java - remove elements in list

Question

Help me with this

 Movie{
    int id;
    String title;
 }

 List<Movie> movies = new ArrayList<Movie>();
 Movie movie1 = new Movie(1,"movie1");
 Movie movie2 = new Movie(2,"movie2");
 Movie movie3 = new Movie(1,"movie3");
 Movie movie4 = new Movie(2,"movie4");
 movies.add(movie1);  movies.add(movie2);  movies.add(movie3);  movies.add(movie4);

Now I have a list of movies including all 4 of above.

 (movies1,movies2,movies3,movies4)

But I want my list of movies contain only the last movies added among the ones that have the same Id which is :

  (movies3,movies4);

Update : Thank @LeffeBrune for the answer but now if I want two or more fields, not just one. What should I do?

  Movie{
     int id; String title ; String plot;
  }
  for example for both id and title field.
    (1,"title1","plot1"),(2,"title2","plot2"),(1,"title3","plot3"),(1,"title1","plot4")

   will become
    (2,"title2","plot2"),(1,"title3","plot3"),(1,"title1","plot4"),

Based on LeffeBrune's answer, Should I put the whole movie object as key and override an equal method.

Do you want to remove movie1 and movie3 from movies itself, or just return a list of movies that has id=2? Your question indicates you want them removed, but then what is the point of adding them in the first place if we're going to never need them again? — Compass
– Compass, Commented Nov 5, 2014 at 15:52
I assume you meant your list to be {movie3, movie4} because those are the two movies with distinct IDs added last. — LeffeBrune
– LeffeBrune, Commented Nov 5, 2014 at 15:55
@Compass I added them just for the example, actually I'm gonna get the list from somewhere else. — user2747502
– user2747502, Commented Nov 5, 2014 at 16:02

LeffeBrune · Accepted Answer · 2014-11-06 01:58:02Z

You need to use a right data structure for this. In this case a map will work:

Map<Integer, Movie> movies = new HashMap<>();
Movie movie1 = new Movie(1,"movie1");
Movie movie2 = new Movie(2,"movie2");
Movie movie3 = new Movie(1,"movie3");
Movie movie4 = new Movie(2,"movie4");
movies.put(movie1.id, movie1);
movies.put(movie2.id, movie2);
movies.put(movie3.id, movie3);
movies.put(movie4.id, movie4);

Now the map contains { movie3, movie4 } or the last added movies with that ID.

If you want to use "composite keys" where integer ID and title identify the movie you need to either use a more complicated data structure or resort to hacks. One easy way to create a composite key is to concatenate fields in it into a string and then use that string as a key in your map:

import java.util.HashMap;
import java.util.Map;

class Movies {
  static class Movie {
    int id;
    String title;
    String plot;

    Movie(int id, String title, String plot) {
      this.id = id;
      this.title = title;
      this.plot = plot;
    }

    @Override
    public String toString() {
      return String.format("{ %d, %s => %s }", id, title, plot);
    }
  }

  public static void main(String[] args) {
    Map<String, Movie> movies = new HashMap<>();
    Movie movie1 = new Movie(1, "title1" ,"plot1");
    Movie movie2 = new Movie(2, "title2", "plot2");
    Movie movie3 = new Movie(1, "title3", "plot3");
    Movie movie4 = new Movie(1, "title1", "plot4");

    // Create composite key by combining id and title into a string. 
    movies.put(movie1.id + movie1.title, movie1);
    movies.put(movie2.id + movie2.title, movie2);
    movies.put(movie3.id + movie3.title, movie3);
    movies.put(movie4.id + movie4.title, movie4);

    for (Map.Entry<String, Movie> entry : movies.entrySet()) {
      System.out.printf(
          "Key: %s Value: %s%n", entry.getKey(), entry.getValue());
    }
  }
}

The proper way to implement such a key would require creating another object and overriding its equals and hashCode methods. This is way more advanced territory.

Now if I want two or more fields, not one ? Should I put the whole movie object as key and override an equal method? See my update
The thing that uniquely identifies a movie is its ID. Keep using that as a key, and add fields for additional data to Movie class.

phil_20686 · Accepted Answer · 2014-11-05 15:55:43Z

Don't use a list, use a Map. Then you can expoit the fact that maps usually only allow one value with agiven key.

Map<Integer, Movie> map = new HashMap<Integer, Movie>();

//for each movie:
Movie movie = new Movie(1,"movie1")
map.put(movie.id, movie.title)

Then map.values() will give you movie 3 and movie 4.

Other classic solution include: Overwrite Movie.equals() to compare ids, and then add them to a set. This will give the same behaviour. All though not sure if sets will keep the first or the last one - so you might have to reverse the order of iteration.

Use a Sorted List with a custom comparator to make a set ordered by id, and then iterate over it pulling out the last move that occurs with every id.

Collectives™ on Stack Overflow

java - remove elements in list

2 Answers 2

2 Comments

Comments

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

Comments

Related