java cast ArrayList<StringBuilder> to ArrayList<String>

Question

I'm writing a method in java which requires ArrayList<String> as its output. In this method, String concatenation is frequently used. I use StringBuilder instead of String since String is more expensive in concatenation.

Before returning result, how could I cast all elements in ArrayList<StringBuilder> to ArrayList<String>?

Do you mean: how can I cast all elements in ArrayList<StringBuilder> to String? You can't cast them. — Sotirios Delimanolis
– Sotirios Delimanolis, Commented Sep 26, 2014 at 1:53
I reopened the question because How do you cast a List of objects from one type to another in Java? concerns casting a list of supertypes to a list of subtypes, whereas StringBuilder and String are unrelated. @SotiriosDelimanolis you could add your Stream#map answer now. — Paul Bellora
– Paul Bellora, Commented Sep 26, 2014 at 2:14

Hovercraft Full Of Eels · Accepted Answer · 2014-09-26 02:04:33Z

7

how could I cast all elements in ArrayList<StringBuilder> to ArrayList<String>?

You can't and you shouldn't. Make it an ArrayList of one or the other.

Or if you must translate, then you'll need a for loop to iterate through the collection, building the new collection.

or I suppose you could declare the variable as an ArrayList<? extends CharSequence> -- nope that wouldn't work as we're nixed since we can't add(...) anything into a covariant structure, so we wouldn't be able to add anything to this list, just read from it.

edited Sep 26, 2014 at 2:04

answered Sep 26, 2014 at 1:54

Hovercraft Full Of Eels

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5 Comments

Sotirios Delimanolis Over a year ago

Use the new Stream#map(Function) functionality!

Hovercraft Full Of Eels Over a year ago

@SotiriosDelimanolis: I'm not familiar with that -- please make an answer with that information so we all can learn!

Pshemo Over a year ago

@HovercraftFullOfEels Sotirios meant something like List<String> listOfString = listOfStringBuffers.stream().map(builder->builder.toString()).collect(Collectors.toList()). BTW lambda expression builder->builder.toString() can be also written as StringBuilder::toString.

Hovercraft Full Of Eels Over a year ago

@Pshemo: Thanks! I've been putting off learning Lambdas and Java 8 for too long.

Sotirios Delimanolis Over a year ago

I was afk. That is what I meant. Your answer is plenty. This is just an implementation of what you described.

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