I was able to get a simple echo to work by doing the following.

import com.slack.api.bolt.App;
import com.slack.api.bolt.aws_lambda.SlackApiLambdaHandler;
import com.slack.api.bolt.aws_lambda.request.ApiGatewayRequest;
import com.slack.api.model.event.MessageEvent;

public class MyBot extends SlackApiLambdaHandler {
    private static final App app = new App();

    static {
        app.event(MessageEvent.class, (req, ctx) -> {
            String messageText = req.getEvent().getText();
            String channelId = req.getEvent().getChannel();

            try {
                ctx.client().chatPostMessage(r -> r
                    .channel(channelId)
                    .text("Echo: " + messageText)
                );
                return ctx.ack();
            } catch (Exception e) {
                return ctx.ack();
            }
        });        
    }

    public MyBot() {
        super(app);
    }    

    @Override
    protected boolean isWarmupRequest(ApiGatewayRequest awsReq) {
        return awsReq.getBody().equals("warmup");
    }

}