How to create PlantUML diagram using Java API without using PlantUML diagram code?

It doesn't look to me like PlantUML is designed to be used in this way: the API isn't documented. Having said that, it's an open source project, so you can download the sources and follow the execution for a given UML string, then recreate that execution with method calls avoiding the UML source, if that's what you need. If you have a debugger that can step through third party code, like IntelliJ, that will help a lot.

I had a little go at that. Here's a class which will generate the "Bob -> Alice : hello" diagram using the Java API without going via UML source string. I included the UML string version for comparison:

import net.sourceforge.plantuml.FileFormat;
import net.sourceforge.plantuml.FileFormatOption;
import net.sourceforge.plantuml.SourceStringReader;
import net.sourceforge.plantuml.core.ImageData;
import net.sourceforge.plantuml.cucadiagram.Display;
import net.sourceforge.plantuml.sequencediagram.Message;
import net.sourceforge.plantuml.sequencediagram.Participant;
import net.sourceforge.plantuml.sequencediagram.SequenceDiagram;
import net.sourceforge.plantuml.sequencediagram.SequenceDiagramFactory;
import net.sourceforge.plantuml.skin.ArrowConfiguration;

import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.OutputStream;

import static com.google.common.base.Preconditions.checkState;

public class PlantUMLDemoMain {
    public static void main(String[] args) throws Exception {
        generateFromStringSource(new File("from-string.png"));
        generateFromApi(new File("from-api.png"));
    }

    private static void generateFromApi(File file) throws IOException {
        // 1. setup:
        SequenceDiagramFactory f = new SequenceDiagramFactory();
        SequenceDiagram diagram = f.createEmptyDiagram();

        // 2. Build the diagram:
        // "Bob -> Alice : hello"
        // See net.sourceforge.plantuml.sequencediagram.command.CommandArrow#executeArg
        Display bobD = Display.getWithNewlines("Bob");
        Participant bobP = diagram.getOrCreateParticipant("Bob", bobD);

        Display aliceD = Display.getWithNewlines("Alice");
        Participant aliceP = diagram.getOrCreateParticipant("Alice", aliceD);

        Display label = Display.getWithNewlines("hello");
        ArrowConfiguration config = ArrowConfiguration.withDirectionNormal();

        Message msg = new Message(bobP, aliceP, label, config, diagram.getNextMessageNumber());

        checkState(null == diagram.addMessage(msg));

        // 3. Output the diagram
        // See net.sourceforge.plantuml.SourceStringReader#generateImage
        diagram.makeDiagramReady();
        checkState(1 == diagram.getNbImages());
        try (OutputStream os = new FileOutputStream(file)) {
            ImageData imageData = diagram.exportDiagram(os, 0, new FileFormatOption(FileFormat.PNG));
            System.out.println("generateFromApi: " + diagram.getDescription().getDescription());
        }
    }

    private static void generateFromStringSource(File file) throws IOException {
        String source = "@startuml\n";
        source += "Bob -> Alice : hello\n";
        source += "@enduml\n";

        SourceStringReader reader = new SourceStringReader(source);
        // Write the first image to "png"
        String desc = reader.generateImage(file);
        // Return a null string if no generation
        System.out.println("generateFromStringSource: " + desc);
    }
}

This will build with a build.sbt file like this:

libraryDependencies += "net.sourceforge.plantuml" % "plantuml" % "8059"
libraryDependencies += "com.google.guava" % "guava" % "31.0.1-jre"

The output of generateFromApi is identical to generateFromStringSource and looks like this: