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);
            }   
        }
}

Nidhish Krishnan · Accepted Answer · 2019-08-15 21:47:26Z

4

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)
]

edited Aug 15, 2019 at 21:47

answered Aug 15, 2019 at 21:42

Nidhish Krishnan

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1 Comment

ak77 Over a year ago

I had to change a little as the model was checking everything instead of ids and I didn't want to update that. but this worked. thank you so much for teaching me how to do this.

randypaq13 · Accepted Answer · 2019-08-14 20:54:59Z

0

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());

answered Aug 14, 2019 at 20:54

randypaq13

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1 Comment

ak77 Over a year ago

no this doesn't replace the list like that.i still have what I started with in list 1

Naman · Accepted Answer · 2019-08-15 02:31:15Z

0

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));

answered Aug 15, 2019 at 2:31

Naman

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1 Comment

ak77 Over a year ago

Thanks for the response. this solved my 1 issue where I am replacing the data from list2 with list 1 when the id is matching. but it doesn't add the data from list2 to list1 when the id is not matching with any of the ids (and it checked all data from list 2)

ak77 · Accepted Answer · 2019-08-16 15:04:27Z

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()+"]");
    }

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