How to compare two different lists in Java?

Question

I have two lists that share a common value between them. I wanted these selected objects to be filtered so that it is added to another list

List<Objeto1> lista1 = new ListaElementos1().getElementos();
List<Objeto2> lista2 = new ListaElementos2().getElementos();

List<Objeto1> destino = new ArrayList<>();



for(Objeto1 item : lista1){
    for(Objeto2 array: lista2){
        if(array.getId() == item.getId()){
            if(!destino.contains(item)){
                destino.add(item);
            }
        }
    }
}

That was the way I found to make the filter so that it met my need. But I don’t think this is the best way to do it (by having to run the two entire lists for each record).

Is there any way to do this without the need to create 2 loops?

Answer 1

You can do this using stream:

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

public class Teste {
    public static void main(String[] args) {
        List<Integer> lista1 = Arrays.asList(1, 2, 3, 4, 5, 6);
        List<String> lista2 = Arrays.asList("5", "6", "7", "8");

        List<Integer> interseccao = lista1.stream().filter(item1 -> {
            return lista2.stream().filter(item2 -> new Integer(item2).equals(item1)).findAny().isPresent();
        }).collect(Collectors.toList());

        System.out.println(interseccao);
    }
}

Answer 2

Maybe that’s what you want, but either way he’ll run the two lists.

I’m practically beginner in Java and I’m not sure if this is the best way.

            List<Object1> arrayObj1 = Arrays.asList(
                                new Object1(1),
                                new Object1(2),
                                new Object1(3),
                                new Object1(4)
                            );

    List<Object2> arrayObj2 = Arrays.asList(
                                new Object2(4),
                                new Object2(5),
                                new Object2(6),
                                new Object2(7)
                            );

    //Array que receberá o resultado da comparação
    List<Object1> array2Obj1;

    array2Obj1 = arrayObj1.stream()
        .filter( o1 -> {
            return arrayObj2.stream()
                    .map(Object2::getId)
                    //Compara se o id do Object2 é igual ao id do Object1
                    .anyMatch(i2 -> i2.equals(o1.getId()));
        }).collect(Collectors.toList());

   array2Obj1.stream().map(Object1::getId).forEach(System.out::println); // Result: 4