Under the assumption that the intention of MessageFormat.toPattern() is to generate a pattern that can be later used to recreate the same `MessageFormat`, then there is a case where MessageFormat.toPattern() is generates the wrong pattern string.

The bug is that MessageFormat.toPattern()` (correctly) quotes plain text opening "{" brace characters in the pattern, it does not quote plain text closing "}" brace characters. This is a problem because the pattern parser interprets a closing "}" brace character as the end of a nested format string.

As a result, when going from MessageFormat -> Pattern String -> MessageFormat, it's possible for the format to change semantically. This is obviously a problem for (for example) tools that store and retrieve MessageFormat's as pattern strings for the purpose of i18n, etc.

Test case:

import java.text.MessageFormat;

public class MessageFormatBug {

    public static void main(String[] args) {

        String pattern1 = "{0,choice,0.0#option A: {1}|1.0#option B: {1}'}'}";
        MessageFormat format1 = new MessageFormat(pattern1);
        String result1 = format1.format(new Object[] { 0, 5 });

        String pattern2 = format1.toPattern();
        MessageFormat format2 = new MessageFormat(pattern2);
        String result2 = format2.format(new Object[] { 0, 5 });

        System.out.println(String.format("pattern1 = \"%s\"", pattern1));
        System.out.println(String.format("pattern2 = \"%s\"", pattern2));
        System.out.println(String.format("result1 = \"%s\"", result1));
        System.out.println(String.format("result2 = \"%s\"", result2));
        System.out.println("results are equal = " + result1.equals(result2));
    }
}

Expected output:

pattern1 = "{0,choice,0.0#option A: {1}|1.0#option B: {1}'}'}"
pattern2 = "{0,choice,0.0#option A: {1}|1.0#option B: {1}'}'}"
result1 = "option A: 5"
result2 = "option A: 5"
results are equal = true

Actual output:

pattern1 = "{0,choice,0.0#option A: {1}|1.0#option B: {1}'}'}"
pattern2 = "{0,choice,0.0#option A: {1}|1.0#option B: {1}}}"
result1 = "option A: 5"
result2 = "option A: 5}"
results are equal = false

Reproducible in JDK 8, 11, 17, and 21.