public synchronized <T extends OffsettedItem> T intern(T item) {
    throwIfPrepared();

    OffsettedItem result = interns.get(item);

    if (result != null) {
      return (T) result;
    }

    add(item);
    interns.put(item, item);
    return item;
  }
Interns an item in this instance, returning the interned instance (which may not be the one
passed in). This will add the item if no equal item has been added.

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