ADDITIONAL SYSTEM INFORMATION :
Java 22, 23, 24

A DESCRIPTION OF THE PROBLEM :
See description inline. Problem: The line "line.isBlank()" does not work because the compiler reports:

% java -version
openjdk version "24-ea" 2025-03-18
OpenJDK Runtime Environment (build 24-ea+6-619)
OpenJDK 64-Bit Server VM (build 24-ea+6-619, mixed mode, sharing)
% javac --release 24 --enable-preview J24Gatherer.java
J24Gatherer.java:43: Fehler: Symbol nicht gefunden
                    if (line.isBlank()) { // does not work
                            ^
  Symbol: Methode isBlank()
  Ort: Variable line von Typ String
  Dabei ist String eine Typvariable:
    String erweitert Object, deklariert in Methode <String>paraGroups()
Hinweis: J24Gatherer.java verwendet Vorschaufeatures von Java SE 24.
Hinweis: Wiederholen Sie die Kompilierung mit -Xlint:preview, um Details zu erhalten.
1 Fehler

The code compiles when commenting out (line 43)

if (line.isBlank()) { // does not work

and commenting out: (line 44)

//if (line.toString().isBlank()) {

At line 50 (para.add(line); everything works fine.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Compile the code with "javac --release 24 --enable-preview J24Gatherer.java"

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
The code must compile.
ACTUAL -
% java -version
openjdk version "24-ea" 2025-03-18
OpenJDK Runtime Environment (build 24-ea+6-619)
OpenJDK 64-Bit Server VM (build 24-ea+6-619, mixed mode, sharing)
% javac --release 24 --enable-preview J24Gatherer.java
J24Gatherer.java:43: Fehler: Symbol nicht gefunden
                    if (line.isBlank()) { // does not work
                            ^
  Symbol: Methode isBlank()
  Ort: Variable line von Typ String
  Dabei ist String eine Typvariable:
    String erweitert Object, deklariert in Methode <String>paraGroups()
Hinweis: J24Gatherer.java verwendet Vorschaufeatures von Java SE 24.
Hinweis: Wiederholen Sie die Kompilierung mit -Xlint:preview, um Details zu erhalten.
1 Fehler


---------- BEGIN SOURCE ----------
import java.util.ArrayList;
import java.util.List;
import java.util.stream.Gatherer;

import static java.lang.System.out;
import static java.util.List.copyOf;


public class J24Gatherer {

    static String MULTILINE =
            """
            The quick brown fox.
            Jumps over.
            
            The laszy dog.
            """;

    public static void main(final String... args) throws Exception {
        // final var paragraphs = MULTILINE.lines().gather(paraGroups()).toList();
        final var paragraphs = MULTILINE.lines().gather(paraGroups()).toList();
        out.format("Paragraphs: %s%n", paragraphs);
    }
    
    static <String> Gatherer<String, List<String>, List<String>> paraGroups() {
        return Gatherer.ofSequential(

                // The current paragraph holder
                () -> new ArrayList<>(),
                

                Gatherer.Integrator.ofGreedy((para, line, downstream) -> {

                    // Looks like there is a compiler bug. "line" is not interpreted as String in line 44
                    // Also casting doesn't work. Only "" + line or line.toString(). Or it's a layer 8 problem.
                    // In line 50 everything is fine.

                    if (line.isBlank()) { // does not work
                    //if (line.toString().isBlank()) {
                        final var result = copyOf(para);
                        para.clear();
                        return downstream.push(result);
                    }

                    para.add(line);
                    return true;
                }),


                (para, downstream) -> {
                    if (!downstream.isRejecting() && !para.isEmpty()) {
                        downstream.push(copyOf(para));
                        para.clear();
                    }
                }
        );
    }
}
---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Dedicated toString() call on input parameter.

FREQUENCY : always