How to filter objects contained in two Sets based on a single property without overriding equals() a-CodePudding

I have a set of objects and trying to find if an object exist in list by id. I do not want to override equals() and hashCode().

class ABC {
    Long id;
    String name;
}

I want to do the below statement without overriding equals() and hashCode() methods

Set<ABC> abcSetOne;
Set<ABC> abcSetTwo;

abcSetOne.stream()
   .filter(abcSetTwo::contains)
   .collect(Collectors.toSet());

ids are guaranteed unique.

I am using Java 8.

CodePudding user response：

Update

When need set intersection (in terms of set theory) of your object based on one attribute only without changing the existing hashCode() and equals() methods, you can create a HashSet of ids.

public static void main(String[] args) {
    Set<ABC> abcSetOne = Set.of(new ABC(1L, "first"), new ABC(2L, "second"), new ABC(3L, "third"));
    Set<ABC> abcSetTwo = Set.of(new ABC(1L, "first"), new ABC(5L, "fifth"), new ABC(3L, "third"));
    
    Set<Long> idsOne = abcSetTwo.stream()
        .map(ABC::getId)
        .collect(Collectors.toSet());
    
    Set<ABC> result = abcSetOne.stream()
        .filter(abc -> idsOne.contains(abc.getId()))
        .collect(Collectors.toSet());
    
    System.out.println(result);
}

Output

[ABC{id=1, name='first'}, ABC{id=3, name='third'}]

When you need to establish uniqueness based on more than one object's property, you can define a wrapper class with the hashCode/equals contract implemented based on these properties.

As a wrapper you can utilize of Java 16 record, that will make the code leaner (hashCode() and equals() will be generated by the compiler)

public record ABCWrapper(Long id, String name) {
    ABCWrapper(Wrapping.ABC abc) {
        this(abc.getId(), abc.getName());
    }
}

For earlier versions, ABCWrapper can be defined as a plain class.

main() - demo

public static void main(String[] args) {
    Set<ABC> abcSetOne = Set.of(new ABC(1L, "first"), new ABC(2L, "second"), new ABC(3L, "third"));
    Set<ABC> abcSetTwo = Set.of(new ABC(1L, "first"), new ABC(5L, "fifth"), new ABC(3L, "third"));
    
    Map<ABCWrapper, ABC> abcByWrapper = abcSetTwo.stream()
        .collect(Collectors.toMap(ABCWrapper::new, Function.identity()));
    
    Set<ABC> result = abcSetOne.stream()
        .map(ABCWrapper::new)
        .filter(abcByWrapper::containsKey)
        .map(abcByWrapper::get)
        .collect(Collectors.toSet());

    System.out.println(result);
}

Output

[ABC{id=1, name='first'}, ABC{id=3, name='third'}]