

This question is specifically on how to use the class java.lang.Enum.
Rather it is true that java.lang.Enum cannot be directly extended nor instantiated, right?
It is true that java.lang.Enum is only for the benefit of enum type and nothing else right?

If so, the variable colours is pretty meaningless, right? Even though the compiler allows it.

final static Enum<colors> colours = null;
enum colors {"R","G","B"};

So, what could I possibly equate the variable colours besides null.

of course, compiler won't allow me to do this

class BaseColour extends Enum<BaseColour>{ ,,,}

but it allows enum an unfair advantage to able to do it as a compiler's behind-the-curtains manipulation.

I am asking this question because I wish to do something like these

class Reddish extends BaseColour {"Crimson", "Blood", "Pink"};
class Purplish extends BaseColour {"Violet", "Evening Sky", "Brinjal"};
class Greenish extends BaseColour {"Algae", "Leaf"};

I wish to pass any of these, and only any of these, enumerated types into a method

void squeezeColour(BaseColour colourConstraint)
{ ... }

Is it possible?
/** If not, and if you happen to be James Gosling et al, would you kindly consider this for the next version of Java specs? **/

Further addition to question: I forgot to say this …
What I am working on is using an EnumMap or EnumSet and exploiting the contains and range methods. Is that the best I could do?

Best Answer

What an enum actually is, internally, is an integer constant. Or at least, the JVM is free to treat it as one, precisely because of the kind of restrictions that are frustrating you. That makes enums efficient, at the cost of breaking object-orientation.

I suggest you revert to the old "type checked enum" pattern that was used before enums had syntax.

class Foo {

  // the constants
  public static final Foo RHUBARB = new Foo();
  public static final Foo CUSTARD = new Foo();

  // only this class and subclasses can create constants
  protected Foo(){ }
}

Related Solutions

Java – How to test a class that has private methods, fields or inner classes

Update:

Some 10 years later perhaps the best way to test a private method, or any inaccessible member, is via @Jailbreak from the Manifold framework.
@Jailbreak Foo foo = new Foo();
// Direct, *type-safe* access to *all* foo's members
foo.privateMethod(x, y, z);
foo.privateField = value;
This way your code remains type-safe and readable. No design compromises, no overexposing methods and fields for the sake of tests.

If you have somewhat of a legacy Java application, and you're not allowed to change the visibility of your methods, the best way to test private methods is to use reflection.

Internally we're using helpers to get/set private and private static variables as well as invoke private and private static methods. The following patterns will let you do pretty much anything related to the private methods and fields. Of course, you can't change private static final variables through reflection.

Method method = TargetClass.getDeclaredMethod(methodName, argClasses);
method.setAccessible(true);
return method.invoke(targetObject, argObjects);

And for fields:

Field field = TargetClass.getDeclaredField(fieldName);
field.setAccessible(true);
field.set(object, value);

Notes:
1. TargetClass.getDeclaredMethod(methodName, argClasses) lets you look into private methods. The same thing applies for getDeclaredField.
2. The setAccessible(true) is required to play around with privates.

Java – How to efficiently iterate over each entry in a Java Map

Map<String, String> map = ...
for (Map.Entry<String, String> entry : map.entrySet()) {
    System.out.println(entry.getKey() + "/" + entry.getValue());
}

On Java 10+:

for (var entry : map.entrySet()) {
    System.out.println(entry.getKey() + "/" + entry.getValue());
}

Best Answer

Related Solutions

Java – How to test a class that has private methods, fields or inner classes

Java – How to efficiently iterate over each entry in a Java Map

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