在Akka Kafka中，可以使用Consumer.committableSource来列出可用主题。下面是一个使用Scala的示例代码：

import akka.kafka.scaladsl.Consumer
import akka.kafka.{ConsumerSettings, Subscriptions}
import akka.stream.scaladsl.Sink
import org.apache.kafka.common.serialization.StringDeserializer

import scala.concurrent.ExecutionContext.Implicits.global

object ListTopicsExample extends App {
  // Kafka consumer settings
  val consumerSettings = ConsumerSettings(system, new StringDeserializer, new StringDeserializer)
    .withBootstrapServers("localhost:9092")
    .withGroupId("group1")

  // Create a source with all available topics
  val topicsSource = Consumer.committableSource(consumerSettings, Subscriptions.topics())

  // Sink that prints out the topics
  val printSink = Sink.foreach { msg =>
    println(s"Received message from topic: ${msg.record.topic()}")
  }

  // Run the stream
  topicsSource.to(printSink).run()
}

上述代码使用Consumer.committableSource创建了一个包含所有可用主题的源，然后使用Sink.foreach将每个主题打印出来。可以根据需要修改代码来实现其他功能。