Compare two array lists of objects by property value

Question

I have two array lists of objects, I need to compare them by the property value and to return unique. Object has two properties, question and answer, only the objects from list one with unique question should be returned.

        *faq(question, answer)*

        List<faq> one = new ArrayList<>();
        one.add(new faq("question 1", "answer 1"));
        one.add(new faq("question 2", "answer 2"));
        one.add(new faq("question 3", "answer 3"));
        
        List<faq> two = new ArrayList<>();
        two.add(new faq("question 4", "answer 4"));
        two.add(new faq("question 5", "answer 5"));
        two.add(new faq("question 1", "answer 6"));
        two.add(new faq("question 7", "answer 7"));
        two.add(new faq("question 8", "answer 8"));

From this code here I want to get objects from list one with questions 2 and 3 since only those two are not contained in list two

Does the class faq implement/overrides the equals (and hashcode) method? — user6073886
– user6073886, Commented Dec 3, 2020 at 15:12
Are you looking for List#removeAll method? javadocs If you want to leave list one without changes, just copy it first. — Amongalen
– Amongalen, Commented Dec 3, 2020 at 15:22

Tushar · Accepted Answer · 2020-12-03 15:27:59Z

You need to implement the equals() and hashCode() in Faq.java

Faq.java

    import java.util.Objects;
    import java.util.StringJoiner;
    
    public class Faq {
        private String questions;
        private String answer;
    
        public Faq(final String questions, final String answer) {
            this.questions = questions;
            this.answer = answer;
        }
    
    
        @Override
        public boolean equals(final Object o) {
            if (this == o) return true;
            if (o == null || getClass() != o.getClass()) return false;
            final Faq faq = (Faq) o;
            return questions.equals(faq.questions);
        }
    
        @Override
        public int hashCode() {
            return Objects.hash(questions);
        }
    
        @Override
        public String toString() {
            return new StringJoiner(", ", Faq.class.getSimpleName() + "[", "]")
                    .add("questions='" + questions + "'")
                    .toString();
        }
    }

Main.java

import java.util.ArrayList;
import java.util.List;

public class Main {

    public static void main(String[] args) {
        List<Faq> one = new ArrayList<>();
        one.add(new Faq("question 1", "answer 1"));
        one.add(new Faq("question 2", "answer 2"));
        one.add(new Faq("question 3", "answer 3"));

        List<Faq> two = new ArrayList<>();
        two.add(new Faq("question 4", "answer 4"));
        two.add(new Faq("question 5", "answer 5"));
        two.add(new Faq("question 1", "answer 6"));
        two.add(new Faq("question 7", "answer 7"));
        two.add(new Faq("question 8", "answer 8"));

        one.removeAll(two);
        System.out.println(one);
    }
}

Output

    [Faq[questions='question 2'], Faq[questions='question 3']]

Worth noting that it modifies list one. If OP wants to leave one as is, he should copy it first.

Collectives™ on Stack Overflow

Compare two array lists of objects by property value

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related