Find element matching in 2 lists using java 8 stream and updating one list value from the other

Question

I have two lists of Objects. Need to traves through both them, match them by id then update list1 values of that object from list 2 if the id matches.

I know how to do it with old java but wondering if it can be done in JAVA8 (stream)

class Person {
    String id ;
    String name;
    String age;
}
p1:{1,test1,3}
p2:{2,test2,6}
p3:{3,test3,8}
p4:{1,test1,33}
p5:{2,test22,16}
p6:{3,test3,18}
p10:{10,test10,8}

List<Person> list1 = {p1,p2, p3, p10};
List<Person> list2 = {p4,p5, p6}; 

wanting the end result like this

list1 = {p1,p2, p3, p10}
p1:{1,test1,33}
p2:{2,test22,16}
p3:{3,test3,18}
p10:{10,test10,8}

basically, I want if the id matches I want the values to be overridden from list2 to list 1 (don't want list 3)

so far I was able to match them by id but then I am lost how to update the fields...

List<CartItemAttribute> test = existingCartItem.get().getAttributes().stream()
                        .filter(x -> (cartItem.getAttributes().stream()
                        .filter(y->y.getCartItemAttributeId()== x.getCartItemAttributeId())
                        .count()<1
)).collect(Collectors.toList());

here is what i have that works as expected

private void upsertAttributes(CartItem cartItem, Optional<CartItem> existingCartItem) {
        boolean addAtribute = true;
        List<CartItemAttribute> existingAttribute = existingCartItem.get().getAttributes();
        for (CartItemAttribute requestingList: cartItem.getAttributes()) {
            addAtribute = true;
            for (CartItemAttribute existingList: existingAttribute) {
                if(StringUtils.isNotBlank(requestingList.getCartItemAttributeId()) 
                              && requestingList.getCartItemAttributeId()
                                .equals(existingList.getCartItemAttributeId())) {

                    existingList.setKey(requestingList.getKey());
                    existingList.setValue(requestingList.getValue());
                    addAtribute = false;
                }
            }
            if(addAtribute) {
                existingAttribute.add(requestingList);
            }   
        }
}

Answer 1

Try this out

List<Person> list1 = getPersonList1();
List<Person> list2 = getPersonList2();

list1.replaceAll(l1 -> list2.stream()
          .filter(l2 -> l2.getId().equals(l1.getId()))
          .findAny()
          .orElse(l1));

list1.addAll(list2.stream()
          .filter(l2 -> !list1.contains(l2))
          .collect(toList()));

getPersonList1

private static List<Person> getPersonList1() {
    return Lists.newArrayList(
        new Person("1", "test1", "3"),
        new Person("2", "test2", "6"),
        new Person("3", "test3", "8"),
        new Person("10", "test10", "8")
    );
}

getPersonList2

private static List<Person> getPersonList2() {
    return Lists.newArrayList(
        new Person("1", "test1", "33"),
        new Person("2", "test22", "16"),
        new Person("3", "test3", "18"),
        new Person("4", "test4", "15")
    );
}

Output

List1

[
Person(id=1, name=test1, age=3), 
Person(id=2, name=test2, age=6), 
Person(id=3, name=test3, age=8), 
Person(id=10, name=test10, age=8)
]

List2

[
Person(id=1, name=test1, age=33), 
Person(id=2, name=test22, age=16), 
Person(id=3, name=test3, age=18), 
Person(id=4, name=test4, age=15)
]

Final List1

[
Person(id=1, name=test1, age=33), 
Person(id=2, name=test22, age=16), 
Person(id=3, name=test3, age=18), 
Person(id=10, name=test10, age=8), 
Person(id=4, name=test4, age=15)
]

Answer 2

You can reassign the list by doing:

 list1 = existingCartItem.get().getAttributes().stream() .filter(x -> (cartItem.getAttributes().stream() .filter(y->y.getCartItemAttributeId()== x.getCartItemAttributeId()) .count()<1 )).collect(Collectors.toList()); 

Answer 3

You could implement the in-place update of the first list using replaceAll as:

list1.replaceAll(l1Person -> list2.stream()
        .filter(l2Person -> l2Person.getId().equals(l1Person.getId()))
        .findAny()
        .orElse(l1Person));

Answer 4

existingCartAttributes.replaceAll(l1 -> requestingCartAttributes.stream()
              .filter(l2 -> l2.getCartItemAttributeId().equals(l1.getCartItemAttributeId()))
              .findAny()
              .orElse(l1));

    System.out.println("*********************************");
    for (CartItemAttribute var: existingCartAttributes) {
        System.out.println("[" +var.getCartItemAttributeId() + "] [" + var.getKey() + "] [" +var.getValue()+"]");
    }
    System.out.println("*********************************");
    existingCartAttributes.addAll(requestingCartAttributes.stream()
              .filter(l2 -> StringUtils.isBlank(l2.getCartItemAttributeId()))
              .collect(Collectors.toList()));
    for (CartItemAttribute var: existingCartAttributes) {
        System.out.println("[" +var.getCartItemAttributeId() + "] [" + var.getKey() + "] [" +var.getValue()+"]");
    }