Converting from List of Object to List of String using streams

Question

I recently started working in Java streams. I was trying to get the String values out of the result set of one of my SQL queries. The result set selects just a String/Varchar column from the DB.

So I did:

List<String> list = query.getResultList().stream().map(Object::toString).collect(Collectors.toList());

or:

List<String> list =  = query.getResultList().stream().map(String::valueOf).collect(Collectors.toList());

I believe the map takes a function to convert data from one type to another. In this case, from Object to String and then collect them in a String list.

But the above code shows compile time error: Cannot convert from Object to List of string.

Please suggest me the correct way of doing this and explain what is wrong with my understanding.

Karol Dowbecki · Accepted Answer · 2019-02-14 13:37:58Z

4

Because Query.getResultList() returns a raw type List it will break stream pipeline which is heavily based on generic type information. Raw type are effectively removing all information about generic types when used so stream collector returns Object.

You can work around it by manually introducing the generic type with a cast to List<?>:

List<String> collect = ((List<?>) query.getResultList()).stream()
    .map(Object::toString)
    .collect(Collectors.toList());

answered Feb 14, 2019 at 13:37

Karol Dowbecki

45.3k9 gold badges83 silver badges118 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Joop Eggen · Accepted Answer · 2019-02-14 13:47:24Z

3

Use a TypedQuery<String> instead of a Query.

This does away with the rather superfluous remapping, and introduces type-safeness.

answered Feb 14, 2019 at 13:47

Joop Eggen

110k8 gold badges89 silver badges142 bronze badges

Collectives™ on Stack Overflow

Converting from List of Object to List of String using streams

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related