Java Stream - 如何将lambda函数传递给方法参数

public class Main {

    public static void main(String[] args) {
        Runnable runnable = andThen(() -> System.out.print("Hello "), () -> System.out.print("World!"));
        new Thread(runnable).start();
    }
    
    static Runnable andThen (Runnable r1, Runnable r2) {
        return () -> {
            r1.run();
            r2.run();
        };
    }
}