[BUG] Java enum values are messed up

Description

When enum values are parsed with the Java parser, they get messed up in the generated code.

For example:

• "R2D2" becomes "R2_D2" in Java code
• "U2F" becomes "U2_F" in Java code

openapi-generator version

Version 7.8.0

Suggested fix

This bug has been introduced with https://github.com/OpenAPITools/openapi-generator/pull/18338

AbstractJavaCodeGen.java, line 1913:

    @Override
    public String toEnumVarName(String value, String datatype) {
        if (enumNameMapping.containsKey(value)) {
            return enumNameMapping.get(value);
        }

        if (value.length() == 0) {
            return "EMPTY";
        }

        // for symbol, e.g. $, #
        if (getSymbolName(value) != null) {
            return getSymbolName(value).toUpperCase(Locale.ROOT);
        }

        if (" ".equals(value)) {
            return "SPACE";
        }

        // number
        if ("Integer".equals(datatype) || "Long".equals(datatype) ||
                "Float".equals(datatype) || "Double".equals(datatype) || "BigDecimal".equals(datatype)) {
            String varName = "NUMBER_" + value;
            varName = varName.replaceAll("-", "MINUS_");
            varName = varName.replaceAll("\\+", "PLUS_");
            varName = varName.replaceAll("\\.", "_DOT_");
            return varName;
        }

        // string
        String var = value.replaceAll("\\W+", "_").toUpperCase(Locale.ROOT);
        if (var.matches("\\d.*")) {
            var = "_" + var;
        }
        return this.toVarName(var); <----------------Should be changed to "return var;"
    }

Related issues/PRs

https://github.com/OpenAPITools/openapi-generator/pull/18338

Start in AbstractJavaCodeGen.java around the toEnumVarName method and review the change introduced by pull request 18338. Verify the behavior with enum values such as R2D2 and U2F; done means generated Java enum names preserve those values without inserting underscores.