java 8 how to get distinct list on more than one property

Question

How can one get the distinct (distinct based on two property) list from a list of objects. for example let there are list of objects with property name and price. Now how can I get a list with distinct name or price.
suppose

list<xyz> l1 = getlist(); // getlist will return the list.

Now let l1 has the following properties(name, price) :-
n1, p1
n1, p2
n2, p1
n2, p3

Now after the filter the list should be-
n1, p1
n2, p3

I tried solving like this -

public List<xyz> getFilteredList(List<xyz> l1) {

        return l1
                .stream()
                .filter(distinctByKey(xyz::getName))
                .filter(distinctByKey(xyz::getPrice))
                .collect(Collectors.toList());
    }

    private static <T> Predicate<T> distinctByKey(Function<? super T, Object> keyExtractor) {
        Map<Object,Boolean> seen = new ConcurrentHashMap<>();
        return t -> seen.putIfAbsent(keyExtractor.apply(t), Boolean.TRUE) == null;
    }

Now the problem is when i did filter on name the list return would be -
n1, p1
n2, p1

and then it would have run filter on price which return -
n1, p1

which is not the expected result.

Answer 1

Almost verbatim from Stuart Marks' answer:

import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;
import java.util.function.Function;
import java.util.function.Predicate;
import java.util.stream.Collectors;

class Class {

  public static <T> Predicate<T> distinctByKey(Function<? super T, Object> keyExtractor) {
    Map<Object, Boolean> seen = new ConcurrentHashMap<>();
    return t -> seen.putIfAbsent(keyExtractor.apply(t), Boolean.TRUE) == null;
  }

  private static List<Pojo> getList() {
    return Arrays.asList(
      new Pojo("123", 100),
      new Pojo("123", 100),
      new Pojo("123", 100),
      new Pojo("456", 200)
    );
  }

  public static void main(String[] args) {

    System.out.println(getList().stream()
      // extract a key for each Pojo in here. 
      // concatenating name and price together works as an example
      .filter(distinctByKey(p -> p.getName() + p.getPrice()))
      .collect(Collectors.toList()));
  }

}

class Pojo {
  private final String name;
  private final Integer price;

  public Pojo(final String name, final Integer price) {
    this.name = name;
    this.price = price;
  }

  public String getName() {
    return name;
  }

  public Integer getPrice() {
    return price;
  }

  @Override
  public String toString() {
    final StringBuilder sb = new StringBuilder("Pojo{");
    sb.append("name='").append(name).append('\'');
    sb.append(", price=").append(price);
    sb.append('}');
    return sb.toString();
  }
}

This main method yields:

[Pojo{name='123', price=100}, Pojo{name='456', price=200}]

Edit

Made price an int per Eugene's prompting.

Note: that you could use something more interesting as a key if you wanted to flesh it out:

import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;
import java.util.function.Function;
import java.util.function.Predicate;
import java.util.stream.Collectors;

class Class {

  public static <T> Predicate<T> distinctByKey(Function<? super T, Object> keyExtractor) {
    Map<Object, Boolean> seen = new ConcurrentHashMap<>();
    return t -> seen.putIfAbsent(keyExtractor.apply(t), Boolean.TRUE) == null;
  }

  private static List<Pojo> getList() {
    return Arrays.asList(
      new Pojo("123", 100),
      new Pojo("123", 100),
      new Pojo("123", 100),
      new Pojo("456", 200)
    );
  }

  private static class NameAndPricePojoKey {
    final String name;
    final int price;

    public NameAndPricePojoKey(final Pojo pojo) {
      this.name = pojo.getName();
      this.price = pojo.getPrice();
    }

    @Override
    public boolean equals(final Object o) {
      if (this == o) return true;
      if (o == null || getClass() != o.getClass()) return false;

      final NameAndPricePojoKey that = (NameAndPricePojoKey) o;

      if (price != that.price) return false;
      return name != null ? name.equals(that.name) : that.name == null;

    }

    @Override
    public int hashCode() {
      int result = name != null ? name.hashCode() : 0;
      result = 31 * result + price;
      return result;
    }
  }

  public static void main(String[] args) {

    System.out.println(getList().stream()
      // extract a key for each Pojo in here. 
      .filter(distinctByKey(NameAndPricePojoKey::new))
      .collect(Collectors.toList()));
  }

}

class Pojo {
  private String name;
  private Integer price;
  private Object otherField1;
  private Object otherField2;

  public Pojo(final String name, final Integer price) {
    this.name = name;
    this.price = price;
  }

  public String getName() {
    return name;
  }

  public void setName(final String name) {
    this.name = name;
  }

  public Integer getPrice() {
    return price;
  }

  public void setPrice(final Integer price) {
    this.price = price;
  }

  public Object getOtherField1() {
    return otherField1;
  }

  public void setOtherField1(final Object otherField1) {
    this.otherField1 = otherField1;
  }

  public Object getOtherField2() {
    return otherField2;
  }

  public void setOtherField2(final Object otherField2) {
    this.otherField2 = otherField2;
  }

  @Override
  public boolean equals(final Object o) {
    if (this == o) return true;
    if (o == null || getClass() != o.getClass()) return false;

    final Pojo pojo = (Pojo) o;

    if (name != null ? !name.equals(pojo.name) : pojo.name != null) return false;
    if (price != null ? !price.equals(pojo.price) : pojo.price != null) return false;
    if (otherField1 != null ? !otherField1.equals(pojo.otherField1) : pojo.otherField1 != null) return false;
    return otherField2 != null ? otherField2.equals(pojo.otherField2) : pojo.otherField2 == null;

  }

  @Override
  public int hashCode() {
    int result = name != null ? name.hashCode() : 0;
    result = 31 * result + (price != null ? price.hashCode() : 0);
    result = 31 * result + (otherField1 != null ? otherField1.hashCode() : 0);
    result = 31 * result + (otherField2 != null ? otherField2.hashCode() : 0);
    return result;
  }

  @Override
  public String toString() {
    final StringBuilder sb = new StringBuilder("Pojo{");
    sb.append("name='").append(name).append('\'');
    sb.append(", price=").append(price);
    sb.append(", otherField1=").append(otherField1);
    sb.append(", otherField2=").append(otherField2);
    sb.append('}');
    return sb.toString();
  }
}

Answer 2

I'd go for something like this, which is fairly simple and flexible, and builds on your example:

public static <T> List<T> distinctList(List<T> list, Function<? super T, ?>... keyExtractors) {

    return list
        .stream()
        .filter(distinctByKeys(keyExtractors))
        .collect(Collectors.toList());
}

private static <T> Predicate<T> distinctByKeys(Function<? super T, ?>... keyExtractors) {

    final Map<List<?>, Boolean> seen = new ConcurrentHashMap<>();

    return t -> {

        final List<?> keys = Arrays.stream(keyExtractors)
            .map(ke -> ke.apply(t))
            .collect(Collectors.toList());

        return seen.putIfAbsent(keys, Boolean.TRUE) == null;

    };

}

This can then be called in the following manner:

final List<Xyz> distinct = distinctList(list, Xyz::getName, Xyz::getPrice)

Answer 3

Here is my solution based on the class Item which defines a name and a price:

public class Item {

    public String name;
    public double price;

    Item(String name, double price) {
        this.name = name;
        this.price = price;
    }
}

The requirement is to obtain only Items from a given List<Item> which have distinct names and distinct prices, in the order in which they occur.

I catch this requirement of being distinct by a class ItemWrapper:

public class ItemWrapper {

    Item item;

    ItemWrapper(Item item) {
        this.item = item;
    }

    @Override
    public boolean equals(Object obj) {
        if (!(obj instanceof ItemWrapper)) return false;
        ItemWrapper other = (ItemWrapper) obj;
        return  Objects.equals(item.name, other.item.name) ||
                item.price == other.item.price;
    }

    @Override
    public int hashCode() {
        return 1;
    }
}

Now we have everything in place to filter a given List<Item> of items:

List<Item> items = Arrays.asList(
        new Item("name-1", 100.00),
        new Item("name-1", 555.00),
        new Item("name-2", 100.00),
        new Item("name-2", 999.99),
        new Item("name-3", 100.00),
        new Item("name-4", 555.00),
        new Item("name-5", 999.99)
);

as following:

items.stream()
     .map(item -> new ItemWrapper(item))
     .distinct()
     .map(wrapper -> wrapper.item)
     .collect(Collectors.toList());
}

The items captured are:

• name=name-1, price=100.0
• name=name-2, price=999.99
• name=name-4, price=555.0

Answer 4

Why not that way:

@Value
static class User {
    String firstName;
    String lastName;
    int age;
}

public static void main(String[] args) {
    var user1 = new User("firstName1", "lastName1", 20);
    var user2 = new User("firstName1", "lastName2", 20);
    var user3 = new User("firstName3", "lastName3", 30);

    var users = List.of(user1, user2, user3);

    var uniqueByFirstNameAndAge = users
        .stream()
        .collect(Collectors.toMap(usr -> Set.of(usr.getFirstName(), usr.getAge()), Function.identity(), (usr1, usr2) -> usr1))
        .values();

    System.out.println(uniqueByFirstNameAndAge);


}