Java forEach 嵌套循环为 stream

Question

Stream instead of nested forEach loops

I'd like to use stream instead of nested forEach loop. I tried to create stream as seen below but it doesn't work. ForEach loops work fine. List of CarWrapper objects should contain as many objects as each of Car objects has strings in its strings list. Could you help?

'''

class Main {
 public static void main(String[] args) {
  new doit();
 }
}

class doit{
 public doit(){
List<CarWrapper> wrappers;
List<Car> cars = new ArrayList<>();
cars.add(new Car());
cars.add(new Car());

  //  cars.forEach(car -> {
  //         car.getDriver().getStrings().forEach(string -> {
  //             wrappers.add(wrapper(car, string));
  //         });
  //     });

  wrappers = cars.stream()
  .map(car->{
     car.getDriver().getStrings().forEach(string->{
      wrapper(car, string);
    });
  })
  .collect(Collectors.toList());
}


 public CarWrapper wrapper(Car car, String s){
   return new CarWrapper.Builder().setBrand(car.getBrand()).setString(s).build();
  }
}


  class Car{
    private Driver driver;
    private String brand;

  public Car(){
     this.brand="bmw";
     this.driver = new Driver();
  }

   public Driver getDriver(){
     return driver;
   }

   public String getBrand(){
     return brand;
    }
   }

   class Driver{
   private List<String> strings;

   public Driver(){
     this.strings = Arrays.asList("qwe","zxc");
   }

    public List<String> getStrings(){
     return strings;
     }
    }

  class CarWrapper{ 
    private String brand;
    private String string;

    private CarWrapper(){
    }

  public static class Builder{
  private String brand;
  private String string;

  public Builder setBrand(String brand){
    this.brand=brand;
    return this;
  }
  public Builder setString(String string){
    this.string=string;
    return this;
  }
  public CarWrapper build(){
    CarWrapper carWrapper = new CarWrapper();
    carWrapper.brand = this.brand;
    carWrapper.string = this.string;
    return carWrapper;
  }
}
}

'''

Answer 1

Try this

wrappers = cars.stream()
              .flatMap(car -> car.getDriver().getStrings().stream()
                      .map(string -> wrapper(car, string)))
              .collect(Collectors.toList());