Question

I have a class declaring constants for my app

public class GroupConstants {
    ..
    public static final int INTEGER_VALUE = 1;
    public static final int LONG_VALUE = 2;
    public static final int STRING_VALUE = 3;
    ..
}

In the code there is a set of switch statements

private static Object getValue(String stringValue, Parameter parameter) throws InvalidPatternException
{
    Object result=null;
    switch (parameter.getDataType())
    {
        case GroupConstants.STRING_VALUE: // String value
            result=stringValue;
        break;
        case GroupConstants.INTEGER_VALUE: // Long value
        case GroupConstants.LONG_VALUE:
        case GroupConstants.BOOLEAN_VALUE:
        case GroupConstants.DATE_VALUE:
..
}

I want to refactor the int constant values to be represented by an enum

public enum DataType {

    UNKNOWN_VALUE(0,"unknown"),
    INTEGER_VALUE(1,"integer"),
    LONG_VALUE(2,"long"),
    STRING_VALUE(3,"string"),
    BOOLEAN_VALUE(4,"boolean"),
..
}

so my code might look like this

@Deprecated
public static final int INTEGER_VALUE = DataType.INTEGER_VALUE.getId();

and overtime i can change my switch statements. When i change the static final int reference to point to the enum all my switch statements break.

[javac] /home/assure/projects/tp/main/src/a/b/c/DDDDDManagerBean.java:1108: constant expression required
[javac]             case GroupConstants.INTEGER_VALUE:
[javac]                                ^
[javac] /home/assure/projects/tp/main/src/a/b/c/ParameterComponent.java:203: constant expression required
[javac]         case GroupConstants.INTEGER_VALUE:
[javac]                            ^
[javac] /home/assure/projects/tp/main/src/a/b/c/ParameterComponent.java:268: constant expression required
[javac]             case GroupConstants.INTEGER_VALUE:
[javac]                                ^
[javac] /home/assure/projects/tp/main/src/a/b/c/ParameterComponent.java:316: constant expression required
[javac]             case GroupConstants.INTEGER_VALUE:
[javac]                                ^
[javac] /home/assure/projects/tp/main/src/a/b/c/ParameterComponent.java:436: constant expression required
[javac]         case GroupConstants.INTEGER_VALUE:

I don’t want to be forced to change all the switches yet, so its there a clean work around?

Answer 1

That won’t work. The problem is that the getId() call means that the constant’s value expression is not a compile-time constant expression according to the JLS. Refer to JLS 15.28 Constant Expressions for the gory details, and you will see that method calls are not allowed in constant expressions.

I don’t think there is any workaround, apart for a large-scale change to all of the switch statements. But I wouldn’t worry. Your IDE should be able help you find and replaces all occurrences of the old constants.

FOLLOWUP

The following code from your comment won’t work either:

private int INTEGER_VALUE_HOLDER = DataType.INTEGER_VALUE.getId(); 
public static final int INTEGER_VALUE = INTEGER_VALUE_HOLDER;

Firstly, INTEGER_VALUE_HOLDER is not a “constant variable” according to the definition in JLS 4.12.4 Final Variables. Therefore neither is INTEGER_VALUE.

Secondly, the initializer expression for a static cannot refer to this, and INTEGER_VALUE_HOLDER is really another way of saying this.INTEGER_VALUE_HOLDER.