Why Groovy Enum Constructors Fail with Char Literals and How to Fix Them

Problem Code

package com.fun.ztest.groovy

import com.fun.moco.MocoServer

class MocoDemo extends MocoServer {
    public static void main(String[] args) {
        println FunTester.FUN
    }

    static enum FunTester {
        FUN('3'), TESTER('2')
        char name
        FunTester(char name) {
            this.name = name
        }
    }
}

Running this class produces an ExceptionInInitializerError with the message

Could not find matching constructor for: com.fun.ztest.groovy.MocoDemo$FunTester(String, Integer, String)

, indicating that Groovy could not locate a suitable constructor for the enum constants.

Root Cause Analysis

Groovy does not distinguish between double quotes ( "") and single quotes ( '') when they are used as character literals; both are interpreted as String objects. The enum constructor expects a char, but the literals are passed as String, causing the initialization failure.

The following snippet demonstrates that both "3" and '3' are treated as String types:

public static void main(String[] args) {
    println "3".class.getName()
    println '3'.class.getName()
}

Console output:

java.lang.String
java.lang.String

Fix

Explicitly cast the literals to char when defining the enum constants. The corrected enum definition looks like this:

class MocoDemo extends MocoServer {
    public static void main(String[] args) {
        println FunTester.FUN
    }

    static enum FunTester {
        FUN((char)'3'), TESTER((char)'2')
        char name
        FunTester(char name) {
            this.name = name
        }
    }
}

Running the corrected code produces the expected output without errors:

INFO-> 当前用户：fv，IP：10.60.131.54，工作目录：/Users/fv/Documents/workspace/fun/,系统编码格式:UTF-8,系统Mac OS X版本:10.16
FUN