The question of where to define constants in Java has appeared numerous times in forums, yet I am struggling to settle on a solution I feel comfortable with.
To make it simple, suppose I have two classes: WriteMyData and ReadMyData. None is a subclass of the other. Both classes share two constants that are vital for their operation: String DELIMITER and int LENGTH. In the future I might want to change the value of these constants so they should be defined somewhere appropriate.
The consensus seems to often favour the enum type. However, there is nothing to enumerate in my case, so I end up with only one item in my enum which I call DEFAULT:
public enum DataSettings {
DEFAULT(",", 32);
private final String delimiter;
private final int length;
DataSettings(String delmiter, int length) {
this.delimiter = delimiter;
this.length = length;
}
public String getDelimiter() { return delimiter; }
public int getLength() { return length; }
}
Thus, in both my classes I access the constants through DataSettings.DEFAULT.getDelimiter() and DataSettings.DEFAULT.getLength().
Is this really good OO style? Is the use of enum perhaps overkill? If I do not use enum, what should I do instead? Creating an interface for constants seems to be frowned upon, and there seems to be no natural superclass for my classes to derive from. Is it a beginners mistake to have only one default item in an enum?
Just create something like Constants.java class where you will put all the constants.
For example:
public class Constants {
public static final String DELIMITER = "-";
public static final int LENGTH = 1;
}
And use them where you want by:
Constants.DELIMITER
Constants.LENGTH
If only those two constans and not going to have more than that, You can have a Interface like
interface DataSetting
{
String DELIMITER = ",";
int LENGTH = 32;
}
And if you need to initilize through property
public class DataSetting {
public static String DELIMITER = ",";
public static int LENGTH = 32;
static {
DELIMITER = System.getProperty("delimiter");
LENGTH = Integer.parseInt(System.getProperty("length"));
// or from config
}
}
Using an enum when there is nothing to enumerate is indeed bad practice.
The other mentioned alternative, using an interface is also a poor choice. Effective Java, Item 19 describes it best:
Item19: Use interfaces only to define types
When a class implements an interface, the interface serves as a type that can be used to refer to instances of the class. That a class implements an interface should therefore say something about what a client can do with instances of the class. It is inappropriate to define an interface for any other purpose.
One kind of interface that fails this test is the so-called constant interface. Such an interface contains no methods; it consists solely of static final fields, each exporting a constant. Classes using these constants implement the interface to avoid the need to qualify constant names with a class name.
The correct implementation is to define a non-instantiable utility class:
public class Constants {
private Constants(){} // Private constructor prevents instantiation AND subclassing
public static final String DELIMITER = "-";
public static final int LENGTH = 1;
}
For convenience you can statically import the constants in your code:
import static com.example.Constants.*;
public class Test {
public static void main(String[] args){
System.out.println(DELIMITER); // prints "-"
}
}
An enumeration with just 1 value might not make much sense, although it might be useful if you plan on extending whatever values it could contain.
An alternative to what you are saying would be to do as follows:
Have a public class which exposes project-wide constants. You could make this class load it's values from some configuration file when your application is starting up so that you can control the values of these constants.
Have a separate set of methods suffixed with WithDefaultValues for your reading and writing methods. These methods will, in turn, call your other methods and pass in the default parameters.
As a side note, it might make sense to simply overload the methods you already have so that you have an implementation which defaults to these constants. If that is the case, be sure to document this in your method's signature.
IMO, Enum is overkill in this case. Enums are made for enumerations.
For global constants, you can just create a public static final class attribute in a java class or java interface, although the latest one is not the usual approach (see Interfaces with static fields in java for sharing 'constants')
I would propose another solution that doesn't use any constants in WriteMyData and ReadMyData.
Do pass delimiter and length as constructor parameters. This way you will be able to unit test these classes with parameters that may make testing more easy.
When it is important that an instance of each uses the same values for delimiter and length, then both should be instantiated at the same location and made available for clients to use. The location where the instances are created is a proper place to have constants for the values of delimiter and length.
You can create an interface.
By default constants in interface are static and final.
And you can user those variable by Referencing interface.
public interface AnyNameInterface {
String AnyVar ="DEMO";
Double AnyVar_2 = 123.00;
}
Use as:
AnyNameInterface.AnyVar
AnyNameInterface.AnyVar_2
I would like to add that even when there are something to enumerate, enums can't always be used as a container for constants, even though they are nice.
For instance, javax.ws.rs.Path defines a single annotation type element of type String. However, this won't compile:
#Path(MyEnum.BASE_PATH)
nor will
#Path(MyEnum.BASE_PATH.name())
Also, in those cases where it is not possible to define the constants on the class itself, or on some common superclass, the discussion often seem to be between defining the constants in a class vs in an interface. A third option could be to define them in an annotation.
public #interface MyAnnotation {
String QUESTIONS = "/questions";
String ANSWERS = "/answers"; }
The annotation approach steers clear of the "constant interface" pitfall. This pitfall is, in my understanding, not that the interface as a constant container in itself is a bad idea, - the problem is when callers decide to implement the interface, rather than accessing the constants through eg. a static import.
This question already has answers here:
Java: Enum vs. Int
(9 answers)
Closed 8 years ago.
Why it is said that use of enums is better in java. In case if you have constants for which datatypes are different I think it is better to use class with constants instead of enums?
Like
Class A {
public static int A = 1;
public static int B = 2;
public static String c = "RADIUS";
public static String d = "LENGTH";
}
instead of
enum ofInts {A(1), B(2)}
enum ofStrings{c("RADUIS"), d("LENGTH")}
One benefit of using enums is that it's strongly typed. Consider two sets of constants, both of which have integer representations. If you just use fields as you've proposed, there's nothing to stop you from passing the wrong kind of value into methods etc. If you use enums, you can't - the two sets of constants are entirely distinct, even though they share the common aspect of "values with underlying integer representations".
Of course, enums provide additional benefits in allowing you to include behaviour for the values, but that's not always necessary - and even when it's not, that doesn't mean that enums aren't useful.
In answer to the question implied by your title Should we use Enums in java with different data types? I must answer a resounding no because actually you can't. Enums have their own data type and each enum is essentially a new data type. This is where one of their their biggest powers lie - they can be strongly type checked at compile time.
The body of your question is a little unfair - firstly you would never consider replacing non-finals with enums so let us pretend you posted:
class A {
public static final int A = 1;
public static final int B = 2;
public static final String c = "RADIUS";
public static final String d = "LENGTH";
}
May I also take a few liberties with your naming and make the sample a little more realistic:
class A {
public static final int ONE = 1;
public static final int TWO = 2;
public static final String RADIUS = "RADIUS";
public static final String LENGTH = "LENGTH";
}
Now - the conversion to enum comes naturally:
class A {
enum Number {
ZERO,
ONE,
TWO;
}
enum Measure {
RADIUS,
LENGTH,
WEIGHT;
}
}
I have added further values to demonstrate the resultant versatility.
Normally, if you need enumerations of different types then just implement different enum types. The unsafe pattern with class with int-constants is not considered as good style (due to inherent unsafety).
Another alternative might be the so-called type-safe enum pattern published by Joshua Bloch. As variation you even could construct type-hierarchies. But this is rather not for beginners. For example with regards to serialization you need special actions (use of readResolve-method).
So standard enums are really quite easy to use and safe.
Enumerated types, on one hand, made the code more self-documenting. Depending on the language, the compiler could automatically assign default values to the enumerators thereby hiding unnecessary detail from the programmer. These values may not even be visible to the programmer (encapsulation).
Enums have a stock value 0,1,2,3...N -1, N so they can be referenced according to respective location.
Enumerated types in Java and other languages can also prevent a programmer from writing illogical code such as performing mathematical operations on the values of the enumerators. Its just more organized.
i was thinking along the line when design some interfaces and classes whether i should be passing enum as a parameter or its literal value.
At this juncture, i am all for passing its literal value as i have read somewhere i could not remember about coupling issues passing the enum.
Does anyone have any opinion on this? :)
public enum Item{
CANDY("candy"), APPLE("apple");
}
Pass by enum
public buy(Item item){}
Pass by literal value
public buy(String itemValue){}
Cheers.
I would recommend passing the enum value as it is more strongly typed. In your example, if "candy" and "apple" are the only valid arguments to the 'buy' method, you can enforce that at compile time by requiring the enum rather than the String as the method argument.
Definitely pass by enum. My reasons being:
It'll be much easier to work with the enum object rather than the string in the implementation of buy. For example, if you do need to do switch etc.
Much better maintainability. You can easily see if you break anything if you do change the string representation, or the available enumeration values, whereas its not as easy to see when its converted to a String.
When looking at the implementation of buy later on, you'll easily be able to see how each of the different enumeration values are handled. When you pass it as a String, you might not know looking at the method what all the possible input values are and how they are handled.
In most cases, I would pass the actual Enum type (Item in this example) rather than a String representation. This ensures proper typing and is the purpose of Enums.
One situation where you might want to pass the String representation (obtained from Enum.name()) is due to serialization/versioning issues.
For example, if you have several instances of an application running and you can't always be sure they are running the same versions, you may want to just pass the String representation between them, and attempt to manually deserialize them (using valueOf()) and handle any differences that way.
Enums are classes but with a set of predefined values. As such you must both declare the enum and give a variable for its instance.
Try this example :
public class ItemDemo {
public enum Item {
CANDY("candy"), APPLE("apple");
}
CANDY c;
APPLE a;
public ItemDemo(String name, int age, double money, CANDY c, APPLE a) {
this.c= c;
this.a= a;
}
}
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I've seen examples like this:
public class MaxSeconds {
public static final int MAX_SECONDS = 25;
}
and supposed that I could have a Constants class to wrap constants in, declaring them static final. I know practically no Java at all and am wondering if this is the best way to create constants.
That is perfectly acceptable, probably even the standard.
(public/private) static final TYPE NAME = VALUE;
where TYPE is the type, NAME is the name in all caps with underscores for spaces, and VALUE is the constant value;
I highly recommend NOT putting your constants in their own classes or interfaces.
As a side note: Variables that are declared final and are mutable can still be changed; however, the variable can never point at a different object.
For example:
public static final Point ORIGIN = new Point(0,0);
public static void main(String[] args){
ORIGIN.x = 3;
}
That is legal and ORIGIN would then be a point at (3, 0).
I would highly advise against having a single constants class. It may seem a good idea at the time, but when developers refuse to document constants and the class grows to encompass upwards of 500 constants which are all not related to each other at all (being related to entirely different aspects of the application), this generally turns into the constants file being completely unreadable. Instead:
If you have access to Java 5+, use enums to define your specific constants for an application area. All parts of the application area should refer to enums, not constant values, for these constants. You may declare an enum similar to how you declare a class. Enums are perhaps the most (and, arguably, only) useful feature of Java 5+.
If you have constants that are only valid to a particular class or one of its subclasses, declare them as either protected or public and place them on the top class in the hierarchy. This way, the subclasses can access these constant values (and if other classes access them via public, the constants aren't only valid to a particular class...which means that the external classes using this constant may be too tightly coupled to the class containing the constant)
If you have an interface with behavior defined, but returned values or argument values should be particular, it is perfectly acceptible to define constants on that interface so that other implementors will have access to them. However, avoid creating an interface just to hold constants: it can become just as bad as a class created just to hold constants.
It is a BAD PRACTICE to use interfaces just to hold constants (named constant interface pattern by Josh Bloch). Here's what Josh advises:
If the constants are strongly tied to
an existing class or interface, you
should add them to the class or
interface. For example, all of the
boxed numerical primitive classes,
such as Integer and Double, export
MIN_VALUE and MAX_VALUE constants. If
the constants are best viewed as
members of an enumerated type, you
should export them with an enum
type. Otherwise, you should export the
constants with a noninstantiable
utility class.
Example:
// Constant utility class
package com.effectivejava.science;
public class PhysicalConstants {
private PhysicalConstants() { } // Prevents instantiation
public static final double AVOGADROS_NUMBER = 6.02214199e23;
public static final double BOLTZMANN_CONSTANT = 1.3806503e-23;
public static final double ELECTRON_MASS = 9.10938188e-31;
}
About the naming convention:
By convention, such fields have names
consisting of capital letters, with
words separated by underscores. It is
critical that these fields contain
either primitive values or references
to immutable objects.
In Effective Java (2nd edition), it's recommended that you use enums instead of static ints for constants.
There's a good writeup on enums in Java here:
http://java.sun.com/j2se/1.5.0/docs/guide/language/enums.html
Note that at the end of that article the question posed is:
So when should you use enums?
With an answer of:
Any time you need a fixed set of constants
Just avoid using an interface:
public interface MyConstants {
String CONSTANT_ONE = "foo";
}
public class NeddsConstant implements MyConstants {
}
It is tempting, but violates encapsulation and blurs the distinction of class definitions.
I use following approach:
public final class Constants {
public final class File {
public static final int MIN_ROWS = 1;
public static final int MAX_ROWS = 1000;
private File() {}
}
public final class DB {
public static final String name = "oups";
public final class Connection {
public static final String URL = "jdbc:tra-ta-ta";
public static final String USER = "testUser";
public static final String PASSWORD = "testPassword";
private Connection() {}
}
private DB() {}
}
private Constants() {}
}
Than, for example, I use Constants.DB.Connection.URL to get constant.
It looks more "object oriently" as for me.
Creating static final constants in a separate class can get you into trouble. The Java compiler will actually optimize this and place the actual value of the constant into any class that references it.
If you later change the 'Constants' class and you don't do a hard re-compile on other classes that reference that class, you will wind up with a combination of old and new values being used.
Instead of thinking of these as constants, think of them as configuration parameters and create a class to manage them. Have the values be non-final, and even consider using getters. In the future, as you determine that some of these parameters actually should be configurable by the user or administrator, it will be much easier to do.
The number one mistake you can make is creating a globally accessible class called with a generic name, like Constants. This simply gets littered with garbage and you lose all ability to figure out what portion of your system uses these constants.
Instead, constants should go into the class which "owns" them. Do you have a constant called TIMEOUT? It should probably go into your Communications() or Connection() class. MAX_BAD_LOGINS_PER_HOUR? Goes into User(). And so on and so forth.
The other possible use is Java .properties files when "constants" can be defined at run-time, but not easily user changeable. You can package these up in your .jars and reference them with the Class resourceLoader.
That's the right way to go.
Generally constants are not kept in separate "Constants" classes because they're not discoverable. If the constant is relevant to the current class, keeping them there helps the next developer.
What about an enumeration?
I prefer to use getters rather than constants. Those getters might return constant values, e.g. public int getMaxConnections() {return 10;}, but anything that needs the constant will go through a getter.
One benefit is that if your program outgrows the constant--you find that it needs to be configurable--you can just change how the getter returns the constant.
The other benefit is that in order to modify the constant you don't have to recompile everything that uses it. When you reference a static final field, the value of that constant is compiled into any bytecode that references it.
I agree that using an interface is not the way to go. Avoiding this pattern even has its own item (#18) in Bloch's Effective Java.
An argument Bloch makes against the constant interface pattern is that use of constants is an implementation detail, but implementing an interface to use them exposes that implementation detail in your exported API.
The public|private static final TYPE NAME = VALUE; pattern is a good way of declaring a constant. Personally, I think it's better to avoid making a separate class to house all of your constants, but I've never seen a reason not to do this, other than personal preference and style.
If your constants can be well-modeled as an enumeration, consider the enum structure available in 1.5 or later.
If you're using a version earlier than 1.5, you can still pull off typesafe enumerations by using normal Java classes. (See this site for more on that).
Based on the comments above I think this is a good approach to change the old-fashioned global constant class (having public static final variables) to its enum-like equivalent in a way like this:
public class Constants {
private Constants() {
throw new AssertionError();
}
public interface ConstantType {}
public enum StringConstant implements ConstantType {
DB_HOST("localhost");
// other String constants come here
private String value;
private StringConstant(String value) {
this.value = value;
}
public String value() {
return value;
}
}
public enum IntConstant implements ConstantType {
DB_PORT(3128),
MAX_PAGE_SIZE(100);
// other int constants come here
private int value;
private IntConstant(int value) {
this.value = value;
}
public int value() {
return value;
}
}
public enum SimpleConstant implements ConstantType {
STATE_INIT,
STATE_START,
STATE_END;
}
}
So then I can refer them to like:
Constants.StringConstant.DB_HOST
A good object oriented design should not need many publicly available constants. Most constants should be encapsulated in the class that needs them to do its job.
There is a certain amount of opinion to answer this. To start with, constants in java are generally declared to be public, static and final. Below are the reasons:
public, so that they are accessible from everywhere
static, so that they can be accessed without any instance. Since they are constants it
makes little sense to duplicate them for every object.
final, since they should not be allowed to change
I would never use an interface for a CONSTANTS accessor/object simply because interfaces are generally expected to be implemented. Wouldn't this look funny:
String myConstant = IMyInterface.CONSTANTX;
Instead I would choose between a few different ways, based on some small trade-offs, and so it depends on what you need:
1. Use a regular enum with a default/private constructor. Most people would define
constants this way, IMHO.
- drawback: cannot effectively Javadoc each constant member
- advantage: var members are implicitly public, static, and final
- advantage: type-safe
- provides "a limited constructor" in a special way that only takes args which match
predefined 'public static final' keys, thus limiting what you can pass to the
constructor
2. Use a altered enum WITHOUT a constructor, having all variables defined with
prefixed 'public static final' .
- looks funny just having a floating semi-colon in the code
- advantage: you can JavaDoc each variable with an explanation
- drawback: you still have to put explicit 'public static final' before each variable
- drawback: not type-safe
- no 'limited constructor'
3. Use a Class with a private constructor:
- advantage: you can JavaDoc each variable with an explanation
- drawback: you have to put explicit 'public static final' before each variable
- you have the option of having a constructor to create an instance
of the class if you want to provide additional functions related
to your constants
(or just keep the constructor private)
- drawback: not type-safe
4. Using interface:
- advantage: you can JavaDoc each variable with an explanation
- advantage: var members are implicitly 'public static final'
- you are able to define default interface methods if you want to provide additional
functions related to your constants (only if you implement the interface)
- drawback: not type-safe
What is the best way to implement constants in Java?
One approach that we should really avoid : using interfaces to define constants.
Creating a interface specifically to declare constants is really the worst thing : it defeats the reason why interfaces were designed : defining method(s) contract.
Even if an interface already exists to address a specific need, declaring the constants in them make really not sense as constants should not make part of the API and the contract provided to client classes.
To simplify, we have broadly 4 valid approaches.
With static final String/Integer field :
1) using a class that declares constants inside but not only.
1 variant) creating a class dedicated to only declare constants.
With Java 5 enum :
2) declaring the enum in a related purpose class (so as a nested class).
2 variant) creating the enum as a standalone class (so defined in its own class file).
TLDR : Which is the best way and where locate the constants ?
In most of cases, the enum way is probably finer than the static final String/Integer way and personally I think that the static final String/Integer way should be used only if we have good reasons to not use enums.
And about where we should declare the constant values, the idea is to search whether there is a single existing class that owns a specific and strong functional cohesion with constant values. If we find such a class, we should use it as the constants holder. Otherwise, the constant should be associated to no one particular class.
static final String/ static final Integer versus enum
Enums usage is really a way to strongly considered.
Enums have a great advantage over String or Integer constant field.
They set a stronger compilation constraint.
If you define a method that takes the enum as parameter, you can only pass a enum value defined in the enum class(or null).
With String and Integer you can substitute them with any values of compatible type and the compilation will be fine even if the value is not a defined constant in the static final String/ static final Integer fields.
For example, below two constants defined in a class as static final String fields :
public class MyClass{
public static final String ONE_CONSTANT = "value";
public static final String ANOTHER_CONSTANT = "other value";
. . .
}
Here a method that expects to have one of these constants as parameter :
public void process(String constantExpected){
...
}
You can invoke it in this way :
process(MyClass.ONE_CONSTANT);
or
process(MyClass.ANOTHER_CONSTANT);
But no compilation constraint prevents you from invoking it in this way :
process("a not defined constant value");
You would have the error only at runtime and only if you do at a time a check on the transmitted value.
With enum, checks are not required as the client could only pass a enum value in a enum parameter.
For example, here two values defined in a enum class (so constant out of the box):
public enum MyEnum {
ONE_CONSTANT("value"), ANOTHER_CONSTANT(" another value");
private String value;
MyEnum(String value) {
this.value = value;
}
...
}
Here a method that expects to have one of these enum values as parameter :
public void process(MyEnum myEnum){
...
}
You can invoke it in this way :
process(MyEnum.ONE_CONSTANT);
or
process(MyEnum.ANOTHER_CONSTANT);
But the compilation will never allow you from invoking it in this way :
process("a not defined constant value");
Where should we declare the constants ?
If your application contains a single existing class that owns a specific and strong functional cohesion with the constant values, the 1) and the 2) appear more intuitive.
Generally, it eases the use of the constants if these are declared in the main class that manipulates them or that has a name very natural to guess that we will find it inside.
For example in the JDK library, the exponential and pi constant values are declared in a class that declare not only constant declarations (java.lang.Math).
public final class Math {
...
public static final double E = 2.7182818284590452354;
public static final double PI = 3.14159265358979323846;
...
}
The clients using mathematics functions rely often on the Math class.
So, they may find constants easily enough and can also remember where E and PI are defined in a very natural way.
If your application doesn't contain an existing class that has a very specific and strong functional cohesion with the constant values, the 1 variant) and the 2 variant) ways appear more intuitive.
Generally, it doesn't ease the use of the constants if these are declared in one class that manipulates them while we have also 3 or 4 other classes that manipulate them as much as and no one of these classes seems be more natural than others to host constant values.
Here, defining a custom class to hold only constant values makes sense.
For example in the JDK library, the java.util.concurrent.TimeUnit enum is not declared in a specific class as there is not really one and only one JDK specific class that appear as the most intuitive to hold it :
public enum TimeUnit {
NANOSECONDS {
.....
},
MICROSECONDS {
.....
},
MILLISECONDS {
.....
},
SECONDS {
.....
},
.....
}
Many classes declared in java.util.concurrent use them :
BlockingQueue, ArrayBlockingQueue<E>, CompletableFuture, ExecutorService , ... and really no one of them seems more appropriate to hold the enum.
A Constant, of any type, can be declared by creating an immutable property that within a class (that is a member variable with the final modifier). Typically the static and public modifiers are also provided.
public class OfficePrinter {
public static final String STATE = "Ready";
}
There are numerous applications where a constant's value indicates a selection from an n-tuple (e.g. enumeration) of choices. In our example, we can choose to define an Enumerated Type that will restrict the possible assigned values (i.e. improved type-safety):
public class OfficePrinter {
public enum PrinterState { Ready, PCLoadLetter, OutOfToner, Offline };
public static final PrinterState STATE = PrinterState.Ready;
}
A single, generic constants class is a bad idea. Constants should be grouped together with the class they're most logically related to.
Rather than using variables of any kind (especially enums), I would suggest that you use methods. Create a method with the same name as the variable and have it return the value you assigned to the variable. Now delete the variable and replace all references to it with calls to the method you just created. If you feel that the constant is generic enough that you shouldn't have to create an instance of the class just to use it, then make the constant method a class method.
FWIW, a timeout in seconds value should probably be a configuration setting (read in from a properties file or through injection as in Spring) and not a constant.
What is the difference
1.
public interface MyGlobalConstants {
public static final int TIMEOUT_IN_SECS = 25;
}
2.
public class MyGlobalConstants {
private MyGlobalConstants () {} // Prevents instantiation
public static final int TIMEOUT_IN_SECS = 25;
}
and using
MyGlobalConstants.TIMEOUT_IN_SECS wherever we need this constant. I think both are same.
I wouldn't call the class the same (aside from casing) as the constant ... I would have at a minimum one class of "Settings", or "Values", or "Constants", where all the constants would live. If I have a large number of them, I'd group them up in logical constant classes (UserSettings, AppSettings, etc.)
To take it a step further, you can place globally used constants in an interface so they can be used system wide. E.g.
public interface MyGlobalConstants {
public static final int TIMEOUT_IN_SECS = 25;
}
But don't then implement it. Just refer to them directly in code via the fully qualified classname.
For Constants, Enum is a better choice IMHO. Here is an example
public class myClass {
public enum myEnum {
Option1("String1", 2),
Option2("String2", 2)
;
String str;
int i;
myEnum(String str1, int i1) { this.str = str1 ; this.i1 = i }
}
One of the way I do it is by creating a 'Global' class with the constant values and do a static import in the classes that need access to the constant.
static final is my preference, I'd only use an enum if the item was indeed enumerable.
I use static final to declare constants and go with the ALL_CAPS naming notation. I have seen quite a few real life instances where all constants are bunched together into an interface. A few posts have rightly called that a bad practice, primarily because that's not what an interface is for. An interface should enforce a contract and should not be a place to put unrelated constants in. Putting it together into a class that cannot be instantiated (through a private constructor) too is fine if the constant semantics don't belong to a specific class(es). I always put a constant in the class that it's most related to, because that makes sense and is also easily maintainable.
Enums are a good choice to represent a range of values, but if you are storing standalone constants with an emphasis on the absolute value (eg. TIMEOUT = 100 ms) you can just go for the static final approach.
I agree with what most are saying, it is best to use enums when dealing with a collection of constants. However, if you are programming in Android there is a better solution: IntDef Annotation.
#Retention(SOURCE)
#IntDef({NAVIGATION_MODE_STANDARD, NAVIGATION_MODE_LIST,NAVIGATION_MODE_TABS})
public #interface NavigationMode {}
public static final int NAVIGATION_MODE_STANDARD = 0;
public static final int NAVIGATION_MODE_LIST = 1;
public static final int NAVIGATION_MODE_TABS = 2;
...
public abstract void setNavigationMode(#NavigationMode int mode);
#NavigationMode
public abstract int getNavigationMode();
IntDef annotation is superior to enums in one simple way, it takes significantly less space as it is simply a compile-time marker. It is not a class, nor does it have the automatic string-conversion property.
It is BAD habit and terribly
ANNOYING practice to quote Joshua Bloch without understanding the basic ground-zero fundamentalism.
I have not read anything Joshua Bloch, so either
he is a terrible programmer
or the people so far whom I find quoting him (Joshua is the name of a boy I presume) are simply using his material as religious scripts to justify their software religious indulgences.
As in Bible fundamentalism all the biblical laws can be summed up by
Love the Fundamental Identity with all your heart and all your mind
Love your neighbour as yourself
and so similarly software engineering fundamentalism can be summed up by
devote yourself to the ground-zero fundamentals with all your programming might and mind
and devote towards the excellence of your fellow-programmers as you would for yourself.
Also, among biblical fundamentalist circles a strong and reasonable corollary is drawn
First love yourself. Because if you don't love yourself much, then the concept "love your neighbour as yourself" doesn't carry much weight, since "how much you love yourself" is the datum line above which you would love others.
Similarly, if you do not respect yourself as a programmer and just accept the pronouncements and prophecies of some programming guru-nath WITHOUT questioning the fundamentals, your quotations and reliance on Joshua Bloch (and the like) is meaningless. And therefore, you would actually have no respect for your fellow-programmers.
The fundamental laws of software programming
laziness is the virtue of a good programmer
you are to make your programming life as easy, as lazy and therefore as effective as possible
you are to make the consequences and entrails of your programming as easy, as lazy and therefore as effective as possible for your neigbour-programmers who work with you and pick up your programming entrails.
Interface-pattern constants is a bad habit ???
Under what laws of fundamentally effective and responsible programming does this religious edict fall into ?
Just read the wikipedia article on interface-pattern constants (https://en.wikipedia.org/wiki/Constant_interface), and the silly excuses it states against interface-pattern constants.
Whatif-No IDE? Who on earth as a software programmer would not use an IDE? Most of us are programmers who prefer not to have to prove having macho aescetic survivalisticism thro avoiding the use of an IDE.
Also - wait a second proponents of micro-functional programming as a means of not needing an IDE. Wait till you read my explanation on data-model normalization.
Pollutes the namespace with variables not used within the current scope? It could be proponents of this opinion
are not aware of, and the need for, data-model normalization
Using interfaces for enforcing constants is an abuse of interfaces. Proponents of such have a bad habit of
not seeing that "constants" must be treated as contract. And interfaces are used for enforcing or projecting compliance to a contract.
It is difficult if not impossible to convert interfaces into implemented classes in the future. Hah .... hmmm ... ???
Why would you want to engage in such pattern of programming as your persistent livelihood? IOW, why devote yourself to such an AMBIVALENT and bad programming habit ?
Whatever the excuses, there is NO VALID EXCUSE when it comes to FUNDAMENTALLY EFFECTIVE software engineering to delegitimize or generally discourage the use of interface constants.
It doesn't matter what the original intents and mental states of the founding fathers who crafted the United States Constitution were. We could debate the original intents of the founding fathers but all I care is the written statements of the US Constitution. And it is the responsibility of every US citizen to exploit the written literary-fundamentalism, not the unwritten founding-intents, of the US Constitution.
Similarly, I do not care what the "original" intents of the founders of the Java platform and programming language had for the interface. What I care are the effective features the Java specification provides, and I intend to exploit those features to the fullest to help me fulfill the fundamental laws of responsible software programming. I don't care if I am perceived to "violate the intention for interfaces". I don't care what Gosling or someone Bloch says about the "proper way to use Java", unless what they say does not violate my need to EFFECTIVE fulfilling fundamentals.
The Fundamental is Data-Model Normalization
It doesn't matter how your data-model is hosted or transmitted. Whether you use interfaces or enums or whatevernots, relational or no-SQL, if you don't understand the need and process of data-model normalization.
We must first define and normalize the data-model of a set of processes. And when we have a coherent data-model, ONLY then can we use the process flow of its components to define the functional behaviour and process blocks a field or realm of applications. And only then can we define the API of each functional process.
Even the facets of data normalization as proposed by EF Codd is now severely challenged and severely-challenged. e.g. his statement on 1NF has been criticized as ambiguous, misaligned and over-simplified, as is the rest of his statements especially in the advent of modern data services, repo-technology and transmission. IMO, the EF Codd statements should be completely ditched and new set of more mathematically plausible statements be designed.
A glaring flaw of EF Codd's and the cause of its misalignment to effective human comprehension is his belief that humanly perceivable multi-dimensional, mutable-dimension data can be efficiently perceived thro a set of piecemeal 2-dimensional mappings.
The Fundamentals of Data Normalization
What EF Codd failed to express.
Within each coherent data-model, these are the sequential graduated order of data-model coherence to achieve.
The Unity and Identity of data instances.
design the granularity of each data component, whereby their granularity is at a level where each instance of a component can be uniquely identified and retrieved.
absence of instance aliasing. i.e., no means exist whereby an identification produces more than one instance of a component.
Absence of instance crosstalk. There does not exist the necessity to use one or more other instances of a component to contribute to identifying an instance of a component.
The unity and identity of data components/dimensions.
Presence of component de-aliasing. There must exist one definition whereby a component/dimension can be uniquely identified. Which is the primary definition of a component;
where the primary definition will not result in exposing sub-dimensions or member-components that are not part of an intended component;
Unique means of component dealiasing. There must exist one, and only one, such component de-aliasing definition for a component.
There exists one, and only one, definition interface or contract to identify a parent component in a hierarchical relationship of components.
Absence of component crosstalk. There does not exist the necessity to use a member of another component to contribute to the definitive identification of a component.
In such a parent-child relationship, the identifying definition of a parent must not depend on part of the set of member components of a child. A member component of a parent's identity must be the complete child identity without resorting to referencing any or all of the children of a child.
Preempt bi-modal or multi-modal appearances of a data-model.
When there exists two candidate definitions of a component, it is an obvious sign that there exists two different data-models being mixed up as one. That means there is incoherence at the data-model level, or the field level.
A field of applications must use one and only one data-model, coherently.
Detect and identify component mutation. Unless you have performed statistical component analysis of huge data, you probably do not see, or see the need to treat, component mutation.
A data-model may have its some of its components mutate cyclically or gradually.
The mode may be member-rotation or transposition-rotation.
Member-rotation mutation could be distinct swapping of child components between components. Or where completely new components would have to be defined.
Transpositional mutation would manifest as a dimensional-member mutating into an attribute, vice versa.
Each mutation cycle must be identified as a distinct data-modal.
Versionize each mutation. Such that you can pull out a previous version of the data model, when perhaps the need arise to treat an 8 year old mutation of the data model.
In a field or grid of inter-servicing component-applications, there must be one and only one coherent data-model or exists a means for a data-model/version to identify itself.
Are we still asking if we could use Interface Constants? Really ?
There are data-normalization issues at stake more consequential than this mundane question. IF you don't solve those issues, the confusion that you think interface constants cause is comparatively nothing. Zilch.
From the data-model normalization then you determine the components as variables, as properties, as contract interface constants.
Then you determine which goes into value injection, property configuration placeholding, interfaces, final strings, etc.
If you have to use the excuse of needing to locate a component easier to dictate against interface constants, it means you are in the bad habit of not practicing data-model normalization.
Perhaps you wish to compile the data-model into a vcs release. That you can pull out a distinctly identifiable version of a data-model.
Values defined in interfaces are completely assured to be non-mutable. And shareable. Why load a set of final strings into your class from another class when all you need is that set of constants ??
So why not this to publish a data-model contract? I mean if you can manage and normalize it coherently, why not? ...
public interface CustomerService {
public interface Label{
char AssignmentCharacter = ':';
public interface Address{
String Street = "Street";
String Unit= "Unit/Suite";
String Municipal = "City";
String County = "County";
String Provincial = "State";
String PostalCode = "Zip"
}
public interface Person {
public interface NameParts{
String Given = "First/Given name"
String Auxiliary = "Middle initial"
String Family = "Last name"
}
}
}
}
Now I can reference my apps' contracted labels in a way such as
CustomerService.Label.Address.Street
CustomerService.Label.Person.NameParts.Family
This confuses the contents of the jar file? As a Java programmer I don't care about the structure of the jar.
This presents complexity to osgi-motivated runtime swapping ? Osgi is an extremely efficient means to allow programmers to continue in their bad habits. There are better alternatives than osgi.
Or why not this? There is no leakage of of the private Constants into published contract. All private constants should be grouped into a private interface named "Constants", because I don't want to have to search for constants and I am too lazy to repeatedly type "private final String".
public class PurchaseRequest {
private interface Constants{
String INTERESTINGName = "Interesting Name";
String OFFICIALLanguage = "Official Language"
int MAXNames = 9;
}
}
Perhaps even this:
public interface PurchaseOrderConstants {
public interface Properties{
default String InterestingName(){
return something();
}
String OFFICIALLanguage = "Official Language"
int MAXNames = 9;
}
}
The only issue with interface constants worth considering is when the interface is implemented.
This is not the "original intention" of interfaces? Like I would care about the "original intention" of the founding fathers in crafting the US Constitution, rather than how the Supreme Court would interpret the written letters of the US Constitution ???
After all, I live in the land of the free, the wild and home of the brave. Be brave, be free, be wild - use the interface. If my fellow-programmers refuse to use efficient and lazy means of programming, am I obliged by the golden rule to lessen my programming efficiency to align with theirs? Perhaps I should, but that is not an ideal situation.