Find elements in a list that are not present in another list using java 8

问题

I have 2 lists. The requirement is to filter out elements in list1 that are not in list2 based on condition.

  Class Fighter
  {
    String name;
    String address;
  }     
  List<Fighter> pairs1 = new ArrayList();
    pairs1.add(new Fighter("a", "a"));
    pairs1.add(new Fighter("b", "a"));

    List<Fighter> pairs2 = new ArrayList();
    pairs2.add(new Fighter("a", "c"));
    pairs2.add(new Fighter("a", "d"));
    Set<Fighter> finalValues = new HashSet<>();


    finalValues = pairs1.stream().filter(firstList -> 
   pairs2.stream().noneMatch(secondList -> 
   firstList.getName().equals(secondList.getName())
            && firstList.getName().equals(secondList.getName()))).collect(Collectors.toSet());

    System.out.println(finalValues);

Expected Output : a=a, b=a

Explanation: Elements in list1 that are not in list2

The above code is not giving the expected output. Please let me know how the above stream code can be corrected to get the output

回答1:

First override the equals and hashcode methods in Fighter class.

@Override
public boolean equals(Object o) {
    if (o == this)
        return true;
    if (!(o instanceof Fighter))
        return false;
    Fighter f = (Fighter) o;
    return f.name.equals(name) && f.address.equals(address);
}

@Override
public int hashCode() {
    int result = name.hashCode();
    result = 31 * result + address.hashCode();
    return result;
}

Then create a Set from pairs2. Finally use it's contains method to get the set difference. Here's how it looks,

Set<Fighter> pairs2Set = new HashSet<>(pairs2);
Set<Fighter> setDiff = pairs1.stream()
    .filter(f -> !pairs2Set.contains(f))
    .collect(Collectors.toSet());

回答2:

You do not use address when matching inside the filter.

List<Fighter> res = pairs1.stream()
                    .filter(f -> !pairs2.stream()
                                        .anyMatch(s -> f.getName().equals(s.getName())
                                                  && f.getAddress().equals(s.getAddress())))
                   .collect(Collectors.toList());

Here filter the element of first list if contains in second list based on condition.

Better approach:

Using List's contains method

List<Fighter> res = pairs1.stream()
                            .filter(e -> !pairs2.contains(e)) 
                            .collect(Collectors.toList());

You need to override equals() method in Fighter class. contains() method you will use the equals() method to evaluate if two objects are the same.

@Override
public boolean equals(Object o) {
    if (o == this)
        return true;
    if (!(o instanceof Fighter))
        return false;
    Fighter f = (Fighter) o;
    return f.name.equals(name) && f.address.equals(address);
}

But using Set it will be more faster. See similar problem solution using Set

来源：https://stackoverflow.com/questions/57621371/find-elements-in-a-list-that-are-not-present-in-another-list-using-java-8