how to can I Add a method on Java.String class

Question

I need to add a custom method on java.lang.String class

I am looking for a java equivalent to the C# extension methods feature.

...is there any language feature that will allow me to write an extension method for a final class ?

my code in c#:

public static string GetLang(this string value, bool isSplited = false, LanguageType? languageType = null)
        {
            if (String.IsNullOrWhiteSpace(value))
                return null;

            var language = (languageType ?? LanguageProvider.Language).ToString().ToLower();

            if (isSplited)
            {
                var items = new List<string>();
                foreach (var item in value.Split('|'))
                {
                    var multiLang = JsonConvert.DeserializeObject<Dictionary<string, string>>(item);

                    if (multiLang.TryGetValue(language, out string textValue))
                    {
                        items.Add(textValue);
                    }
                    else
                    {
                        items.Add($"[{language}]");
                    }
                }

                return string.Join(" ", items);
            }
            else
            {
                var multiLang = JsonConvert.DeserializeObject<Dictionary<string, string>>(value);

                if (multiLang.TryGetValue(language, out string textValue))
                {
                    return textValue;
                }
                else
                {
                    return $"[{language}]";
                }

            }
        }

i want use this similar feature in Java

No, there isn't. An alternative would be to instead write static methods accepting the string as first argument. — MC Emperor, Sep 29 '19 at 14:07
Groovy and Kotlin provide this ability, but plain Java does not. — chrylis -cautiouslyoptimistic-, Sep 29 '19 at 14:14
As question is mapped to android, you can do this in kotlin `fun String.removeFirstLastChar(): String = this.substring(1, this.length - 1)` — Govinda Sakhare, Sep 29 '19 at 14:29

score 2 · Answer 1 · answered Sep 29 '19 at 14:06

2

You cannot. Java Strings are immutable, and to ensure people do not break immutability doing what you have asked, the class is final. What need do you have of adding a new method on strings?

Immutability of Strings in Java

answered Sep 29 '19 at 14:06

Martin'sRun

522
3
11

score 0 · Answer 2 · answered Sep 29 '19 at 14:06

0

Unfortunately, Java does not support extension methods.

answered Sep 29 '19 at 14:06

shy4blue

46
5

[Java equivalent to C# extension methods](https://stackoverflow.com/questions/4359979/java-equivalent-to-c-sharp-extension-methods) – shy4blue Sep 29 '19 at 14:12

score 0 · Answer 3 · answered Sep 29 '19 at 14:17

You cannot extend String class or any other final class but you can always use composition to make use of String class and add your own functionality to it -

   Class CustomString {
      private String str; // composition
      CustomString(String s) {
        str = s;
      }
      public String getStr() {
         return str;
      }
      // .. your custom method 
      public String anyLogicGoesHere() {
      }
}

how to can I Add a method on Java.String class

3 Answers3