Can I add new methods to the String class in Java?

Question

I\'d like to add a method AddDefaultNamespace() to the String class in Java so that I can type \"myString\".AddDefaultNamespace() instead of

Answer 1

Well, actually everyone is being unimaginative. I needed to write my own version of startsWith method because I needed one that was case insensitive.

class MyString{
    public String str;
    public MyString(String str){
        this.str = str;
    }
    // Your methods.
}

Then it's quite simple, you make your String as such:

MyString StringOne = new MyString("Stringy stuff");

and when you need to call a method in the String library, simple do so like this:

StringOne.str.equals("");

or something similar, and there you have it...extending of the String class.

Answer 2

All is said by the other contributors before. You can not extend String directly because it is final.

If you would use Scala, you can use implicit conversions like this:

object Snippet {
  class MyString(s:String) {
    def addDefaultNamespace = println("AddDefaultNamespace called")
  }
  implicit def wrapIt(s:String) = new MyString(s)

  /** test driver */
  def main(args:Array[String]):Unit = {
    "any java.io.String".addDefaultNamespace // !!! THAT is IT! OR?
  }

Answer 3

People searching with keywords "add method to built in class" might end up here. If you're looking to add method to a non final class such as HashMap, you can do something like this.

public class ObjectMap extends HashMap<String, Object> {

    public Map<String, Object> map;

    public ObjectMap(Map<String, Object> map){
        this.map = map;
    }

    public int getInt(String K) {
        return Integer.valueOf(map.get(K).toString());
    }

    public String getString(String K) {
        return String.valueOf(map.get(K));
    }

    public boolean getBoolean(String K) {
        return Boolean.valueOf(map.get(K).toString());
    }

    @SuppressWarnings("unchecked")
    public List<String> getListOfStrings(String K) {
        return (List<String>) map.get(K);
    }

    @SuppressWarnings("unchecked")
    public List<Integer> getListOfIntegers(String K) {
        return (List<Integer>) map.get(K);
    }

    @SuppressWarnings("unchecked")
    public List<Map<String, String>> getListOfMapString(String K) {
        return (List<Map<String, String>>) map.get(K);
    }

    @SuppressWarnings("unchecked")
    public List<Map<String, Object>> getListOfMapObject(String K) {
        return (List<Map<String, Object>>) map.get(K);
    }

    @SuppressWarnings("unchecked")
    public Map<String, Object> getMapOfObjects(String K) {
        return (Map<String, Object>) map.get(K);
    }

    @SuppressWarnings("unchecked")
    public Map<String, String> getMapOfStrings(String K) {
        return (Map<String, String>) map.get(K);
    }
}

Now define a new Instance of this class as:

ObjectMap objectMap = new ObjectMap(new HashMap<String, Object>();

Now you can access all the method of the built-in Map class, and also the newly implemented methods.

objectMap.getInt("KEY");

EDIT:

In the above code, for accessing the built-in methods of map class, you'd have to use

objectMap.map.get("KEY");

Here's an even better solution:

public class ObjectMap extends HashMap<String, Object> {

    public ObjectMap() {

    }

    public ObjectMap(Map<String, Object> map){
        this.putAll(map);
    }

    public int getInt(String K) {
        return Integer.valueOf(this.get(K).toString());
    }

    public String getString(String K) {
        return String.valueOf(this.get(K));
    }

    public boolean getBoolean(String K) {
        return Boolean.valueOf(this.get(K).toString());
    }

    @SuppressWarnings("unchecked")
    public List<String> getListOfStrings(String K) {
        return (List<String>) this.get(K);
    }

    @SuppressWarnings("unchecked")
    public List<Integer> getListOfIntegers(String K) {
        return (List<Integer>) this.get(K);
    }

    @SuppressWarnings("unchecked")
    public List<Map<String, String>> getListOfMapString(String K) {
        return (List<Map<String, String>>) this.get(K);
    }

    @SuppressWarnings("unchecked")
    public List<Map<String, Object>> getListOfMapObject(String K) {
        return (List<Map<String, Object>>) this.get(K);
    }

    @SuppressWarnings("unchecked")
    public Map<String, Object> getMapOfObjects(String K) {
        return (Map<String, Object>) this.get(K);
    }

    @SuppressWarnings("unchecked")
    public Map<String, String> getMapOfStrings(String K) {
        return (Map<String, String>) this.get(K);
    }

    @SuppressWarnings("unchecked")
    public boolean getBooleanForInt(String K) {
        return Integer.valueOf(this.get(K).toString()) == 1 ? true : false;
    }
}

Now you don't have to call

objectMap.map.get("KEY");

simply call

objectMap.get("KEY");

Answer 4

Actually , you can modify the String class . If you edit the String.java file located in src.zip , and then rebuild the rt.jar , the String class will have more methods added by you . The downside is that that code will only work on your computer , or if you provide your String.class , and place it in the classpath before the default one .

Answer 5

Better use StringBuilder, which has method append() and does the job you want. The String class is final and can not be extended.

Answer 6

String is a final class which means it cannot be extended to work on your own implementation.