Best practice for defining a constant - within a class or internal interface?

I have a class that only has a constant. Snippets of code:

public class CommonConstant

    public static final String  DAY                 = "DAY";
    public static final String  WEEK                = "WEEK";
    public static final String  MONTH               = "MONTH";
    public static final String  YEAR                = "YEAR";
    public static final String  ACCEPTED            = "accepted";
    public static final String  REJECTED            = "rejected";
    public static final String  ADDED               = "added";
    public static final String  MODIFIED            = "modified";



Is it okay to specify constants in the class file or should it be in the interface? What is good practice?


source to share

2 answers

What is a good practice

This is really good practice.

public enum EnumTest {

    private String value;

    private EnumTest(String s) {
        value = s;

    public String getValue() {
        return value;



Then elsewhere

System.out.println(EnumTest.DAY.getValue()); // give you the value



for(EnumTest e :EnumTest.values()){ //values return an array of enum values




Since constants are implementation specific, you must put them in a class with a private constructor, or alternatively an enum. Joshua Bloch in Effective Java 2nd Edition discourages the use of "persistent interfaces".

Bloch says that interfaces should only be used to define types, and that when a class implements an interface, that one serves as a type to refer to an instance - it is inappropriate to use the interface for any other purpose. As mentioned earlier, constants are implemented

and more relatively persistent interfaces

if a class is changed in a future version so that it is no longer needed to use constants, it must still implement the interface to ensure binary compatibility. If a non-final class implements an interface constant, all of its subclasses will have a polluted namespace over the constants in the interface

BlocHquoted :)

HTH, Carlo



All Articles