public synchronized FieldIdItem intern(CstFieldRef field) {
    if (field == null) {
      throw new NullPointerException("field == null");
    }

    throwIfPrepared();

    FieldIdItem result = fieldIds.get(field);

    if (result == null) {
      result = new FieldIdItem(field);
      fieldIds.put(field, result);
    }

    return result;
  }
Interns an element into this instance.

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