public synchronized StringIdItem intern(StringIdItem string) {
    if (string == null) {
      throw new NullPointerException("string == null");
    }

    throwIfPrepared();

    CstString value = string.getValue();
    StringIdItem already = strings.get(value);

    if (already != null) {
      return already;
    }

    strings.put(value, string);
    return string;
  }
Interns an element into this instance.

@param string {@code non-null;} the string to intern
@return {@code non-null;} the interned string
android/android_framework/dex-maker/src/main/java/com/android/dx/dex/file/StringIdsSection.java