如何获取 HashMap 中包含的所有键的特定属性值？

Question

I have a Hashmap: Map<String, Paper> numberMapping = new HashMap<>();

class Paper String[] author; String[] scores; String[] author; String[] scores;

I want to retrieve all the author of each object contained in the hashmap.

I tried System.out.print(numberMapping.values().getAuthor()); but it's not working, how can I do that? Thanks in advance.

Answer 1

You are not seeing a user friendly output cause you are printing String[] objects, which does not have a friendly toString() method.

Either use use Set<String> instead of String[] , which has a better implementation, or else assuming Java 8+, a functional way could be:

Set<String> allAuthors = numberMapping.values()
        .stream()
        .map(paper -> paper.author)
        .flatMap(author -> Arrays.stream(author))
        .collect(Collectors.toSet());
System.out.println(allAuthors);

Also it's common practice to use plurals for variables that refers to Collections.

I'm assuming the overall order of the output doesn't matter to you since you are using a Hashmap to store the values, as pointed out in the comments

Answer 2

You could do it in following ways:

List<String[]> authors = ...
for (Paper paper : numberMapping.values()) {
    authors.add(paper.getAuthor());
}
//print authors

Alternatively, you could use Stream api like below:

System.out.println(numberMapping.values().stream().
                       map(paper -> paper.getAuthor()).
                       collect(Collectors::asList));

Now remember, you have array of authors, hence it might not print correctly as arrays doesn't have inbuilt toString implemented, hence you may need to iterate over each elements and print the authors using Arrays.toString(authors) .