Java 使用 stream 将 object 列表过滤为字符串数组

Question

I have a class like this

public class Host {

  private HTMLToken.TokenType type;
  private StringBuilder content;

  public Host(HTMLToken.TokenType type) {
    this.type = type;
    content = new StringBuilder();
  }


  String contentAsString() {
    return content.toString();
  }

  void addStrig(String x) {
    content.append(x);
  }

  public enum TokenType {
    TAG, TEXT
  }

}

and static function like this

 public static String[] filterLength(List<Host> hostList, int length) {
    return hostList.stream().filter(str -> str.contentAsString().length() > length)
        .toArray(String[]::new);
  }

The problem is inside of filterLength function I can't return stream as a String array(I want to return content in the Host class) because List holds Host object how can I return as a string in 1 line code?

Answer 1

Transform each object in stream

You are successfully streaming, filtering, and collecting a bunch of Host objects into an array. But you seem to be asking how to instead collect a string representation of each Host object.

So, you need to add a step to your streaming operations. For each Host object, we want to generate and collect a String object.

Stream#map

To transform from the objects being streamed to a different kind of object, use Stream#map .

Here is an analog of your code, using LocalDate . We transform each filtered LocalDate object to a String object by passing a method reference : LocalDate:: toString .

List < LocalDate > dates = new ArrayList<>( 4 ) ;
dates.add( LocalDate.of( 2022 , Month.JANUARY, 11 ) ) ;
dates.add( LocalDate.of( 2022 , Month.JANUARY, 12 ) ) ;
dates.add( LocalDate.of( 2022 , Month.JANUARY, 13 ) ) ;
dates.add( LocalDate.of( 2022 , Month.JANUARY, 14 ) ) ;

String[] results = 
    dates
    .stream()
    .filter( localDate -> localDate.getDayOfMonth() > 12 )
    .map( LocalDate :: toString )
    .toArray( String[] :: new ) ;

System.out.println( "results = " + String.join( ", " , results ) ) ;

See this code run live at IdeOne.com .

results = 2022-01-13, 2022-01-14

I see you have a contentAsString method. I will guess that method returns the kind of text you want to collect.

So your code should be something like the following, passing method reference Host:: contentAsString to Stream#map .

return 
    hostList
    .stream()
    .filter( str -> str.contentAsString().length() > length )
    .map( host -> Host::contentAsString ) 
    .toArray( String[]::new );